variation

Evolution
I   INTRODUCTION

Purple and Yellow Tube Sponge
Sponges, considered to be the most primitive of the multicellular animals, are represented in the fossil record back to the Cambrian Period, at least 600 million years ago. The purple and yellow tube sponge displays one of the many different body forms typical of sponges. The interior body cavities of sponges provide shelter for a variety of small crabs, sea stars, and other marine invertebrates.
Joe Dorsey/Oxford Scientific Films

Evolution, in biology, complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over the course of time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees that span millions of years.

Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, birds, and mammals, share a common ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately 6 million to 7 million years ago (see Human Evolution). On the other hand, an ancestor common to humans and reptiles lived some 300 million years ago. And the common ancestor to even more distantly related forms lived even further in the past. Evolutionary biologists attempt to determine the history of lineages as they diverge and how differences in characteristics developed over time.

Throughout history, philosophers, religious thinkers, and scientists have attempted to explain the history and variety of life on Earth. During the rise of modern science in western Europe in the 17th and 18th centuries, a predominant view held that God created every organism on Earth more or less as it now exists. But in that time of burgeoning interest in the study of fossils and natural history, the beginnings of a modern evolutionary theory began to take shape. Early evolutionary theorists proposed that all of life on Earth evolved gradually from simple organisms. Their knowledge of science was incomplete, however, and their theories left too many questions unanswered. Most prominent scientists of the day remained convinced that the variety of life on Earth could only result from an act of divine creation.

In the mid-19th century a modern theory of evolution took hold, thanks to British naturalist Charles Darwin. In his book On the Origin of Species by Means of Natural Selection, published in 1859, Darwin described the evolution of life as a process of natural selection. Life, he suggested, is a competitive struggle to survive, often in the face of limited resources. Living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment, such as changes in climate. Darwin offered that, within a given population in a given environment, certain individuals possess characteristics that make them more likely to survive and reproduce. These individuals will pass these critical characteristics on to their offspring. The number of organisms with these traits increases as each generation passes on the advantageous combination of traits. Outmatched, individuals lacking the beneficial traits gradually decrease in number. Slowly, Darwin argued, natural selection tips the balance in a population toward those with the combination of traits, or adaptations, best suited to their environment.

Animal Kingdom
Kingdom Animalia includes more than one million living species, grouped into more than 30 phyla. Vertebrates, members of the phylum Chordata, comprise only one percent of these organisms. Phylum Arthropoda is more successful in sheer numbers, total mass, and distribution than all other groups of animals combined. The remaining animal phyla are composed of mostly marine-dwelling organisms. Illustrated here is the evolutionary relationship between all of these groups.
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While On the Origin of Species was an instant sensation and best-seller, Darwin’s theories faced hostile reception by critics who railed against his blasphemous ideas. Other critics pointed to questions left unresolved by Darwin’s careful arguments. For instance, Darwin could not explain the mechanism that caused life forms to change from generation to generation.

Hostility gave way to acclaim as scientists vigorously debated, explored, and built on Darwin’s theory of natural selection. As the 20th century unfolded, scientific advances revealed the detailed mechanisms missing from Darwin’s theory. Study of the complex chemistry of all organisms unveiled the structure of genes as well as how they are duplicated, altered, and passed from generation to generation. New statistical methods helped explain how genes in specific populations change over generations. These new methods provided insight into how populations remain adaptable to changing environmental circumstances and broadened our understanding of the genetic structure of populations. Advances in techniques used to determine the age of fossils provided clues about when extinct organisms existed and details about the circumstances surrounding their extinction. And new molecular biology techniques compare the genetic structures of different species, enabling scientists to determine heretofore undetectable evolutionary relationships between species. Today, evolution is recognized as the cornerstone of modern biology. Uniting such diverse scientific fields as cell biology, genetics, paleontology, and even geology and statistics, the study of evolution reveals an exquisitely complex interaction of the forces that act upon every life form on Earth.

II   GENETIC BASIS OF EVOLUTION

Structure of DNA
 
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Natural selection is tied to traits that organisms pass from one generation to the next (see Heredity). In humans, these traits include hundreds of features such as eye color, blood type, and height. Nature offers countless other examples of traits in living things, such as the pattern on a butterfly’s wings, the markings on a snail’s shell, the shape of a bird’s beak, or the color of a flower’s petals.

Recessive Gene Transmission
Some genes that cause genetic diseases interact in a dominant-recessive pattern. In these cases, two copies of the recessive gene are required for the disease to occur. A person who has just one copy of the recessive gene is termed a carrier, since he or she carries the gene but is not affected by it. In this illustration, the dominant gene is represented in green, and the recessive in blue. For the couple on the left, the father has one copy of the dominant gene and one copy of the recessive gene. The mother has two copies of the dominant gene. Each parent can contribute just one gene to the child. The four children shown on the lower left represent the probabilities (not the actual children) for the combinations that can result from their parents. The children on the far left received the recessive gene from their father and the dominant gene from their mother, and are therefore carriers. For any child born to these parents, there is a 50 percent chance that the child will be a carrier. Since none of the children can inherit two copies of the recessive gene, none of the children will develop the disease. When both parents are carriers, however, as shown by the couple on the right, there is a 25 percent chance that any child born has the disease, a 50 percent chance that a child is a carrier, and a 25 percent chance that a child does not have the disease and is not a carrier.
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Such traits are controlled by specific bits of biochemical instructions known as genes. Genes are composed of individual segments of the long, coiled molecule called deoxyribonucleic acid (DNA). They direct the synthesis of proteins, molecular laborers that serve as building blocks of cells, control chemical reactions, and transport materials to and from cells. Proteins are themselves composed of long chains of amino acids, and the biochemical instructions found in DNA determine the arrangement of amino acids in a chain. The specific sequence of amino acids dictates the structure and resulting function of each protein.

All genetic traits result from different combinations of gene pairs, one gene inherited from the mother and one from the father. Each trait is thus represented by two genes, often in different forms. Different forms of the same gene are called alleles. Traits depend on very precise rules governing how genetic units are expressed through generations. For example, some people have the ability to roll their tongue into a U-shape, while others can only curve their tongue slightly. A single gene with two alleles controls this heritable trait. If a child inherits the allele for tongue rolling from one parent and the allele for no tongue rolling from the other parent, she will be able to roll her tongue. The allele for tongue rolling dominates the gene pair, and so its trait is expressed. According to the laws governing heredity, when a dominant allele (in this case, tongue rolling) and a recessive allele (no tongue rolling) combine, the trait will always be dictated by the dominant allele. The no tongue rolling trait, or any other recessive trait, will only occur in an individual who inherits the two recessive alleles.

A   Genetic Variation in Populations

Human Fertilization
 
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Evolutionary change takes place in populations over the course of many generations. Since individual organisms cannot evolve in a single lifetime, evolutionary science focuses on a population of interbreeding individuals. All populations contain some variations in traits. In humans, for example, some people are tall, some are short, and some are of medium height (see Population Biology).

In interbreeding populations, genes are randomly shuffled among members of the population through sexual reproduction, the process that produces genetically unique offspring. Individuals of different sexes develop specialized sex cells called gametes. In humans and other vertebrates (animals with backbones), these gametes are sperm in males and eggs in females. When males and females mate, these sex cells join in fertilization. A series of cell divisions creates individuals with a unique assembly of genes. No individual members of any population (except identical twins, which develop from a single egg) are exactly alike in their genetic makeup. This diversity, referred to as genetic diversity or variation, is essential to evolution. The greater a population’s genetic diversity, the more likely it is to evolve specific traits that enable it to adapt to new environmental pressures, such as climate change or disease. In contrast, such pressures might drive a population with a low degree of genetic diversity to extinction.

Sexual reproduction ensures that the genes in a population are rearranged in each generation, a process termed recombination. Although the combinations of genes in individuals change with each new generation, the gene frequency, or ratio of different alleles in the entire population, remains relatively constant if no evolutionary forces act on the population. One such force is the introduction of new genes into the genetic material of the population, or gene pool.

A1   Gene Flow

When individuals move between one population and another, new genes may be introduced to populations. This phenomenon, known as gene flow, results from chance dispersal as well as intentional migration. Take, for example, two populations of related wildflowers, one red and one white, separated by a large tract of land. Under normal circumstances, the two groups do not interbreed because the wind does not blow hard enough to carry pollen between the populations so that pollination can occur. If one day an unusually strong wind carries pollen from the red wildflower population to the white wildflower population, the gene for red flowers may be introduced to the white population’s gene pool.

In many animals, gene flow results when individuals from one population migrate to another population. The Lake Erie water snake provides an excellent example. Although skilled swimmers, these snakes spend much of their day basking in the sun on overhanging vegetation or rocks. In Lake Erie, water snakes form distinct populations—snakes that live on the rock islands in the lake, and others that live in the vegetation close to the shore. Shore populations have gray bodies with black bands, a coloration that helps them evade hungry seagulls by blending with the shoreline vegetation. Island snakes are primarily light gray with no banding, coloring that helps them to blend in to their rocky surroundings. An easy target for seagulls, banded island snakes rarely survive to reproductive age. Yet every year, biologists count banded newborns among the island litters. What at first appeared to be a mystery was later revealed to be gene flow at work. Although the populations do not regularly interbreed, once in a while shore snakes, carried by currents or winds, migrate to the islands. Once there, they mate with island snakes, reintroducing the gene for banding into the island population as they do.

