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Natural Selection

Natural Selection

NATURAL SELECTION APPLIED TO HUMAN BEHAVIOR

CRITIQUES OF NATURAL SELECTION IN HUMAN BEHAVIOR

BIBLIOGRAPHY

Natural selection is the central process of evolutionary theory, presented by Charles Darwin (18091882) in his 1859 book The Origin of Species. Darwins theory of natural selection is really a simple idea. It states that (1) if there is variation among members of a species in their hereditary traits and (2) some of those traits are more conducive to survival and reproduction than others, then (3) the frequency of individuals carrying those traits will gradually increase in the population. The result is that the species total pool of hereditary traits will gradually change over generations, so long as environmental conditions do not dramatically change. Thus, natural selection is crucial to how a species adapts to its environment. Evolutionary theory describes how these functional, problem-solving adaptations originate and are maintained.

Theorists and researchers in the social sciences have increasingly applied the concept of natural selection to their explanations of human individual and social behaviors. A major impetus was the writings of twentieth-century evolutionary biologists and sociobiologists such as George Williams, Robert Trivers, Edward O. Wilson, and Richard Dawkins. The 1980s and 1990s saw many social scientists, particularly psychologists, incorporating the theory of natural selection into accounts of human behavior. This effort has led to a new approach in psychology, called evolutionary psychology (EP).

NATURAL SELECTION APPLIED TO HUMAN BEHAVIOR

Unlike sociobiologists, who use natural selection to explain the behavior of social animals, evolutionary psychologists (EPs) focus primarily on how certain human behaviors may have evolved, how they are interrelated, and how or why they survive in the population. Whereas behavior geneticists are interested in how individual differences in human behavior can be explained by differences in genes, EPs are more interested in the evolved neural architecture that is shared by all humans, much of which may be outside of conscious awareness.

In 1992 anthropologist Jerome Barkow, psychologist Leda Cosmides, and anthropologist John Tooby edited The Adapted Mind: Evolutionary Psychology and the Generation of Culture. This book energized the EP field. It helped to popularize the idea that humans evolved distinct brain circuitry or information processing modules adapted to solve problems faced by our hunter-gatherer ancestors. Researchers have proposed brain modules for speech and language, facial recognition, the recognition of emotional expressions, social reasoning, and many other aspects of information processing. A popular EP description of the brain is that it is analogous to a Swiss Army knife with its various specialized modules, a reflection of successful problem-solving adaptations during human evolutionary history.

EPs have applied the theory of natural selection to human behavior in several ways. These scientists argue that the majority of human cultural universals, including social traditions, laws, religions, and ethical positions arose out of the reproductive imperative to reproduce and leave behind as many offspring as possible. EPs argue that humans are neurologically predisposed to develop certain phobias (e.g., snakes, enclosed spaces, heights) that were presumably tied to the greatest dangers present in ancient ancestral environments, rather than developing fears toward the dangers of current technologically advanced society (e.g., guns, automobiles, electric sockets).

A good example of the EP application of selection is in the controversial area of gender differences. EPs have suggested that sexual selection (processes relating to how males and females compete for mates) has played an important role in human patterns of mate selection and jealousy. With regard to mate selection, these researchers argue that men have lower overall parental investment in offspring than women. If men are more interested in reproducing than in investing parental resources, then they will tend to find the reproductive potential of a prospective partner to be particularly attractive. Because they have greater parental investment, women will find a partners potential for providing resources and protection for offspring to be relatively more attractive. This analysis has been used to account for mens greater emphasis on their resources, such as occupation or income, and womens greater emphasis on factors related to their ability to reproduce, such as their age or appearance, when trying to attract a prospective mate.

With regard to jealousy, EPs argue that whereas women are always sure they are the mothers of their offspring (due to internal gestation), men can have doubts about fathering offspring. The implication of such differences in parental certainty is that males are more likely to take steps to make sure their investment of resources is legitimate. Thus, they will be more concerned with possible sexual rivals and place a high value on the chastity of a prospective mate. Male parental uncertainty predicts that men will attempt to coercively control female reproductive capacity by showing vigilance, mate concealment, violence, derogation of competitors, and threats to others.

