Evolution

Evolution

It is a process of change over a long period. The word evolution may refer to various types of change. For example, scientists generally describe the formation of the universe as having occurred through evolution. Many astronomers think that the stars and planets evolved from a huge cloud of hot gases. Anthropologists study the evolution of human culture from hunting and gathering societies to complex, industrialized societies.

Most commonly, however, evolution refers to the formation and development of life on earth. The idea that all living things evolved from simple organisms and changed through the ages to produce millions of species is known as the theory of organic evolution. Most people call it simply the theory of evolution.

The French naturalist Chevalier de Lamarck proposed a theory of evolution in 1809. But evolution did not receive widespread scientific consideration until 1858, when the British naturalist Charles R. Darwin presented his theory of evolution. Since then, advances in various scientific fields have resulted in refinements of the theory. The main ideas of evolution, however, have remained largely unchanged.

This article discusses the main ideas of evolutionary theory and the scientific evidence that supports the theory. For information about other types of evolution, see the World Book articles on UNIVERSE (Changing views of the universe) and EARTH (How the earth began).

Main ideas of evolutionary theory

The theory of evolution is actually a set of several interrelated ideas. The basic idea is that species undergo changes in their inherited characteristics over time. These changes transformed some of the species that lived long ago into the species that are alive today. In the last few million years--a relatively brief period in the history of the earth--thousands of species have become extinct and thousands of other species have evolved.

Evolutionary theory holds that all species probably evolved from a single form of life which lived about 31/2 billion years ago. Over time, the basic life form evolved into two or more species. These species, in turn, developed into many other species. This branching process, called speciation, produced the more than 10 million species that inhabit the earth today.

Related to speciation is the idea of common ancestry. Because all organisms evolved from one basic life form, any two species once had a common ancestor. Closely related species share a more recent common ancestor, but distantly related species must trace their ancestry far into the past to find a common ancestor. For example, human beings, chimpanzees, and gorillas evolved from a common ancestor that lived between 4 million and 10 million years ago, while the common ancestor of human beings and reptiles lived about 300 million years ago.

Another idea related to evolution is gradualism. Gradualism is the idea that evolutionary changes do not occur suddenly but over large stretches of time, ranging from decades to millions of years. Scientists think that evolution continues today at rates comparable to those of the past.

Still another idea is natural selection, a process by which the organisms best suited to their environment tend to leave the most descendants. All living things must compete for a limited supply of food, water, space, and other necessities. The individual plants and animals whose variations are best adapted to conditions have an advantage in this struggle. These organisms, on average, tend to leave a larger number of offspring than other members of their group. As a result, the proportion of the group sharing the traits of the best-adapted organisms increases from generation to generation. Scientists use the term fitness to refer to the ability of an organism to reproduce. For this reason, natural selection is often called the "survival of the fittest."

Although evolution is called a "theory," this does not mean that evolutionary biology is guesswork or is not supported by evidence. In science, a theory is a set of ideas based on observations about nature that explains many related facts. The theory of evolution is supported by evidence from many scientific fields. When a theory is supported by so much evidence, it becomes accepted as a scientific fact. Almost all scientists consider the theory of evolution to be a scientific fact.

Many people, however, reject the theory of evolution because of their religious beliefs. They believe the theory conflicts with the Biblical account of the Creation, which states that all forms of life were created essentially as they exist today.

Causes of evolutionary change

Most evolutionary change is caused by the interaction of two processes: (1) mutation and (2) natural selection. Mutation produces random (chance) variation in the genetic makeup of a species or a population--that is, individuals of the same species living in the same area. Natural selection sorts out these random changes according to their value in enhancing the individual's reproduction and survival. Such selection ensures that variations that make a species better adapted to its environment will pass on to future generations. At the same time, natural selection eliminates variations that make a species less able to survive and reproduce.

Another process that causes evolutionary change in populations is called genetic drift. Some scientists think it is much less important than natural selection.

Mutation is a permanent change in the hereditary material of an organism. Mutations may produce changes in the inherited characteristics of an organism. To understand how mutations produce these changes, it is necessary to understand how characteristics are inherited.