A2   Mutation

Sickled Red Blood Cells
A mutation in the gene responsible for producing hemoglobin in the blood causes a disease known as sickle-cell anemia. In this disease the structure of the oxygen-carrying protein in the human bloodstream is severely altered. The mutation changes the structure of red blood cells to a slender sickle shape.
London Scientific Films/Oxford Scientific Films

Genes themselves are constantly being modified through a process called mutation—a change in the structure of the DNA in an individual’s cells. Mutations can occur during replication, the process in which a cell splits itself into two identical copies known as daughter cells. Normally each daughter cell receives an exact copy of the DNA from the parent cell. Occasionally, however, errors occur, resulting in a change in the gene. Such a change may affect the protein that the gene produces and, ultimately, change an individual’s traits. While some mutations occur spontaneously, others are caused by factors in the environment, known as mutagens. Examples of mutagens that affect human DNA include ultraviolet light, X rays, and various chemicals.

Whatever their cause, mutations are a rare but slow and continuous source of new genes in a gene pool. Most mutations are neutral—that is, they have no effect. Other mutations are detrimental to life, causing the immediate death of any organism that inherits them. Once in a great while, however, a mutation provides an organism with an advantageous trait. A single organism with an advantageous trait is only half of the equation, however. For evolution to occur, the forces of natural selection must distribute that trait to other members of a population.

III   NATURAL SELECTION IN POPULATIONS

Stonefish
Stonefish have evolved an ornate coloration that blends well against the coral and mud of their ocean bottom habitat. Natural selection may have favored this coloration in stonefish because well camouflaged members of the population were better able to surprise small fish and other prey.
Carl Roessler/Bruce Coleman, Inc.

Natural selection sorts out the useful changes in the gene pool. When this happens, populations evolve. Beneficial new genes quickly spread through a population because members who carry them have a greater reproductive success, or evolutionary fitness, and consequently pass the beneficial genes to more offspring. Conversely, genes that are not as good for an organism are eliminated from the population—sometimes quickly and sometimes more gradually, depending on the severity of the gene—because the individuals who carry them do not survive and reproduce as well as individuals without the bad gene. Over the course of several generations, the gene and most of its carriers are eliminated from the population. Severely detrimental genes may persist at very low levels in a population, however, because they can be reintroduced each generation by mutation.

Natural selection only allows organisms to adapt to their current environment. Should environmental conditions change, new traits may prevail. Moreover, natural selection does not always favor a single version of a trait. In some cases, multiple versions of the same trait may instill their carriers with equal evolutionary benefit. Nor does natural selection always favor change. If environmental conditions so dictate, natural selection maintains the status quo by eliminating extreme versions of a particular trait from the population.

A   Directional Selection

English Peppered Moths
The color change that English peppered moths underwent during the 1800s is a classic example of directional selection. Before the Industrial Revolution took place in England in the late 1700s, light-colored English peppered moths that blended with the lichen-covered bark of trees were far more prevalent than dark-colored English peppered moths. However, pollution from the Industrial Revolution killed the lichen on trees, leaving their dark bark exposed, and the contrasting light-colored moths became easy prey for birds. The dark English peppered moths, camouflaged on the dark bark, soon became far more common than the lighter varieties in polluted areas.
David Fox/Oxford Scientific Films

Often, shifts in environmental conditions, such as climate change or the presence of a new disease or predator, can push a population toward one extreme for a trait. In periods of prolonged cold temperatures, for example, natural selection may favor larger animals because they are better able to withstand extreme temperatures. This mode of natural selection, known as directional selection, is evident in cheetahs. About 4 million years ago, cheetahs were more than twice as heavy as modern cheetahs. But quicker and lighter members of the population had greater reproductive success than did larger members of the population. Over time, natural selection favored smaller and smaller cheetahs.

B   Stabilizing Selection

Horseshoe Crab
Sometimes natural selection operates to prevent change in a population. This type of natural selection, called stabilizing selection, may explain why some organisms have remained virtually the same for hundreds of millions of years. For example, the horseshoe crab, often referred to as a living fossil, has remained essentially unchanged for approximately 500 million years.
David Cayless/Oxford Scientific Films

Sometimes natural selection acts to preserve the status quo by favoring the intermediate version of a characteristic instead of one of two extremes. An example of this selective force, known as stabilizing selection, was evident in a study of the birth weight of human babies published in the middle of the 20th century. It showed that babies of intermediate weight, about 3.5 kg (8 lb), were more likely to survive. Babies with a heftier birth weight had lower chances for survival because they were more likely to cause complications during the delivery process, and lightweight babies were often born premature or with other health problems. Babies of intermediate birth weight, then, were more likely to survive to reproductive age.

C   Disruptive Selection

Occasionally natural selection favors two extremes, causing alleles for intermediate forms of a trait to become less common in the gene pool. The African Mocker swallowtail butterfly has undergone this form of selection, known as disruptive selection. The Mocker swallowtail evades its predators by resembling poisonous butterflies in its ecosystem. Predators have learned to avoid these poisonous butterflies and also to steer away from the look-alike Mocker swallowtails. The Mocker swallowtail has a large range, and in different regions, the Mocker swallowtail looks very different, depending on which species of poisonous butterfly it mimics. In some areas the butterfly displays black markings on a white background; in others the markings float on an orange background. As long as a Mocker swallowtail appears poisonous to predators, it has a greater chance of survival and therefore a higher evolutionary fitness. Mocker swallowtails that do not look poisonous have a much lower evolutionary fitness because predators quickly eat them. Disruptive selection, then, favors the extreme color patterns of white or orange, and nothing in between.

D   Sexual Selection

Elk Rivalry
During the mating season, a bull elk, or wapiti, must defend his harem of females from rival males. The opponents lock antlers and attempt to push each other away. With their large size and sharp points, the antlers are formidable weapons capable of inflicting serious injury to the loser.
BBC Worldwide Americas, Inc.

Sexual selection operates on factors that contribute to an organism’s mating success. In many animals, sexual attractiveness is an important component of selection because it increases the likelihood of mating. Sexual selection rarely affects females, because the duration of pregnancy and infant care limits the number of babies they can have. Males, on the other hand, have few limitations on the number of offspring they can father, and a male who produces many offspring has a high level of evolutionary fitness. Males of many species, then, must compete with other males to mate with females. Some males win females’ attention more often than others and, as a result, pass their genes to more offspring.

Peacock
The male peacock’s elongated upper tail feathers and brilliant colors are used to attract females during courtship.
BBC Natural History Sound Library. All rights reserved./Corbis

In many species, sexual selection results in males with elaborate features. Many male birds, such as peacocks, have colorful and showy plumage. Male fiddler crabs have one greatly enlarged claw, and large skin flaps frame the face of the male frilled lizards. In some species, males perform elaborate courtship dances designed to demonstrate their virility and physical fitness.

Fiddler Crab
The fiddler crab, characterized by the enlargement of one of the claws in males, is commonly found on beaches and in marshes throughout the temperate and tropical regions of the world. Although greatly enlarged, the male’s claw is not used for combat but rather for attracting females during mating season. As at home on exposed beaches as it is under water, the fiddler crab prefers the shelter of a shallow burrow when covered by the tide. Fiddler crabs feed on plant debris, decomposed organic matter, and even carrion.
P. and W. Ward/Oxford Scientific Films

Many such traits are a liability to survival, making them counter to the principles of natural selection. For instance, bright coloration and elaborate courtship dances draw the attention of predators. The fiddler crab’s large claw is cumbersome, as are the frilled lizard’s skin flaps. The huge tail feathers of the male peacock give it an awkward, bumbling gait. All of these features undoubtedly slow the animals down, making them less capable of evading predators or securing prey. Nevertheless, the increased reproductive success these showy characteristics instill makes them worth the risk.

IV   GENETIC DRIFT

Northern Elephant Seal
Heavy hunting reduced the number of northern elephant seals to as few as 20 individuals in the 19th century. Their numbers have rebounded significantly since a ban on seal hunting became effective in the early 20th century. However, all living northern elephant seals descended from the small group that survived the severe hunting of the 19th century. This so-called population bottleneck drastically reduced the genetic diversity of northern elephant seal populations, leaving the animals vulnerable to disease and other environmental pressures. The male northern elephant seal, right, typically weighs more than three times the female seal, left.
Patti Murray/Animals Animals

Natural selection is not the only force that changes the ratio of alleles present in a population. Sometimes the frequency of particular alleles may be altered drastically by chance alone. This phenomenon, known as genetic drift, can cause the loss of an allele in a population, even if the allele leads to greater evolutionary fitness. Conversely, genetic drift can cause an allele to become fixed in a population—that is, the allele can be found in every member of the population, even if the allele decreases fitness.