CRITIQUES OF NATURAL SELECTION IN HUMAN BEHAVIOR

There are many criticisms of the application of the theory of natural selection to human behavior. One of the main criticisms is that EP is overly deterministic in its focus on biological or genetic destiny. Much human social, group, and cultural behavior is thought to have emergent properties that cannot be traced back to the evolved structure of individual brains. Thus, culture and socialization may be better explanations for gender differences than natural selection.

Critics argue that natural selection did not necessarily create human brain specialization for any adaptive purpose. Many aspects of modern human behavior can be described as non-adaptive side consequences of natural selection. Critics of the EP approach also argue that empirical evidence is not only lacking but is often impossible to obtain for many of its claims. Because researchers cannot recreate the evolutionary forces that affected ancient human ancestors (what EPs call the environment of evolutionary adaptation), these claims become nothing more than just-so stories telling us that people are this way because they are this way.

Another area of controversy concerns the rate and extent of natural selection when applied to humans. Several lines of research suggest that natural selection in humans can occur quite rapidly. For example, researchers have identified increases in recent gene variants related to brain development and size, resistance to HIV infection in parts of Africa, lactose tolerance (i.e., the ability for humans to digest milk sugars), intelligence increases among certain ethnic groups (such as Ashkenazi Jews), and salt retention and hypertension among African slave descendants. In these cases advocates of rapid selection propose a relatively recent appearance of genetic changes or rapid proliferation of those changes throughout the human population. However, the possibility of rapid selection has been questioned by epidemiologists and evolutionary biologists. Critics argue that in many cases the relevant genetic mechanisms or variants that can account for such rapid evolutionary changes have not been identified. Rather, environmental, cultural, or more complicated biocultural influences may account for the observed changes.

Some social scientists argue that natural selection has decreased in importance for humans. Rapid cultural, medical, and technological changes are thought to be more strongly linked to contemporary human survival and reproduction than genetic factors. For example, improvements in public health mean that newborns are much more likely to survive to reproductive age today than they were 500 years ago. Such improvements are thought to neutralize the process by which less adaptive genes are removed from the population. In addition, successful scientific efforts to manipulate the human genome are likely to replace or compete with natural or sexual selection pressures in the future.

The application of the theory of natural selection to the social sciences has made great strides since the 1980s. A good deal of social, political, and scientific controversy and criticism has accompanied social scientists efforts. Establishing whether and to what extent natural selection applies to human behavior is an ongoing and difficult process. Whether the criticisms will hold up or merely reflect the growing pains of a new discipline remains to be seen.

SEE ALSO Darwin, Charles; Darwinism, Social; Determinism; Determinism, Biological; Durkheim, Émile; Enlightenment; Evolutionary Psychology; Functionalism; Hypertension; Nature vs. Nurture; Popper, Karl; Racism; Slavery; Sociobiology; Teleology; Weber, Max

BIBLIOGRAPHY

Barkow, Jerome H., Leda Cosmides, and John Tooby, eds. 1992. The Adapted Mind: Evolutionary Psychology and the Generation of Culture. New York: Oxford University Press.

Cosmides, Leda, and John Tooby. 1997. Evolutionary Psychology: A Primer. Center for Evolutionary Psychology. http://www.psych.ucsb.edu/research/cep/primer.html.

Darwin, Charles. 1859. On the Origin of Species by Means of Natural Selection. London: John Murray.

Rose, Hilary, and Steven Rose, eds. 2000. Alas Poor Darwin: Arguments Against Evolutionary Psychology. New York: Harmony Books.

Wilson, Edward O. 1998. Consilience: The Unity of Knowledge. New York: Knopf.

Thomas M. Brinthaupt

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Natural Selection

Natural Selection

Natural selection is the process by which individuals with characteristics that are advantageous for reproduction in a specific environment leave more offspring in the next generation, thereby increasing the proportion of their genes in the population gene pool over time. Natural selection is the principal mechanism of evolutionary change, and is the most important idea in all biology. Natural selection, the unifying concept of life, was first proposed by Charles Darwin, and represents his single greatest contribution to science.