How characteristics are inherited. Hereditary characteristics of organisms are carried by threadlike structures called chromosomes in cells. Chromosomes carry large numbers of genes, the basic units of heredity. Genes consist of a substance called DNA (deoxyribonucleic acid). DNA contains the coded information that determines hereditary characteristics.

Among most animals and plants, each body cell has a full set of paired chromosomes. Human body cells, for example, have 46 chromosomes, arranged in 23 pairs. Offspring inherit half a set of chromosomes from each parent. Parents pass on their chromosomes to their offspring during sexual reproduction. Egg cells and sperm cells form in a special process that gives them one chromosome at random from each pair of the parent's set. As a result, egg and sperm cells have half the number of chromosomes found in all other cells in the body. During reproduction, a sperm and an egg unite in the process called fertilization, and the fertilized egg then has the full number of chromosomes.

Sometimes, the genes from one of a pair of chromosomes change places with genes on the other chromosome as a sperm or egg cell is formed. This change in the arrangement of genes, called recombination, can result in new combinations of inherited traits.

As the fertilized egg cell begins to grow, each chromosome in the nucleus of the cell duplicates itself. The chromosome and its duplicate lie next to each other in pairs. During normal cell division, one of each pair of chromosomes goes into each of two new cells. Thus, the new cells contain chromosomes that are identical with those in the original cell. This process of growth through cell division continues until it has produced all the cells that make up an organism.

How mutations change a species. Mutations may be caused by environmental factors, such as chemicals and radiation, which alter the DNA in genes, or by errors in the copying of DNA during cell division. After a gene has changed, it duplicates itself in its changed form. If these mutant genes are present in the egg or sperm cells of an organism, they may alter some inherited characteristics. Only this mutation can introduce new hereditary characteristics. For this reason, mutations are the building blocks of evolutionary change and of the development of new species.

Mutations occur regularly but infrequently, and most of them produce unfavorable traits. Albinism is one such mutation. Albino animals have mutant genes that lack the ability to produce normal skin pigment. These animals do not survive and reproduce as well as nonmutant animals. In most cases, such mutant genes are eliminated by natural selection because most of the organisms that have them die before producing any offspring. Some mutations, however, help organisms adapt better to their environment. A plant in a dry area might have a mutant gene that causes it to grow longer roots. The plant would have a better chance of survival than others of its species because its roots could reach deeper for water. This type of beneficial mutation provides the raw material for evolutionary change.

Natural selection can involve any feature that affects an individual's ability to leave offspring. These features include appearance, body chemistry, and physiology (how an organism functions), as well as behavior.

For natural selection to operate, two biological conditions must be met. First, the individuals of a population must differ in their hereditary characteristics. Humans, for example, vary in almost every aspect of their appearance, including height, weight, and eye color. People also differ in not-so-obvious features, such as brain size, thickness of bones, and amount of fat in the blood. Many of these differences have some genetic basis.

The second requirement for natural selection is that some of the inherited differences must affect chances for survival and reproduction. When this occurs, the fittest individuals will pass on more copies of their genes to future generations than will other individuals. Over time, a species accumulates genes that increase its ability to survive and reproduce in its environment.

Natural selection is a group process. It causes the evolution of a population or a species as a whole--not the evolution of an individual--by gradually shifting the average characteristics of the group over time.

Natural selection can be illustrated by a cactus called the prickly pear, which normally grows close to the ground and has soft spines. On the Galapagos Islands, in the Pacific Ocean off South America, prickly pears are the chief food of giant tortoises. A tortoise is more likely to eat an ordinary prickly pear than a tall one with tough spines. As a result, tall, tough-spined prickly pears have evolved from their short, soft-spined ancestors and reproduced in greater numbers over the years. Today, they are the most common form of prickly pears on the islands. But on the islands with no tortoises, almost all the prickly pears are short and have soft spines.

There are several types of natural selection. They include (1) directional selection, (2) stabilizing selection, and (3) sexual selection.