Although any population can fall victim to genetic drift, small populations are more vulnerable than larger populations. Imagine that a particular allele is present in 25 percent of a population of worms. If a flood occurs and randomly eliminates half of the population, the laws of probability predict that approximately 25 percent of the surviving population will carry the allele. In a population of 120,000 worms, this means that about 15,000 of the surviving 60,000 worms will carry the allele. Even if, by chance, the flood claimed the lives of an additional 10 percent of the carriers, thousands of copies of the allele would still remain in the population. But in a population of only 12 worms, the laws of probability predict that only 1.5 of the surviving 6 worms would carry the allele. If, by chance, the flood claimed more of the carriers of the allele than the noncarriers, the allele could be eliminated.

The hypothetical flood created what is called a population bottleneck. It reduced the genetic variation in the smaller population such that, even if the group’s number again reached 12 members, its genetic diversity might very well be lower than the genetic diversity of the original population. All of the descendants came from just a few surviving individuals, who carried just a fraction of the alleles present in the former population. Likewise, when a few individuals leave a large population and establish a new one, they bring only a fraction of the genetic diversity of the original population with them. Any descendants of the founding members face the possibility of a drastically reduced genetic diversity. An example of this principle, known as the founder effect, is evident in the Amish community in Pennsylvania. All of the people in this community are descendants of about 200 individuals who established the community after leaving Europe in the early 1700s. One of these founders carried an unusual allele that causes a rare kind of dwarfism. As a result, in the Pennsylvania Amish community today the frequency of this rare allele is 1 in 14 individuals. In the general population this allele appears in 1 in 1,000 individuals.

V   ORIGIN OF NEW SPECIES

The forces of natural selection and genetic drift continuously influence and change the characteristics of a population. However, most often these forces are not sufficient to create an entirely new species. Different species arise when, for one reason or another, members of a population cease to interbreed. When something prevents populations from mating, they are said to be reproductively isolated from one another. Two reproductively isolated populations cannot randomly exchange genetic material with each other, and as a result, the groups diverge as they evolve independently of one another. In this process, called speciation, the members of each group become so different that they can no longer successfully interbreed. At this point, a new species has formed.

Interbreeding normally continues if there is nothing to stop it. Anything that hinders interbreeding is called an isolating mechanism. Geographic barriers isolate populations, leading to the formation of entirely new species in a process called allopatric speciation. Less frequently, mutations or subtle changes in behavior prevent individuals living in close proximity from reproducing. This may lead to sympatric speciation, in which two distinct subgroups of a population cease exchanging genetic material and evolve into two or more distinct species.

A   Allopatric Speciation

Toroweap Point, Grand Canyon National Park
When a geographic barrier, such as the Grand Canyon, prevents members of a population from interbreeding, the groups evolve independently of one another and may eventually constitute two or more species. For example, although they descended from the same original population, the squirrels on the south rim of the Grand Canyon now belong to an entirely different species than the squirrels on the north rim. By comparison, birds and other animals capable of crossing the divide continue to interbreed and so remain members of the same species.
Pat O’Hara Photography

When a barrier, such as a stretch of sea or a mountain range, separates different populations of a particular species, the populations may no longer be capable of crossing the barrier to interbreed. Speciation caused by geographic isolating mechanisms, or allopatric speciation, is evident in the many different populations of pupfish that live in the Death Valley region of California and Nevada. About 50,000 years ago this region had a damp, rainy climate and was peppered by lakes and ponds connected by streams and rivers. Over time, rainfall decreased significantly, and by about 4,000 years ago, this region was a desert. The interconnected lakes and streams dried up, and in their place remained a series of small, isolated stream-fed ponds. Each pond is home to a different species of pupfish, specially adapted to its pond’s unique temperature and mineral composition. Biologists speculate that all of these species of pupfish descended from a single species that inhabited the interconnected lakes and streams of the region about 50,000 years ago. As the lakes and streams dried up, the dry ground that separated them became a geographical isolating mechanism that prevented the individual populations from interbreeding. Consequently, the many pupfish populations evolved independently.

B   Sympatric Speciation

Western Grebe Courtship
During the spring, western grebes perform spectacular courtship dances. In the “rushing” display, the mating pair swim side-by-side with their wings held back, their long necks arched, and their yellow beaks angled upward. They swim so quickly that their bodies are pushed up out of the water and they appear to run across the surface. This courtship ritual functions as a reproductive isolating mechanism, one way of preventing western grebes from exchanging genetic material with birds of other species.
BBC Worldwide Americas, Inc.

In sympatric speciation, isolating mechanisms may be triggered by differences in habitat, sexual reproduction, or heredity. Similar plants may fail to breed together because their flowering seasons are different. Many different types of rain forest orchids, for example, cannot interbreed because they flower on different days. Some animals mate only if they recognize characteristic color patterns or scents of their own group. Other organisms, particularly birds, are stimulated to breed only after witnessing a song, display, or other courtship ritual that is characteristic in their group (see Animal Courtship and Mating).

Mule
A horse can mate with a donkey to produce a mule. The two do not blend into a single species, however, because mules are sterile—they cannot pass their hybrid genes to another generation. Strong and good-tempered, mules are used worldwide in agriculture and as work animals.
M.J. Adams/Oxford Scientific Films

Sometimes two subpopulations of the same species do not produce offspring with one another, even though they come into breeding contact. This may be due, for example, to reproductive incongruities between two subpopulations that cause embryos to die before development and birth. In other instances, if viable offspring are produced, reproductive isolation is still maintained because the offspring are sterile. For example, asses and horses are capable of mating, but their offspring are usually sterile. Both types of reproductive dysfunction occur when the hereditary factors of the two groups have become incompatible in some way and cannot combine to produce normal offspring.

C   Gradual Change

Speciation may occur even when no isolating mechanism is present. In this case, a new species may form through the slow modification of a single group of organisms into an entirely new group. The evolving population gradually changes over the course of generations until the organisms at the end of the line appear very different from the first. Foraminifera, a tiny species of marine animals that live in the Indian Ocean, demonstrate this process, known as vertical or phyletic evolution. From about 10 million to 6 million years ago, the species remained relatively unchanged. These organisms then began a slow and gradual change, lasting about 600,000 years, that left them so unlike their ancestors that biologists consider them an entirely new species.

VI   PATTERNS OF DESCENT

Whatever the cause of their reproductive isolation, independently evolving populations tend to adhere to general patterns of evolutionary descent. Most often, environmental factors determine the pattern followed. A gradually cooling climate, for example, may result in a population of foxes developing progressively thicker coats over successive generations. This pattern of gradual evolutionary change occurs in a population of interbreeding organisms evolving together. When two or more populations diverge, they may evolve to be less alike or more alike, depending on the conditions of their divergence.

A   Divergent Evolution

In the pattern known as divergent evolution, after two segments of a population diverge, each group follows an independent and gradual process of evolutionary change, leading them to grow increasingly different from each other over time. Over the course of many generations, the two segments of the population look less and less like each other and their ancestor species. For example, when the Colorado River formed the Grand Canyon, a geographic barrier developed between two populations of antelope-squirrels. The groups diverged, resulting in two distinct species of antelope-squirrel that have different physical characteristics. On the south rim of the canyon is Harris’s antelope-squirrel, while just across the river on the north rim is the smaller, white-tailed antelope-squirrel.

B   Adaptive Radiation

Galápagos Finches
The fourteen species of finch that inhabit the Galápagos Islands are believed to have evolved from a single species resembling the blue-black grassquit, Volatinia jacarina, abundant in Latin America and the Pacific coast of South America. The ancestral finch, with its short, stout, conical bill specialized for crushing seeds, probably migrated from the mainland to the Galápagos Islands. Its descendants, free to exploit the resources they would otherwise share with warblers, woodpeckers, and other birds, adapted to the available range of habitats (tree, cactus, or ground) and food (seeds, cactus, fruit, or insects). The size and shape of their bills reflect these specializations, an example of adaptive radiation.
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Sometimes divergence occurs simultaneously among a number of populations of a single species. In this process, known as adaptive radiation, members of the species quickly disperse to take advantage of the many different types of habitat niches—that is, the different ways of obtaining food and shelter in their environment. Such specialization ultimately results in a number of genetically distinct but similar-looking species. This commonly occurs when a species colonizes a new habitat in which it has little or no competition. For example, a flock of one species of bird may arrive on some sparsely populated islands. Finding little or no competition, the birds may evolve rapidly into a number of species, each adapted to one of the available niches. Charles Darwin noted an instance of adaptive radiation on his visit to the Galápagos Islands off the coast of South America. He surmised that one species of finch colonized the islands thousands of years ago and gave rise to the 14 species of finchlike birds that exist there now. Darwin observed that the greatest differences in their appearance lay in the shapes of the bills, adapted for their mode of eating. Some species possessed large beaks for cracking seeds. Others had smaller beaks for eating vegetation, and still others featured long, thin beaks for eating insects.

C   Convergent Evolution

Cactus and Spurge Plants
Although all three of the plants in this picture appear to be cactus plants, the one to the far left is a spurge, not a true cactus. While both the cactus and the spurge have reduced, spiny leaves to prevent water loss and a large, succulent stem for water storage, they are quite different plants, distinguished by the presence of a milky latexlike fluid in the stem of the spurge. This similar adaptation to their arid environment is an example of convergent evolution.
Dorling Kindersley

Sometimes distantly related species evolve in ways that make them appear more closely related. This pattern, known as convergent evolution, occurs when members of distantly related species occupy similar ecological niches. Natural selection favors similar adaptations in each population.