Natural selection occurs in any reproducing population faced with a changing or variable environment. The environment includes not only physical factors such as climate or terrain, but also living factors such as predators, prey, and other members of a population.

Mechanism of Natural Selection

The mechanism of natural selection depends on several phenomena:

  • Heredity: Offspring inherit their traits from their parents, in the form of genes.
  • Heritable individual variation: Members of a population have slight differences among them, whether in height, eyesight acuity, beak shape, rate of egg production, or other traits that may affect survival and reproduction. If a trait has a genetic basis, it can be passed on to offspring.
  • Overproduction of offspring: In any given generation, populations tend to create more progeny than can survive to reproductive age.
  • Competition for resources: Because of excess population, individuals must compete for food, nesting sites, mates, or other resources that affect their ability to successfully reproduce.

Given all these factors, natural selection unavoidably occurs. Those members of a population that reproduce the most will, by definition, leave more offspring for the next generation. These offspring inherit their parents' traits, and are therefore also likely to succeed in competition for resources (assuming the environment continues to pose the same challenges as those faced by parents). Over several generations, the proportion of offspring in a population that are descended from the successful ancestor increases, and traits that made the ancestor successful therefore also increase in frequency. Natural selection leads to adaptation, in which an organism's traits conform to the environment's conditions for existence.

Consequences of Natural Selection

Natural selection is truly the ultimate inventor. A short list of some of its many "inventions" includes flight, celestial navigation, echolocation, insulation, infrared sensors, hypodermic needles, plus all sorts of useful biologically active chemicals such as antibiotics, analgesics, emetics, diuretics, laxatives, tranquilizers, contraceptives, hallucinogens, pain killers, and many, many more. Each of these has been fashioned by natural selection to meet the needs of particular organisms in specific environments.

Pesticide-resistant insects and antibiotic-resistant bacteria are well-documented examples of natural selection in action. In each case, humans have provided the environmental challenge in the form of poisons acting on the population. Preexisting variations in susceptibility to the poison mean that some organisms survive while others die without reproducing. Offspring of survivors have the same variation, and the most resistant of those survive best to reproduce. Over time, populations of resistant insects or bacteria are formed. (This is why taking the full prescription of an antibiotic is important; it kills the entire microbe population, preventing any from reproducing.)

Misconceptions About Natural Selection

Natural selection is easy to understand, but it is misunderstood much too often. Natural selection is not synonymous with evolution. Evolution refers to any genetic change in a population, whereas natural selection specifies one particular way in which such changes are brought about. Natural selection is the most important agent of evolutionary change simply because it results in adaptation of an organism to its environment. Other possible mechanisms of evolution besides natural selection include gene flow, meiotic drive, and genetic drift.

A persistent misconception is that natural selection occurs mainly through differences between organisms in death rates, or differential mortality. Differential mortality can be selective but only to the degree that it creates differences between individuals in the number of reproductive offspring they produce. Reproductive rate, rather than death rate, drives natural selection. A cautious tomcat that seldom crosses busy streets might live to a ripe old age without leaving behind as many descendent kittens as another less staid tomcat killed on a highway at a much younger age. If the short-lived cat leaves more descendants, its genes will spread faster than those of the long-lived cat, and natural selection will favor a short life span. Unless living longer allows or results in higher reproductive success, long life is not favored by natural selection.

Adaptations fashioned by natural selection suit an organism to its particular environment. For instance, a maple tree's broad leaves are well adapted to temperate climates, but unsuited to arctic cold. Similarly, a human's ability to store fat is an adaptation to environments in which fat is scarce, but is poorly suited to the modern fast-food environment. In this respect, natural selection is somewhat shortsighted, since it cannot "see" beyond the next generation.

Natural selection cannot preferentially create favorable variations, but instead must work with what is at hand. For instance, treatment with antibiotics does not create antibiotic-resistant mutants. Instead, it favors microbes that, by chance, already have genes for resistance.

Phrases such as "the struggle for existence" and "survival of the fittest" have had an unfortunate consequence. They tend to emphasize predation and fighting for food as the prevalent means of selection. This reinforces erroneous emphasis on differential death rates, with the strongest and fastest individuals being considered as having a selective advantage over weaker and slower individuals. But if this were true, every species would continually gain in strength and speed.