Directional selection produces new features that help a species adapt to its environment. This type of selection is what most people think of as natural selection. The evolution of the prickly pear is an example of directional selection. The individuals that differ most from the population average of a characteristic--in this case, the spiniest individuals--leave the most offspring. This causes a continual change in the species toward the more extreme characteristic.

Stabilizing selection occurs if a species is already well adapted to its environment. In such cases, the individuals with average characteristics leave the most offspring, and individuals that differ most from the average leave fewest. One example of stabilizing selection is the survival rate of human babies according to birth weight. Babies of average weight tend to survive better than those who are either heavier or lighter. Unlike directional selection, stabilizing selection eliminates extreme characteristics, reducing the amount of variation in a population. Stabilizing selection may actually be the most common type of natural selection.

Sexual selection occurs primarily among animals. Adults of many species prefer mates who display certain behaviors or have certain external features. Over time, this process can lead to the evolution of complicated courtship rituals, bright coloring to attract a mate, and other features. Sexual selection explains, for example, why males of many bird species have more-colorful feathers than the females.

Genetic drift is a random change in the frequencies of genes in populations. It is caused by the random way that egg and sperm cells receive some chromosomes from each parent as they form. Because these reproductive cells contain only half a set of chromosomes, only half of a parent's genes are present in an egg or sperm. If the parents produce a limited number of offspring, some of their genes may not be passed on.

Genetic drift does not enable species to evolve to adapt to their environment because it causes random changes in the frequencies of traits. Over time, however, genetic drift can gradually change the genetic makeup of a population.

Evolution of new species

Among sexually reproducing plants and animals, a species is a group of plants or animals whose members can breed with one another. Members of different species cannot produce fertile offspring together.

Various devices in nature that keep species distinct are called reproductive isolating factors. They include factors that prevent different species living in the same area from mating with each other. For example, many species of birds have unique courtship rituals, and females of one species will not respond to the courtship of males from other species. Although courtship and mating may occur between different species, other reproductive isolating factors make any offspring of such matings unable to survive or to reproduce. A well-known example is the mule. A mule is the offspring of a female horse and a male donkey, and is sterile.

A group of plants or animals that develop reproductive isolating factors, preventing them from breeding outside the group, have evolved into a new species. Most biologists believe that this process, called speciation, usually begins when a species is separated into two or more groups that are geographically isolated.

The geographic isolation of land species may result from the movement of continents over millions of years or from the division of habitats by such features as glaciers and rivers. The rise of land bridges, such as the Isthmus of Panama, may separate marine species.

Over time, isolated populations evolve in different ways because their environments differ and because different mutations occur in each population. If the geographic isolation lasts long enough, the populations may become so dissimilar that members of one population cannot breed with members of another. Reproductive isolating factors would then have evolved, and the populations would have become distinct species.

Speciation is normally extremely slow, taking millions of years. But in certain cases it can occur more rapidly. Rapid speciation is especially likely after a species settles in a new habitat, such as an unpopulated island. The species then becomes subject to strong, new forces of natural selection, such as a different climate or food supply. There is also a unique form of rapid speciation in plants called allopolyploidy in which major increases in the number of chromosomes can give rise to new species within two generations.

Evidence of evolution

Most evolutionary change occurs too slowly to be observed directly. But direct observation is not the only way to determine whether a process has taken place. Scientists have accumulated a tremendous amount of evidence documenting the occurrence of evolution. This evidence comes from six principal sources: (1) the fossil record, (2) the geographic distribution of species, (3) embryology, (4) vestigial organs, (5) direct observation of evolution, and (6) artificial selection.

The fossil record provides some of the strongest evidence for evolution. Most organisms preserved as fossils were buried under layers of mud or sand that later turned into rock. Scientists determine the age of fossils by means of radiocarbon dating and other methods of dating.

The fossil record has many gaps because relatively few species were preserved. Nevertheless, paleontologists (scientists who study prehistoric life) have found enough fossils to form a fairly complete record that documents much of the history of life on earth.