Convergent Evolution
Although marsupial mammals once populated all land masses, they remain diversified only on the isolated Australian continent, where they have evolved to fill the same ecological niches that placental mammals occupy elsewhere. The Tasmanian wolf, for example, closely resembles the doglike carnivores of other continents. More specialized parallel adaptations include those of the marsupial and placental anteaters, the marsupial sugar glider and placental flying squirrels, and the burrowing marsupial wombat and placental ground hog. In this illustration, placental mammals are in the top row, and their marsupial equivalents are in the bottom row.
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Some of the best examples of convergent evolution are the marsupial mammals of Australia and their placental mammal counterparts on other continents. About 50 million years ago the Australian continent separated from the rest of the Earth’s continents. Biologists speculate that few if any placental mammals had migrated to Australia by the time the continents split. They also surmise that neither marsupial mammals, nor their placental counterparts were capable of crossing the ocean after the landmasses drifted apart. As a result, the animals evolved entirely independently. Yet many of the marsupial mammals in Australia strongly resemble many of the placental mammals found on other continents.

For example, the marsupial mole of Australia looks very much like the placental moles found on other continents, yet these animals have evolved entirely independent of one another. The explanation for the moles’ similar appearances lies in the principles of convergent evolution. Both species evolved to exploit similar ecological niches—in this case, the realm just beneath the surface of the ground. Over the course of millions of generations in both marsupial and placental moles, natural selection favored adaptations suited for a life of burrowing: tube-shaped bodies, broad, shovel-like feet, and short, silky fur that sheds dirt or sand easily. The most striking difference between placental moles and marsupial moles is the color of their fur. Placental moles are usually dark brown or gray, a coloration that enables them to blend in with the soil in their habitat. Marsupial moles burrow in the golden or reddish sand of Australia, so natural selection produced golden or golden-red fur.

D   Coevolution

Often two or more organisms in an ecosystem fall into evolutionary step with one another, each adapting to changes in the other, a pattern known as coevolution. Coevolution is often apparent in flowers and their pollinators. Hummingbirds, for example, have long, narrow beaks and a relatively poor sense of smell, and they are attracted to the color red. Fuchsias, flowering plants that rely on hummingbirds for pollination, usually have long, slender flowers in various shades of red, and they have little or no fragrance. What at first appears to be a remarkable coincidence is, in fact, the product of thousands of generations of evolutionary fine-tuning. More likely to attract hummingbirds than fuchsias with different coloration, red-flowered individuals had greater reproductive success. And hummingbirds tended to spend more time extracting nectar from the flowers of fuchsias with shapes that matched the size of their slender beaks, thus increasing the likelihood of successful pollination. By the same token, those hummingbirds with long, slender beaks were best able to collect nectar from the long-necked flower. Over many generations, long-beaked hummingbirds became the rule, rather than the exception, in hummingbird populations.

VII   HOW SCIENTISTS STUDY EVOLUTION

Species do not change overnight, or even in the course of one lifetime. Rather, evolutionary change usually occurs in tiny, almost imperceptible increments over the course of thousands of generations—periods that range from decades to millions of years. To study the evolutionary relationships among organisms, scientists must perform complex detective work, deriving indirect clues from the fossil record, patterns of animal distribution, comparative anatomy, molecular biology, and finally, direct observation in laboratories and the natural environment.

A   Fossils

Midge Fly Caught in Amber
Paleontologists can learn about prehistoric life by studying the remains of ancient insects, such as this midge fly, trapped in tree resin when they were alive. The resin eventually hardens and fossilizes into amber. Occasionally whole organisms are preserved in this manner.
John Koivula/Photo Researchers, Inc.

One way biologists learn about the evolutionary relationships between species is by examining fossils. These ancient remains of living things are created when a dead plant or animal is buried under layers of mud or sand that gradually turn into stone. Over time, the organism remains themselves may turn to stone, becoming preserved within the rock layer in which they came to rest. By measuring radioactivity in the rock in which a fossil is embedded, paleontologists (scientists who study the fossil record) can determine the age of a fossil (see Dating Methods).

Fossilized Trilobites
Although trilobites became extinct about 250 million years ago, they left detailed casts in the rocks where they died. This silica shale formation shows several trilobites. Because these primitive arthropods were typical organisms of the Paleozoic era, a paleontologist may use them to determine the relative age of the rock strata.
James L. Amos/Photo Researchers, Inc.

Fossils present a vivid record of the earliest life on Earth, and of a progression over time from simple to more-complex life forms. The earliest fossils, for example, are those of primitive bacteria some 3.5 billion years old. In more recent layers of rock, the first animal fossils appear—primitive jellyfish that date from 680 million years ago. Still more-complex forms, such as the first vertebrates (animals with backbones), are documented by fossils some 570 million years old. Fossils also indicate that the first mammals appeared roughly 200 million years ago.

Archaeopteryx
This fossil cast shows the remains of an archaeopteryx, a birdlike dinosaur. It had teeth and a long, reptilelike tail covered with feathers. The archaeopteryx lived about 150 million years ago during the Jurassic Period of the Mesozoic Era.
Sinclair Stammers/Science Library/Photo Researchers, Inc.

Although these ancient forms of life have not existed on Earth for millions of years, scientists have been able, in many instances, to show a clear evolutionary line between extinct animals and their modern descendants. The horse’s lineage, for example, can be traced back about 50 million years to a four-toed animal about the size of a dog. Fossils provide evidence of several different transitional forms between this ancient horselike animal and the modern species. In another example, the extinct, winged creature Archaeopteryx lived about 145 million years ago. Its fossil shows the skeleton of a dinosaur and the feathers of a bird. Many paleontologists consider this creature an intermediate step in the evolution of reptilian dinosaurs into modern birds. Fossils show clear evidence that the earliest human species had many apelike features. These features included large, strong jaws and teeth; short stature, long, curved fingers; and faces that protruded outward from the forehead. Later species evolved progressively more humanlike features.

B   Distribution of Species

Ostriches
The distribution of the world’s flightless birds, called ratites, is indicative of the shifting of continents in the Earth’s distant past. Ratites evolved on a landmass called Gondwanaland, which broke apart to give rise to Africa, South America, and Australia. Following this separation, the ratites of each of these continents gradually evolved into one or more distinct species—the ostrich of Africa (shown here), the rhea of South America, and the cassowary and emu of Australia.
Stan Osolinski/Oxford Scientific Films

Scientists also learn about evolution by studying how different species of plants and animals are geographically distributed in nature, and how they relate to their environment and to each other. In particular, populations that exist on islands provide living clues of patterns of evolution. The study of these evolutionary relationships, known as island biogeography, has its roots in Darwin’s observations of the adaptive radiation of the Galapagos finches. The Hawaiian Islands provide similar examples, particularly in the species of birds known as honeycreepers. Like the Galapagos finches, the honeycreepers of Hawaii evolved from a common ancestor and branched into several species, showing a striking variety of beak shapes adapted for obtaining different food sources in their various niches.

C   Anatomical Similarities

Analogous and Homologous Structures
Structures that are similar due to evolutionary origin, such as the forearm bones of humans, birds, porpoises, and elephants, are called homologous. Structures that evolve separately to perform a similar function are analogous. The wings of birds, bats, and insects, for example, have different embryological origins but are all designed for flight.
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Detailed study of the internal and external features of different living things, a discipline known as comparative anatomy, also provides a wealth of information about evolution. The arm of a human, the flipper of a whale, the foreleg of a horse, and the wing of a bird have different forms and are adapted to different functions. Yet they correspond in some way, and this correspondence extends to many details. In the case of the arm, flipper, foreleg, and wing, for example, each appendage shows a similar bone structure. The study of comparative anatomy has revealed many instances of correspondence within various groups of organisms and these bodily structures are said to be homologous. Evolutionary biologists suggest that such homologous structures originated in a common ancestor. The differences arose as each group diverged from the common ancestor and adapted to different ways of life. The more recent the common ancestor, the more similar the species.

Vertebrate Brains
Although all vertebrate brains share the same basic three-part structure, the development of their constituent parts varies across the evolutionary scale. In fish, the cerebrum is dwarfed by the rest of the brain and serves mostly to process input from the senses. In reptiles and amphibians, the cerebrum is proportionally larger and begins to connect and form conclusions about this input. Birds have well-developed optic lobes, making the cerebrum even larger. Among mammals, the cerebrum dominates the brain. It is most developed among primates, in whom cognitive ability is the highest.
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The skeletons of humans, for instance, retain evidence of a tail-like structure that is probably a relic from previous mammalian ancestors. This feature, called the coccyx, or more commonly, the tailbone, has little apparent function in modern humans. Relic features such as the coccyx are called vestigial organs. Another vestigial organ in humans is the appendix, a narrow tube attached to the large intestine. In some plant-eating mammals, the appendix is a functioning organ that helps to digest plant material. In humans, however, the organ lacks this purpose and is considerably reduced in size, serving only as a minor source of certain white blood cells that guard against infection.

Vertebrate Embryos
Vertebrates that evolved from fish pass through similar embryonic stages. As a flexible notochord develops in the back, blocks of tissue called somites form along each side of it. These somites will become major structures, such as muscle, vertebrae, connective tissue, and, later, the larger glands of the body. Just above the notochord lies a hollow nerve cord. Such similarities formed the basis for German biologist Ernst Haeckel’s biogenetic law, which states that an animal’s embryonic development recapitulates its evolution. Although scientists now know that this law does not hold absolutely, Haeckel’s idea has remained influential.
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The field of embryology, the study of how organisms develop from a fertilized egg until they are ready for birth or hatching, also provides evolutionary clues. Scientists have noted that in the earliest stages of development, the embryos of organisms that share a recent common ancestor are very similar in appearance. As the embryos develop, they grow less similar. For example, the embryos of dogs and cats, both members of the mammal order Carnivora, are more similar in the early stages of development than just before birth. The same is true of human and ape embryos.

D   Molecular Similarities

DNA Molecule
A DNA molecule consists of a ladder, formed of sugars and phosphates, and four nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The genetic code is specified by the order of the nucleotide bases, and each gene possesses a unique sequence of base pairs. Scientists use these base sequences to locate the position of genes on chromosomes and to construct a map of an organism’s genome.
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With advances in molecular biology in the last few decades, researchers seek evolutionary clues at the smallest level: within the molecules of living organisms. Despite the enormous variety of form and function seen in living things, the underlying genetic code—the molecular building material of life—displays a striking uniformity. Almost all living organisms have DNA, and in each case it consists of different pairings of the same building blocks: four nucleotide bases called adenine, thymine, guanine, and cytosine. Using different combinations of these bases, DNA directs the assembly of amino acids into functional proteins. The same uniform code operates within all living things.

Genetic Mapping
This gel scan showing the arrangement of chromosomes within a cell enables scientists to take a closer look at the genetic makeup of each individual. By comparing the genetic makeup of different species, researchers can determine how they are related.
Perkin Elmer/Applied Biosytems Division

These molecules contain more than the master plan for living organisms—each is a record of an organism’s evolutionary history. By examining the makeup of such molecules, scientists gain insights into how different species are related. For example, scientists compare the protein cytochrome c from different species. In closely related species, the proteins have amino-acid sequences that are very similar, perhaps varying by one or a few amino acids. More distantly related organisms generally have proteins with fewer similarities. The more distant the relationship, the less alike the proteins.

The idea that species become genetically more different as they diverge from a common ancestor laid the groundwork for the concept of the molecular clock. Scientists know that, statistically, neutral mutations tend to accumulate at a regular rate, like ticks of a clock. Therefore, the number of molecular differences in a shared molecule is proportional to the amount of time that has elapsed since the species shared a common ancestor. This calculation has provided new knowledge of the evolutionary relationship between modern apes and modern humans. The molecular clock concept is controversial, however, and has caused much disagreement between evolutionary scientists who study molecules and those who study fossils. This disagreement arises particularly when the molecular clock time estimates do not agree with the estimates derived from studying the fossil record.

E   Direct Observation

Fruit Fly Laying Egg
The small vinegar flies of the family Drosophilidae, commonly called fruit flies, have been exceptionally useful in scientific research. Their short reproductive cycle (a new generation of adults develops in only two weeks) and uncomplicated genetics make them ideal subjects for studies of heredity. This fly lays its eggs in rotting fruit and therefore does not need the stiff, sharp ovipositor that other fruit flies use to drill into fruits and stems.
J.A.L. Cooke/Oxford Scientific Films

Information about evolutionary processes is also obtained by direct observation of species that undergo rapid modification in only a few generations. One of the most powerful tools in the study of evolutionary mechanisms is also one of the tiniest—the common fruit fly. These insects have short life spans and, therefore, short generations. This enables researchers to observe and manipulate fruit fly reproduction in the laboratory and learn about evolutionary change in the process.

Scientists also study organisms in their natural environments to learn about evolutionary processes—for example, how insects develop genetic resistance to human-made pesticides, such as DDT. While pesticides are often initially effective in killing crop-destroying pests, sometimes the insect populations bounce back. In every insect population there are a few individual insects that are not affected by the pesticide. The pesticide wipes out most of the population, leaving only the genetically resistant individuals to multiply and flourish. Gradually, resistant individuals predominate in the population, and the pesticide loses its effectiveness. The same phenomenon has been observed in strains of disease-causing bacteria that have become resistant to even the most powerful antibiotics. Bacterial resistance forces scientists to continuously develop new antibacterial compounds. Scientists have hoped that curbing overuse of antibiotics might cause the drugs to become effective again. Recent research, however, suggests that bacteria may retain their resistance to antibiotics over many generations, even if they have not been exposed to the agent.

F   Determining Life’s Origins

Evolution of Air-Breathing Organisms
Both the lung structure of air-breathing organisms and the swim bladders of most modern fishes evolved from paired air sacs of primitive bony fishes. In the primitive fish, as in the modern bony fishes, these sacs served as a buoyancy device that inflated and deflated to alter the fish’s depth in the water. In other fish, these sacs became primitive lung structures, repeatedly folding inward to maximize oxygen uptake in an oxygen-deprived environment. Both kinds of fishes improved upon a preexisting adaptation but in so doing evolved into very different groups of organisms.
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In addition to studying how life changes and diversifies over time, some evolutionary biologists are trying to understand how life originated on Earth. This too requires the careful examination and interpretation of many indirect clues. In one well-known series of experiments in 1953, American chemists Stanley L. Miller and Harold C. Urey attempted to reproduce the atmosphere of the primitive Earth nearly 4 billion years ago. They circulated a mixture of gases believed to have been present at the time (hydrogen, methane, ammonia, and water vapor) over water in a sterile glass container. They then subjected the gases to the energy of electrical sparks, simulating the action of lightning on the primitive Earth. After about a week, the fluid turned brown and was found to contain amino acids—the building blocks of proteins. Subsequent work by these scientists and others also succeeded in producing nucleotides, the building blocks of DNA and other nucleic acids. While the artificial generation of these molecules in laboratories did not produce a living organism, this research offers some support that the first building blocks of life could have arisen from raw materials that were present in the environment of the primitive Earth.

Other theories regarding the origin of life on Earth point to outer space. Molecules formerly believed to be produced only by living systems have been found to spontaneously form in great abundance in space. Some scientists speculate that the building blocks of early life might have reached the primitive Earth on meteorites or from the dust of a comet tail.

Once all the raw materials were in place—nucleic acids, proteins, and the other components of simple cells—it is not clear how the first self-replicating life forms actually came about. Recent theories center on the role of a particular nucleic acid—ribonucleic acid (RNA), which, in modern cells, carries out the task of translating the instructions coded in DNA for the assembling of proteins. RNA also acts as a catalyst—that is, to cause other chemical reactions—and perhaps most significantly, to make copies of itself. Some scientists believe that the first self-replicating organisms were based on RNA.

According to the fossil record, the first single-celled bacteria appeared some 3.5 billion to 3.9 billion years ago. These microscopic creatures lived in the water, converting the Sun’s light into chemical energy. This metabolic process, called photosynthesis, released oxygen gas as a byproduct. Photosynthesis slowly changed the composition of the early atmosphere, adding more oxygen to what scientists believe was a mixture of sulfur and carbon gases and water vapor. Perhaps 2 billion years ago, more-complex cells appeared. These were the first eukaryotic cells, containing a nucleus and other organized internal structures. At around the same time, the level of oxygen in Earth’s atmosphere increased to nearly what it is today—another step that was crucial to the development of early life. Around 1 billion years ago, the first multicellular life forms began to appear. The beginning of the Cambrian Period (around 540 million years ago), known as the Cambrian explosion, marked an enormous expansion in the diversity and complexity of life. Subsequent to this great diversification, plant life found its way to land, while the first fishes evolved, ultimately giving rise to amphibians. Later came reptiles and, later still, mammals. The tumult of evolution was in full swing, as it remains today.

VIII   DEVELOPMENT OF EVOLUTIONARY THEORY

The origins of life on Earth have been a source of speculation among philosophers, religious thinkers, and scientists for thousands of years. Many human civilizations used rich and complex creation stories and myths to explain the presence of living organisms. Ancient Greek philosophers and scientists were among the earliest to apply the principles of modern science to the mysterious complexity and variety of life around them. During early Christian times, ancient Greek ideas gave way to Creationism, the view that a single God created the universe, the world, and all life on Earth. For the next 1,500 years, evolutionary science was at a standstill. The dawn of the Renaissance in the early 14th century brought a renewed interest in science and medicine. Advances in anatomy highlighted physical similarities in the features of widely different organisms. Fossils provided evidence that life on this planet was vastly different millions of years ago. With each new development came new ideas and theories about the nature of life.