The fossil record shows a progression from the earliest types of one-celled life to the first simple, multicelled organisms, and from these organisms to the many simple and complex organisms living today. The fossils found in ancient layers of rock include the simplest forms of life and differ greatly from many organisms that exist today, while fossils in recently formed layers of rock include complex forms of life and are more similar to living plants and animals. Thus, the fossil record shows that many species became extinct and that the species alive today have not always lived on earth.

The fossil record also documents many examples of continuous evolutionary change and speciation. A famous example is the evolution of mammals from reptiles. The fossil record contains no mammals before 250 million years ago but has many species of reptiles from that period. Mammals first appear as fossils about 200 million years ago. Between these two periods, scientists have found many remains of mammallike reptiles. The skeletons of the first mammallike reptiles are nearly identical to those of reptiles, but later skeletons resemble those of mammals. In between occur creatures with a mixture of skeletal characteristics of reptiles and mammals. The transition from reptiles to mammals is so gradual that it is impossible to fix a point when reptiles became mammals. These fossils clearly document the evolution of mammals from reptile ancestors.

Another example of continuous evolution found in the fossil record is that of the horse.

Other fossil evidence for evolution includes transitional forms--that is, organisms that show common ancestry between groups of animals living today. Many transitional forms have a combination of the characteristics of living species. For example, Archaeopteryx is a fossil that may be closely related to the common ancestor of birds and reptiles. This fossil had a skeleton nearly identical to that of a small dinosaur but had such birdlike features as feathers, a beak, and an early form of wings. For an illustration of an Archaeopteryx.

Other transitional forms include the early ancestors of human beings. Since the 1920's, paleontologists have assembled a collection of fossils showing the evolution of modern human beings from apelike ancestors called australopithecines.

Geographic distribution of species, also known as biogeography, provides important evidence for the theory of evolution. Certain island groups, called oceanic islands, arose from the sea floor and have never been connected to continents. They include Hawaii, Tahiti, and the Galapagos Islands. The species found on oceanic islands are those that can travel easily, particularly over large stretches of water. These islands are rich in flying insects, bats, birds, and certain types of plants that floated to the islands as seeds. But oceanic islands lack many major types of animals and plants that live on continents. For example, the Galapagos Islands have no native land mammals. The islands also have no amphibians--animals that live part of their life in water and part on land, such as frogs and toads. Mammals and amphibians cannot easily migrate from continents to islands.

In addition, the majority of species found on oceanic islands are most similar to those on the nearest mainland, even if the environment and climate are different. The Galapagos Islands, for example, lie off the coast of mainland Ecuador in South America. The islands are much drier and rockier than the coast, which has a humid climate and lush tropical forests. But the plants and birds on the Galapagos Islands are more similar to those of the wet, tropical coast than to the plants and birds of other arid islands. This suggests that the first species to inhabit the Galapagos Islands came from South America rather than originating on the islands.

The species on oceanic islands also make up a much larger part of the plant or animal population than similar species do on continents, and some species are found nowhere else. The Galapagos Islands, for example, have 21 native species of land birds. Of these, 13 species are finches--a much higher proportion of finches than is found on any continent. These finches all belong to species found only on the Galapagos Islands. Thus, the distribution of species supports the idea that a limited number of species came to the islands from the nearest mainland and then evolved into new species.

Embryology is the study of the way organisms develop during the earliest stages of life. The embryonic development of many organisms includes peculiar events that can only be explained by the evolution of the organism from another species. For example, a mammal embryo forms three types of kidneys in succession during its development. The first two kidneys perform no function and break down and disappear shortly after they form. The third type of kidney then takes shape and develops into the mature, functioning kidney of the mammal. In the embryos of fishes, amphibians, and reptiles, however, one of the first two types of kidneys becomes the mature kidneys of these animals. These events suggest that mammals have retained some of the developmental features of their evolutionary ancestors. The repetition of embryonic features of evolutionary ancestors during the early development of an organism is called recapitulation.