A   Ancient Views

The Greek philosopher Anaximander, who lived in the 500s bc, is generally credited as the earliest evolutionist. Anaximander believed that the Earth first existed in a liquid state. Further, he believed that humans evolved from fishlike aquatic beings who left the water once they had developed sufficiently to survive on land. Greek scientist Empedocles speculated in the 400s bc that plant life arose first on Earth, followed by animals. Empedocles proposed that humans and animals arose not as complete individuals but as various body parts that joined together randomly to form strange, fantastic creatures. Some of these creatures, being unable to reproduce, became extinct, while others thrived. Outlandish as his ideas seem today, Empedocles’ thinking anticipates the fundamental principles of natural selection.

The Greek philosopher and scientist Aristotle, who lived in the 300s bc, referred to a “ladder of nature”—a progression of life forms from lower to higher—but his ladder was a static hierarchy of levels of perfection, not an evolutionary concept. Each rung on this ladder was occupied by organisms of higher complexity than the rung before it, with humans occupying the top rung. Aristotle acknowledged that some organisms are incapable of meeting the challenges of nature and so cease to exist. As he saw it, successful creatures possessed a gift, or perfecting principle, that enabled them to rise to meet the demands of their world. Creatures without the perfecting principle died out. In Aristotle’s view it was this principle—not evolution—that accounted for the progression of forms in nature.

B   Linnaeus and Scientific Classification

Carolus Linnaeus
Swedish physician and naturalist Carolus Linnaeus had a keen interest in botany and developed a method for classifying plants using a binomial method of scientific nomenclature. His classification system greatly simplified the manner in which organisms were named by organizing them into meaningful groups based upon their physical similarities. Many of his classifications persist unchanged to the present day.
Culver Pictures

Many centuries later, the idea of a perfect and unchanging natural world—the product of divine creation—was predominant not only in religion and philosophy, but in science. Gradually, however, as knowledge accumulated from seemingly disparate areas, the beginnings of modern evolutionary theory began to take shape. A key figure in this regard was the Swedish naturalist Carolus Linnaeus, who became known as the father of modern taxonomy, the science of classifying organisms.

In his major work Systema Naturae (The System of Nature), first published in 1735, Linnaeus devised a system of classification of organisms that is still in use today. This system places living things within increasingly specific categories based on common attributes—from a general grouping (kingdom) down to the specific individual (species). Using this system, Linnaeus named nearly 10,000 plant and animal species in his lifetime. Not an evolutionist by any means, Linnaeus believed that each species was created by God and was incapable of change. Nevertheless, his orderly groupings of living things provided important insights for later theorists.

C   19th-Century Foundations

Perhaps the most prominent of those who embraced the idea of progressive change in the living world was the early 19th-century French biologist Jean-Baptiste Lamarck. Lamarck’s theory, now known as Lamarckism and based in part on his study of the fossils of marine invertebrates, was that species do change over time. He believed, furthermore, that animals evolve because unfavorable conditions produce needs that animals try to satisfy. For example, short-necked ancestors of the modern giraffe voluntarily stretched their necks to reach leaves high in trees during times when food was scarce. Proponents of Lamarckism thought this voluntary use slightly changed the hereditary characteristics controlling neck growth; the giraffe then transmitted these alterations to its offspring as what Lamarck called acquired characteristics. Modern scientists know that adaptation and natural selection are far more complicated than Lamarck supposed, having nothing to do with an animal’s voluntary efforts. Nevertheless, the idea of acquired characteristics, with Lamarck as its most famous proponent, persisted for many years.

French naturalist and paleontologist Georges Cuvier feuded with Lamarck. Unearthing the fossils of mastodons and other vanished species, Cuvier produced proof of long-extinct life forms on Earth. Unlike Lamarck, however, Cuvier did not believe in evolution. Instead, Cuvier believed that floods and other cataclysms destroyed such ancient species. He suggested that after each cataclysmic event, God created a new set of organisms.

At around the same time that Cuvier and Lamarck were squabbling, British economist Thomas Robert Malthus proposed ideas extremely influential in evolutionary theory. In his 1798 work An Essay on the Principle of Population, Malthus theorized that the human population would increase at a much greater rate than its food sources. This theory introduced the key idea of competition for limited resources—that is, there is not enough food, water, and living space to go around, and organisms must somehow compete with each other to obtain resources necessary for survival. Another key idea came from Scottish geologist Charles Lyell, who supplied a deeper understanding of Earth’s history. In his book Principles of Geology (1830), Lyell set forth his case that the Earth was millions of years old rather than only a few thousand years old, as was maintained by those who accepted the biblical story of divine creation as fact.

D   Darwin and Natural Selection

Alfred Russel Wallace
British naturalist Alfred Russel Wallace shares credit for the revolutionary theory of evolution by natural selection with another British naturalist, Charles Darwin. Though the two scientists arrived at their conclusions independently, excerpts of their papers were presented simultaneously at a now famous meeting of the Linnean Society in London in 1858. In addition to his contribution to the theory of natural selection, Wallace is also recognized for developing the concept of Wallace’s line, an invisible border dividing two regions of dissimilar groups of plants and animals. The first of many such biogeographical borders to be identified, Wallace’s line pioneered the field of biogeography.
E.O. Hoppé/Corbis

In 1831 Charles Darwin, who was intending to become a country minister, had an opportunity to sail as ship’s naturalist aboard the HMS Beagle on a five-year, round-the-world mapmaking voyage. During the journey, as the ship anchored off South America and other distant shores, Darwin had the opportunity to travel inland and make observations of the natural world. In the Galápagos Islands, he noted how species on the various islands were similar but distinct from one another. He also observed fossils and other geological evidence of the Earth’s great age. The observations Darwin made on that voyage seemed to suggest the evolution, rather than the creation, of the many local forms of life.

In 1837, shortly after returning to England, Darwin began a notebook of his observations and thoughts on evolution. Although Darwin had developed the major components of his theory of evolution by natural selection in an 1842 unpublished paper circulated among his friends, he was unwilling to publish the results until he could present as complete a case as possible. He labored for almost 20 additional years on his theory of evolution and on its primary mechanism, natural selection. In 1858 he received a letter from British naturalist Alfred Russel Wallace, a professional collector of wildlife specimens. Much to Darwin’s surprise, Wallace had independently hit upon the idea of natural selection to explain how species are modified by adapting to different conditions. Not wanting Darwin to be unfairly deprived of his share of the credit for the theory, some of Darwin’s scientific colleagues presented extracts of Darwin’s work along with Wallace’s paper at a meeting of the Linnean Society, a London-based science organization, in June 1858. Wallace’s paper stimulated Darwin to finish his work and get it into print. Darwin published On the Origin of Species by Means of Natural Selection on November 24, 1859. All 1,250 copies of the first printing were sold on that day.

Darwin’s book and the theory it popularized—evolution through natural selection—set off a storm of controversy. Some of the protest came from the clergy and other religious thinkers. Other objections came from scientists. Many scientists continued to believe in Lamarckism, the idea that living things could consciously strive to accumulate modifications during a lifetime and could pass these traits on to their offspring. Other scientists objected to the seemingly random quality of natural selection. If natural selection depended upon random combinations of traits and variations, critics asked, how could it account for such refined and complex structures as the human eye? Perhaps the most serious question—one for which Wallace and Darwin had no answer—concerned the inheritance of traits. How exactly were traits passed along to offspring?

E   Mendel and Early Genetics

Gregor Mendel
Known as the father of modern genetics, Austrian monk Gregor Mendel developed the principles of heredity by studying the variation and heredity of seven pairs of inherited characteristics in pea plants. Although the significance of his work was not recognized during his lifetime, it became the basis for the present day field of genetics.
Culver Pictures

Darwin did not know it, but the answer was at hand—although it would not be acknowledged in his lifetime. In the Augustinian monastery at Brünn (now Brno in the Czech Republic), Austrian monk Gregor Mendel experimented with the breeding of garden peas, observing how their traits were passed down through generations. In crossbreeding pea plants to produce different combinations of traits—color, height, smoothness, and other characteristics—Mendel noted that although a given trait might not appear in every generation, the trait did not disappear. Mendel discovered that the expression of traits hinged on whether the traits were dominant or recessive, and on how these dominant and recessive traits combined. He learned that contrary to what most scientists believed at the time, the mixing of traits in sexual reproduction did not result in a random blending. Traits were passed along in discrete units. These units are now known as genes. Mendel performed hundreds of experiments and produced precise statistical models and principles of heredity, now known as Mendel’s Laws, showing how dominant and recessive traits are expressed over generations. However, no one appreciated the significance of Mendel’s work until after his death. But his work ultimately gave birth to the modern field of genetics.

Mendelian Genetics
Austrian monk Gregor Mendel formulated the hereditary principles by which parents pass physical characteristics to their offspring. He derived these principles through experiments with crossbreeding pea plants that differed in one essential and easily observed trait. He explained the results he obtained and the transmission of the trait in terms of hereditary units. These units later became known as genes.
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In 1900, Dutch botanist Hugo Marie de Vries and others independently discovered Mendel’s laws. The following year, de Vries’s book The Mutation Theory challenged Darwin’s concept of gradual changes over long periods by proposing that evolution occurred in abrupt, radical steps. Having observed new varieties of the evening primrose plant coming into existence in a single generation, de Vries had subsequently determined that sudden change, or mutation, in the genetic material was responsible. As the debate over evolution continued in the early 20th century, some scientists came to believe that mutation, and not natural selection, was the driving force in evolution. In the face of these mutationists, Darwin’s central theory threatened to fall out of favor.

F   Population Genetics and the Modern Synthesis

Theodosius Dobzhansky
American geneticist and zoologist Theodosius Dobzhansky was an important contributor to the field of population genetics. Dobzhansky’s studies of fruit flies and other organisms helped explain the role of adaptation in the evolution of races and species.
Science Photo Library/Photo Researchers, Inc.

As the science of genetics advanced during the 1920s and 1930s, several key scientists forged a link between Mendel’s laws of inheritance and the theory of natural selection proposed by Darwin and Wallace. British mathematician Sir Ronald Fisher, British geneticist J.B.S. Haldane, and American geneticist Sewall Wright pioneered the field of population genetics. By mathematically analyzing the genetic variation in entire populations, these scientists demonstrated that natural selection, and not just mutation, could result in evolutionary change.

Further investigation into population genetics and such fields as paleontology, taxonomy, biogeography, and the biochemistry of genes eventually led to what is called the modern synthesis. This modern view of evolution integrated discoveries and ideas from many different disciplines. In so doing, this view reconciled the many disparate ideas about evolution into the all-encompassing evolutionary science studied today. The modern synthesis was advanced in such books as Genetics and the Origin of Species, published in 1937 by Russian-born American geneticist Theodosius Dobzhansky; Evolution: The Modern Synthesis (1942) by British biologist Sir Julian Huxley; and Systematics and the Origin of Species (1942) by German-born American evolutionary biologist Ernst Mayr. In 1942, American paleontologist George Gaylord Simpson demonstrated from the fossil record that rates and modes of evolution are correlated: New kinds of organisms arise when their ancestors invade a new niche, and evolve rapidly to best exploit the conditions in the new environment. In the late 1940s American botanist G. Ledyard Stebbins showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has demonstrated diverse adaptive responses to environmental pressures and opportunities.

In addition, biologists reviewed a broad range of genetic, ecological, and anatomical evidence to show that observation and experimental evidence strongly supported the modern synthesis. The theory has formed the basis of evolutionary science since the 1950s. It has also led to an effort to classify organisms according to their evolutionary history, as well as their physical similarities. Modern scientists use the principles of genetics and molecular biology to study relationships first proposed by Carolus Linnaeus more than 200 years ago.

G   New Techniques in Molecular Biology

Francis Crick and James Watson
The deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and ultimately the blueprint that determines every characteristic of a living organism. In 1953 American biochemist James Watson, left, and British biophysicist Francis Crick, right, described the structure of the DNA molecule as a double helix, somewhat like a spiral staircase with many individual steps. Their work was aided by X-ray diffraction pictures of the DNA molecule taken by British biophysicist Maurice Wilkins and British physical chemist Rosalind Franklin. In 1962 Crick, Watson, and Wilkins received the Nobel Prize for their pioneering work on the structure of the DNA molecule.
Photo Researchers, Inc.

In 1953, American biochemist James Watson and British biophysicist Francis Crick described the three-dimensional shape of DNA, the molecule that contains hereditary information in nearly all living organisms. In the following decade, geneticists developed techniques to rapidly compare DNA and proteins from different organisms. In one such procedure, electrophoresis, geneticists evaluate different specimens of DNA or proteins by observing how they behave in the presence of a slight electric charge. Such techniques opened up entirely new ways to study evolution. For the first time geneticists could quantitatively determine, for example, the genetic change that occurs during the formation of new species.

DNA Strands
Nucleic acids are complex molecules produced by living cells and are essential to all living organisms. These acids govern the body’s development and specific characteristics by providing hereditary information and triggering the production of proteins within the body. This computer-generated model shows two strands of deoxyribonucleic acid (DNA) and the double-helical structure typical of this class of nucleic acids.
Ken Eward/Photo Researchers, Inc.

Electrophoresis and other biochemical techniques also demonstrated to geneticists that populations varied extensively at the molecular level. They learned that much of population variation at the molecular or biochemical level has no apparent benefit. In 1968 Japanese geneticist Motoo Kimura proposed that much of the variation at the molecular level results not from the forces of natural selection, but from chance mutations that do not affect an organism’s fitness. Not all scientists agree with the neutral gene theory.

H   Sociobiology

Ant Nest
Inside an ant nest, colony members perform their specialized tasks. The queen resides in the core of the nest, where she lays the eggs that are the colony”s sole source of new individuals. Specialized workers care for the queen and the developing young. Other ants bring seeds to the nest and expand the nest by excavating new chambers and tunnels. Among ants and other social insects, only queens reproduce. These apparently altruistic behaviors, in which workers expend energy to support the queen, yet derive no apparent evolutionary benefit, are of interest to sociobiologists.
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In recent decades, another branch of evolutionary theory has appeared, as researchers have explored the possibility that not only physical traits, but behavior itself, might be inherited. Behavioral geneticists have studied how genes influence behavior, and more recently, the role of biology in social behavior has been explored. This field of investigation, known as sociobiology, was inaugurated in 1975 with the publication of the book Sociobiology: The New Synthesis by American evolutionary biologist Edward O. Wilson. In this book, Wilson proposed that genes influence much of animal and human behavior, and that these characteristics are also subject to natural selection.

Sociobiologists examine animal behaviors that are called altruistic—that is, unselfish, or demonstrating concern for the welfare of others. When birds feed on the ground, for example, one individual may notice a predator and sound an alarm. In so doing, the bird also calls the predator’s attention to itself. What can account for the behavior of such a sentry, who would seem to derive no evolutionary benefit from its unselfish behavior and so seem to defy the laws of natural selection?

Darwin was aware of altruistic social behavior in animals, and of how this phenomenon challenged his theory of natural selection. Among the different types of bees in a colony, for example, worker bees are responsible for collecting food, defending the colony, and caring for the nest and the young, but they are sterile and create no offspring. Only the queen bees reproduce. If natural selection rewards those who have the highest reproductive success, how could sterile worker bees come about by natural selection when worker bees devote themselves to others and do not reproduce?

Scientists now recognize that among social insects, such as bees, wasps, and ants, the sterile workers are actually more closely related genetically to one another and to their fertile sisters, the queens, than brothers and sisters are among other organisms. By helping to protect or nurture their sisters, the sterile worker bees preserve their own genes—more so than if they actually reproduced themselves. Thus, the altruistic behavior evolved by natural selection.

I   Punctuated Equilibria

Evolutionary theory has undergone many further refinements in recent years. One such theory challenges the central idea that evolution proceeds by gradual change. In 1972 American paleontologists Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibria. According to this theory, trends in the fossil record cannot be attributed to gradual transformation within a lineage, but rather result from quick bursts of rapid evolutionary change. In Darwinian theory, new species arise by gradual, but not necessarily uniform, accumulation of many small genetic changes over long periods of geologic time. In the fossil record, however, new species generally appear suddenly after long periods of stasis—that is, no change. Gould and Eldredge recognized that speciation more likely occurs in small, isolated, peripheral populations than in the main population of the species, and that the unchanging nature of large populations contributes to the stasis of most fossil species over millions of years. Occasionally, when conditions are right, the equilibrium state becomes “punctuated” by one or more speciation events. While these events probably require thousands or tens of thousands of years to establish effective reproductive isolation and distinctive characteristics, this is but an instant in geologic time compared with an average life span of more than ten million years for most fossil species. Proponents of this theory envision a trend in evolutionary development to be more like climbing a flight of stairs (punctuations followed by stasis) than rolling up an inclined plane (Darwinian gradualism).

J   Role of Extinction

Ammonite Fossil
Now extinct, the ammonite mollusk was a shelled cephalopod, usually appearing in a coiled, spiral shape. The extinction of ammonites coincided with the extinction of dinosaurs, and more than 75 percent of all the species that lived at the time. By eliminating dominant species and so opening possibilities for different organisms to thrive, mass extinctions change the make up of life on Earth.
Jonathan Blair/Woodfin Camp and Associates, Inc.

In the last several decades, scientists have questioned the role of extinction in evolution. Of the millions of species that have existed on this planet, more than 99 percent are extinct. Historically, biologists regarded extinction as a natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors. Recently, however, paleontologists have discovered that many different, unrelated species living in large ecosystems tend to become extinct at nearly the same time. The cause is always some sort of climate change or catastrophic event that produces conditions too severe for most organisms to endure. Moreover, new species evolve after the wave of extinction removes many of the species that previously occupied a region for millions of years. Thus extinction does not result from evolution, but actually causes it.

Scientists have identified several instances of mass extinction, when species apparently died out on a huge scale. The greatest of these episodes occurred during the end of the Permian Period, some 245 million years ago. At that time, according to estimates, more than 95 percent of species—nearly all life on the planet—died out. Another extensively studied extinction took place at the boundary of the Cretaceous Period and the Tertiary Period, roughly 65 million years ago, when the dinosaurs disappeared. In all, more than 20 global mass extinctions have been identified. Some scientists theorize that such events may even be cyclical, occurring at regular intervals.