Another example of recapitulation occurs during the development of human beings. A human fetus (unborn child) grows a coat of fine hair called the lanugo, which is shed before or shortly after birth. The lanugo is almost certainly a developmental feature that has remained from the time of the apelike ancestors of human beings, because monkey and ape fetuses also develop a coat of hair but do not shed it.

Vestigial organs are the useless remains of organs that were once useful in an evolutionary ancestor. For example, many species of fish, amphibians, and other animals that live in caves are blind but still have eyes. Some species have nearly complete eyes but lack an optic nerve, while others have tiny, malformed eyes. Some cave-dwelling crayfish have eyestalks but no eyes. These species evolved from ancestors with functioning eyes. Because eyes are useless in a dark habitat, mutations that damaged the vision of these species did not decrease their fitness, and these species gradually lost their sight. Similarly, many whales have tiny vestigial leg bones, the useless evolutionary remains of their land-dwelling ancestors.

One of the best-known vestigial organs is the human appendix, a narrow tube attached to the large intestine. In orangutans and other apes, the appendix is a fully formed intestinal sac that helps digest plant material. In human beings, it serves no known purpose and is, in fact, often harmful to them.

Direct observation of evolution. Evolutionary change is normally extremely slow. But in some cases it is rapid and can be seen as it occurs in living species. This happens most often when a species undergoes genetic change in response to a disturbance of its environment by human beings.

The peppered moth is an example of an organism that evolved rapidly in response to environmental change. At one time, nearly all peppered moths in industrial areas of the United Kingdom were white with black spots. Only a few mutant ones were black. The light-colored moths blended in with the light-colored lichens that covered the trunks of trees, but the black ones could easily be seen--and then eaten--by birds. In the mid-1800's, soot from newly built factories began to kill the lichens and blacken many trees. As a result of the change in tree color, the light-colored moths became easier for the birds to spot than the black ones. The number of light-colored moths in industrial areas soon declined, and the mutation for black coloring became widespread.

Other examples of rapid, observable evolutionary change have occurred among certain insects and disease-causing bacteria. In areas where DDT and other insecticides were used, some insects developed immunity to the chemicals within a few years. Some disease-causing bacteria have become resistant to antibiotics in a similar way. Such resistance may develop so quickly that researchers must continually come up with new antibiotics to replace those no longer effective.

Artificial selection. In the mid-1800's, Darwin noted that animal and plant breeders use a method similar to natural selection to produce new varieties. Breeders commonly breed only the individuals in a species that show desired characteristics. This process, called artificial selection, eventually leads to large changes in a species. For example, the various breeds of dogs differ widely in size, appearance, and behavior. They probably descended, however, from one or a few dog species that were bred to develop various traits. Many of these traits helped the dog perform a specific job, such as hunting badgers or herding sheep.

Plant breeders developed most food crops from wild ancestors by the same process. For example, cabbage, broccoli, kohlrabi, cauliflower, and Brussels sprouts all belong to a single species that was selectively bred to develop different characteristics.

Artificial selection differs from natural selection only because human beings--instead of the natural environment--determine which characteristics give individuals an advantage in reproduction. The ability of artificial selection to cause dramatic changes in a short time leaves little doubt that natural selection could cause larger changes over the vast spans of earth's history.

History of the theory of evolution

Early theories. Some of the first scientific investigations of evolution were conducted in the 1700's by two French naturalists, Comte de Buffon and Baron Cuvier. They concluded from their studies of fossils and comparative anatomy that life on earth had undergone a series of changes. But neither Buffon nor Cuvier had any idea how long ago these changes had occurred because they knew little about the earth's history.

In 1809, the French naturalist Lamarck formulated the first comprehensive theory of evolution. He observed that an animal's body parts could change during its lifetime, depending on the extent to which it used them. Organs and muscles that were frequently used became larger and stronger, but those that were rarely used tended to shrink. According to Lamarck, such acquired traits became hereditary. His theory of the inheritance of acquired characteristics influenced many scientists. Later discoveries in genetics disproved his theory.