In the view of many scientists, mass extinctions can be explained by changes in climate—episodes of global warming or cooling that destroy sensitive ecosystems, such as tropical or marine habitats. Other theories have centered on abrupt changes in the levels of the world’s oceans, for example, or on the effect of changing salinity on early sea life. Another theory blames catastrophic events for mass extinction. Strong evidence, for example, supports the theory that a meteorite some 10 km (6 mi) in diameter struck the Earth 65 million years ago. The dust cloud from the collision, according to this impact theory, shrouded Earth for months, blocking the sunlight that plants need to survive. Without plants to eat, the dinosaurs and many other species of land animals were wiped out.

Extinction as a cause of evolution rather than the result of it is perhaps best demonstrated in terms of our own ancestors—ancient mammals. During the time of the dinosaurs, mammals constituted only a small percentage of the animals that roamed the planet. The demise of dinosaurs provided an opportunity for mammals to expand their numbers and ultimately to become the dominant land animal. Without the catastrophe that took place 65 million years ago, mammals may have remained in the shadow of the dinosaurs.

IX   HUMAN IMPACT

Infant Incubator
An infant incubator provides a controlled environment for newborns needing special care, such as those born prematurely. By placing an infant in an incubator, doctors and nurses can set and monitor different aspects of the child’s environment in order to create ideal conditions for survival. In addition to regulating oxygen, temperature, and humidity levels, incubators protect infants from pollutants and infection. Infant incubators and other advances in medical technology have made it possible for small or premature babies to survive in higher numbers than they did in the middle of the 20th century.
Steve Grand/Photo Researchers, Inc.

Extinction is not exclusively a natural phenomenon. For thousands of years, as the human species has grown in number and technological sophistication, we have demonstrated our power to cause extinction and to upset the world’s ecological balance. In North America alone, for example, about 40 species of birds and more than 35 species of mammals have become extinct in the last few hundred years—mostly as a result of human activity. Humans drive plants and animals to extinction by relentlessly hunting or harvesting them, by destroying and replacing their habitat with farms and other forms of development, by introducing foreign species that hunt or compete with local species, and by poisoning them with chemicals and other pollutants.

The rain forests of South America and other tropical regions offer a particularly troubling scenario. Upwards of 50 million acres of rain forest disappear every year as humans raze trees to make room for agriculture and livestock. Given that a single acre of rain forest may contain thousands of irreplaceable species of plant and animal life, the threat to biodiversity is severe. The conservation of wildlife is now an international concern—as evidenced by treaties and agreements enacted at the 1992 Earth Summit in Rio De Janeiro, Brazil. In the United States, federal laws protect endangered species. But the problem of dwindling biodiversity seems certain to worsen as the human population continues to expand, and no one knows for sure how it will affect evolution.

Correcting Genetic Diseases
Gene therapy may someday be able to cure hereditary diseases, such as hemophilia and cystic fibrosis, which are caused by missing or defective genes. In one type of gene therapy, genetically engineered viruses are used to insert new, functioning genes into the cells of people who are unable to produce certain hormones or proteins necessary for the body to function normally.
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Advances in medical technology may also affect natural selection. The study from the mid-20th century showing that babies of medium birth weights were more likely to survive than their heavier or lighter counterparts would be difficult to reproduce today. Advances in neonatal medical technology have made it possible for small or premature babies to survive in much higher numbers.

Recent genetic analysis shows the human population contains harmful mutations in unprecedented levels. Researchers attribute this to genetic drift acting on small human populations throughout history. They also expect that improved medical technology may exacerbate the problem. Better medicine enables more people to survive to reproductive age, even if they carry mutations that in past generations would have caused their early death. The genetic repercussions of this are still unknown, but biologists speculate that many minor problems, such as poor eyesight, headaches, and stomach upsets may be attributable to our collection of harmful mutations.

Humans have also developed the potential to affect evolution at the most basic level—the genes. The techniques of genetic engineering have become commonplace. Scientists can extract genes from living things, alter them by combining them with another segment of DNA, and then place this recombinant DNA back inside the organism. Genetic engineering has produced pest-resistant crops as well as larger cows and other livestock. To an increasing extent, genetic engineers fight human disease, such as cancer and heart disease. The investigation of gene therapy, in which scientists substitute functioning copies of a given gene for a defective gene, is an active field of research. The way this tinkering with genetic material will affect evolution remains to be determined.

X   RELIGIOUS DEBATE

The most contentious debates over evolution have involved religion. From Darwin’s day to the present, members of some religious faiths have perceived the scientific theory of evolution to be in direct and objectionable conflict with religious doctrine regarding the creation of the world. Most religious denominations, however, see no conflict between the scientific study of evolution and religious teachings about creation. Christian Fundamentalists and others who believe literally in the biblical story of creation choose to reject evolutionary theory because it contradicts the book of Genesis, which describes how God created the world and all its plant and animal life in six days. Many such people maintain that the Earth is relatively young—perhaps 6,000 to 8,000 years old—and that humans and all the world’s species have remained unchanged since their recent creation by a divine hand.

Opponents of evolution argue that only a divine intelligence, and not some comparatively random, undirected process, could have created the variety of the world’s species, not to mention an organism as complex as a human being. Some people are upset by the oversimplification that humans evolved from monkeys. In the eyes of some, a divine being placed humans apart from the animal world. Proponents of this view find any attempt to place humans within the context of natural history deeply insulting.

For decades, the teaching of evolution in schools has been a flash point in the conflict between religious fundamentalism and science. During the 1920s, Fundamentalists lobbied against the teaching of evolution in public schools. Four states—Arkansas, Mississippi, Oklahoma, and Tennessee—passed laws outlawing public-school instruction in the principles of Darwinian evolution. In 1925 John Scopes, a biology teacher in Dayton, Tennessee, assigned his students readings about Darwinism, in direct violation of state law. Scopes was arrested and placed on trial. In what was the major trial of its time, American defense attorney Clarence Darrow represented Scopes, while American politician William Jennings Bryan argued for the prosecution. Ultimately, Scopes was convicted and received a small fine. However, the ‘Monkey Trial,’ as it came to be called, was seen as a victory for evolution, since Darrow, in cross-examining Bryan, succeeded in pointing out several serious inconsistencies in Fundamentalist belief.

Laws against the teaching of evolution were upheld for another 40 years, until the Supreme Court of the United States, in a 1968 decision in the case Epperson v. Arkansas, ruled that such laws were an unconstitutional violation of the legally required separation of church and state. Over the next decades, Fundamentalists responded by de-emphasizing the religious content in their doctrine and instead casting their arguments as a scientific alternative to evolution called creation science (see Creationism) or intelligent design theory. In response to Fundamentalist pressure, numerous states debated laws that would require teachers to spend equal amounts of time teaching creation science and evolution. Only two states, Arkansas and Louisiana, passed such laws. The Arkansas law was struck down in federal district court, while proponents of the Louisiana law appealed all the way to the Supreme Court. In its 1987 decision in Edwards v. Aquillard, the Court struck down such equal time laws, ruling that creation science is a religious idea and thus an illegal violation of the church-state separation. Despite these rulings, school board members and other government officials continue to grapple with the long-standing debate between creation and evolution scientists. Courts generally struck down school board decisions to mandate the teaching of intelligent design in place of evolution. The official Vatican newspaper, L’Osservatore, entered the debate in 2006, calling intelligent design “unscientific” and adding that it should not be taught as an alternative to evolution.

XI   COMMON MISCONCEPTIONS

For more than 100 years, scientists have sought—and found—evidence for evolution. The fossil record demonstrates that life on this planet was vastly different millions of years ago. Fossils, furthermore, provide evidence of how species change over time. The study of comparative anatomy has highlighted physical similarities in the features of widely different species—proof of common ancestry. Bacteria that mutate and develop resistance to antibiotics, along with other observable instances of adaptation, demonstrate evolutionary principles at work. And the study of genes, proteins, and other molecular evidence has added to the understanding of evolutionary descent and the relationship between all living things. Research in all these areas has led to overwhelming support for evolution among scientists.

Nevertheless, evolutionary theory is still, in some cases, the cause of misconception or misunderstanding. People often misconstrue the phrase ‘survival of the fittest.’ Some people interpret this to mean that survival is the reward for the strongest, the most vigorous, or the most dominant. In the Darwinian sense, however, fitness does not necessarily mean strength so much as the capacity to adapt successfully. This might mean developing adaptations for more efficiently obtaining food, or escaping predators, or enduring climate change—in short, for thriving in a given set of circumstances.

But it bears repeating that organisms do not change their characteristics in direct response to the environment. The key is genetic variation within a population—and the potential for new combinations of traits. Nature will select those individuals that have developed the ideal characteristics with which to flourish in a given environment or niche. These individuals will have the greatest degree of reproductive success, passing their successful traits on to their descendants.

Another misconception is that evolution always progresses to better creatures. In fact, if species become too narrowly adapted to a given environment, they may ultimately lose the genetic variation necessary to survive sudden changes. Evolution, in such cases, will lead to extinction.

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