Darwin's theory. In 1858, Darwin introduced a theory of evolution that, in modified form, is accepted by almost all scientists today. Darwin's theory states that all species evolved from a few common ancestors by means of natural selection. He set forth his theory in The Origin of Species (1859). Another British naturalist, Alfred R. Wallace, proposed an identical theory at about the same time. However, Darwin's ideas were developed much more thoroughly in a best-selling book, and his work has become better known.

Darwin used three principal sources in developing his theory. These were (1) his personal observations, (2) the geological theory of the British scientist Sir Charles Lyell, and (3) the population theory of the British economist Thomas Robert Malthus. Darwin made many of his observations as a member of a scientific expedition aboard the H.M.S. Beagle from 1831 to 1836. The ship made stops along the coast of South America, and Darwin collected many specimens of plants and animals and wrote detailed notes.

Darwin was particularly impressed by the variety of species on the Galapagos Islands. He found striking differences not only between species on the islands and those on the mainland, but also among those on each island. Darwin's findings led him to reject the idea of divine creation and to search for another explanation for the origin of species.

The theories of Lyell and Malthus influenced Darwin's ideas about the earth's history and the relationship between living things and their environment. Lyell's Principles of Geology, published in the early 1830's, stated that the earth had been formed by natural processes over long periods of time. Darwin wondered whether life on earth had also developed gradually as a result of natural processes. In 1798, Malthus wrote that the growth of the human population would someday exceed the food supply unless checked by such factors as war and disease. Darwin assumed that some environmental factor also regulated the population of all other living things. He concluded that only the individuals most fit for their environment would tend to survive and pass on their characteristics to their offspring.

The synthetic theory was formulated during the 1930's and 1940's by a number of scientists, including two American biologists, Russian-born Theodosius Dobzhansky and German-born Ernst Mayr, and the British geneticist and statistician Ronald A. Fisher. The theory is called synthetic because it synthesizes (combines) Darwin's theory of natural selection with the principles of genetics and of certain other sciences. Darwin had observed that the characteristics of organisms may change during the process of being passed on to offspring. However, he could not explain how or why these changes took place because the principles of genetics were not yet known.

The genetic principles of variation and mutation filled this gap in Darwin's theory. Gregor Mendel, an Austrian monk, had discovered the principles of genetics in the 1860's. Mendel's findings remained unnoticed until the early 1900's, when the science of genetics was established. About 1910, the American biologist Thomas Hunt Morgan discovered that genes are carried by chromosomes. Morgan also described the process of recombination, in which genes are exchanged from one chromosome to another, producing new combinations of hereditary traits.

Recent developments. The theory of evolution has not changed substantially since the 1940's. But evolutionary biologists have continued to discover more about how this process works, particularly through discoveries in other fields. Some of the most important contributions have come from molecular biology, which deals with the genetic processes involved in evolution. In the 1940's, biologists identified DNA as the substance in chromosomes that carries hereditary information. In the 1950's and 1960's, studies of DNA revealed much about its structure and its role in evolutionary changes. These studies have led scientists to believe that, at the molecular level, evolution occurs through the substitution of one nucleotide (a small bit of DNA) or one amino acid for another.

In the 1970's, molecular biologists developed methods to determine the complete sequences of DNA molecules. This discovery enabled scientists to directly measure the amount of genetic variation among individuals of a species. Biologists also developed methods to estimate the amount of genetic similarity between species and thus to measure the evolutionary relatedness of one species to another. This measurement makes it possible to reconstruct the evolutionary history of organisms by comparing the DNA of existing species. For example, scientists did not know whether the giant panda was more closely related to the raccoon or to bears. But DNA analysis has led most scientists to think that the panda is actually more closely related to bears.

Similar analysis of DNA has led scientists to revise the timeline of earth's evolutionary history. For example, the explosive growth of multicellular life once thought to have occurred at the beginning of the Cambrian Period (544 million years ago) is now believed to have taken place at least 1 billion years ago, during Precambrian time.

In the 1990's, scientists studying animal embryos at the molecular level found that a common set of genes controls basic aspects of development in different animals. These aspects include the formation of body segments in animals as diverse as worms and human beings. Such findings imply that parts of the developmental plan were present long ago in the common ancestors of most species, and have remained unchanged.

Recent discoveries in molecular biology and paleontology have led to controversy about human evolution. Many scientists believe that modern human beings evolved in Africa and then migrated to other continents, displacing earlier hominids (humanlike creatures) who lived in these places. Others think that earlier hominids evolved into modern human beings at the same time in different places. Resolving such conflicting views will require the discovery of more fossils.

Scientists also remain uncertain about the relative importance of genetic drift and natural selection in explaining evolutionary change. Many species have characteristics that have no obvious value in adapting to the environment. Scientists disagree whether these characteristics, such as differences in the shape of leaves among various species of trees, affect an individual's survival and reproduction or are merely random variations caused by genetic drift.

Acceptance of evolution

Today, the theory of evolution is considered the most important fundamental concept in the biological sciences. Nearly all scientists accept it. However, large numbers of people opposed the theory when it was introduced. Many people still do not accept it today.

In Darwin's time, the theory of evolution was attacked by many scientists, religious leaders, and other groups. Biologists argued that the evolutionary concept of hereditary variations within species contradicted the theory of blending inheritance. According to this theory, which was popular during the 1800's, hereditary characteristics became mixed and diluted as the blood carried them from one generation to another.

By the early 1900's, discoveries in genetics and other fields had resolved virtually all the original scientific objections to evolution. But other philosophical or religious objections remained. Many Christian leaders denounced the idea of evolution because it conflicted with the Biblical account of the Creation and suggested that human beings had evolved from apelike ancestors.

In the United States, much of the controversy centered on whether evolution should be taught in schools. In the 1920's, some states passed laws that banned such teaching in the public schools. In 1925, John T. Scopes, a Tennessee high-school teacher, was convicted in the famous "monkey trial" of teaching Darwin's theory. Although Scopes's conviction was later overturned because of a legal error, few public schools included evolution in the biology curriculum for many years after the trial.

In 1968, the Supreme Court of the United States ruled that laws banning the teaching of evolution were unconstitutional. The ruling stated that such laws made religious considerations part of the curriculum and thus violated the First Amendment to the Constitution. During the 1970's and 1980's, many religious groups proposed legislation that would require evolution to be taught along with an opposing view called creationism. Creationists believe that each species has remained relatively unchanged since the Creation and that no species has evolved from any other. Strict creationists accept the Bible's account of the Creation as literal truth. They believe the earth is only thousands of years old. They also hold that all species were created simultaneously and that much of early life was destroyed by a worldwide flood.

In 1981, Arkansas became the first state to enact a law requiring public schools to teach creationism whenever evolution is taught. However, a federal court declared this law unconstitutional before it went into effect. The court ruled that there was no scientific evidence for creationism and that these views constituted a religious and not a scientific explanation of life. Therefore, the court held that the Arkansas law violated the separation of church and state guaranteed by the First Amendment.

Because of these court decisions, opponents of evolution have largely abandoned their efforts to promote laws banning its teaching. However, such opponents have turned their attentions toward influencing local school boards to reduce or eliminate the teaching of evolution in biology classes.

Evolution and religion

Many people--including some Christians, Muslims, and Orthodox Jews--do not accept the theory of evolution because it conflicts with their religious beliefs. For example, the Biblical account of the Creation states that God created all living things, including human beings, within a short time. A literal reading of this account contradicts the idea that organisms evolved over millions of years. The Bible also states that human beings were created in the image of God and thus were elevated above all other forms of life. Some people find it difficult to reconcile this view with the idea that human beings evolved through natural processes.

Many other people, however, accept the basic principles of evolution within the framework of their religious beliefs. For example, some people interpret the story of the Creation as a symbolic and not a literal account of the origin of life. They find this symbolic interpretation compatible with the discoveries of evolutionary biology. For many people, the idea that human beings evolved from other forms of life does not diminish the uniqueness of human capabilities and the accomplishments of human civilization.