Because this is not happening, selection against increased strength and speed (counterselection) must be occurring and must limit the process. Animals can sometimes be too aggressive for their own good; an extremely aggressive individual may spend so much time and energy chasing its prey that it spends less than average time and energy on mating and reproduction, and as a result, leaves fewer offspring than average. Likewise, an individual could be too submissive and spend too much time and energy running away from others. Usually, intermediate levels of aggressiveness result in the highest fitness.

Natural selection does not operate "for the benefit of the species." Birds lay fewer eggs during drought years. Is this because competition for limited food supplies would be detrimental to the species, and do birds hold back "for the good of their species"? Such arguments have a fatal flaw: "cheaters" that laid as many eggs as possible would reap a higher reproductive success than individuals that voluntarily decreased their clutch size. Over time, cheater genes would spread through a population, and genes for holding-back would become rare.

However, the same phenomenon can be interpreted more plausibly in terms of natural selection at the level of individuals. During droughts, parental birds cannot bring as many insects to their nest and therefore cannot feed and fledge as many chicks as they can when food supplies are more ample. Laying extra eggs means most chicks would die of starvation. Birds can actually leave more surviving offspring to breed in the next generation by laying fewer eggs.

Any individual that sacrifices its own reproductive success for the benefit of a group is at a selective disadvantage within that group to any other individual not making such a sacrifice. Classical selection will always favor individuals that maximize their own selfish reproductive success. Natural selection recognizes only one currency: babies. Although we might wish otherwise, beauty, brains, or brawn need not be favored unless such traits are translated into more offspring than average. If ugly, dumb, weak individuals pass on more genes, those traits will prevail in future generations.

Whenever one organism leaves more successful offspring than others, in time its genes will come to dominate the population gene pool. Ultimately, natural selection operates only by differential reproductive success. An individual's ability to perpetuate itself as measured by its reproductive success is known as its Darwinian fitness.

see also Adaptation; Convergent Evolution; Evolution; Population Genetics; Sexual Selection

Eric R. Pianka

Bibliography

Pianka, Eric R. Evolutionary Ecology, 6th ed. San Francisco, CA: Addison-Wesley-Longman, 2000.

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Natural Selection

Natural Selection

Natural selection is the differential survival and reproduction of individuals with particular phenotypes , the physical manifestation of genotypes . Natural selection works only on the phenotypes of individuals. Natural selection produces adaptation when the phenotype is heritable. Natural selection is the most important cause of biological evolution.

Charles Darwin was the creator of the concept of evolution by natural selection. In his 1859 book, On the Origin of Species, the most important book on evolution, Darwin put forth his argument and supported it with multiple examples. Darwin's idea of natural selection was heavily influenced by an essay on human population growth written in 1798 by English economist Thomas Malthus. Malthus pointed out that every organism has the ability to produce more individuals than the environment can support, and that many individuals die without reproducing. Darwin recognized that variation among individuals is always present, and that some individuals with particular combinations of traits are more likely to survive than other individuals with different combinations of traits. With so much variation, and more individuals being produced than can survive, the individuals with the combination of traits that are best suited to their environment will survive better and reproduce more than other individuals. This is natural selection as Darwin described it.

Darwin also identified artificial selection, which works in the same way as natural selection except that humans are the selective force rather than the environment. Artificial selection, for example, has produced domesticated animals. One of the best illustrations of artificial selection is the breeding of dogs, as humans selectively bred dogs to have specific characteristics. Beagles were bred to bark as they chased after foxes. Labrador retrievers were bred to swim and to carry game birds that had been shot down over water back to shore. Other characteristics selected for included body size, color, hair length, and personality. Dogs are all the same species, but there are clearly huge amounts of variation among breeds.

Darwin also identified a third type of selection, sexual selection . Sexual selection is the differential ability of individuals to win mates and reproduce. Most animal species have sexual dimorphism, that is, the different sexes have different traits. Sexual dimorphism results from sexual selection. Bird songs, elaborate coloration, and the other characteristics that help males attract mates are sexually selected traits. For example, male guppies have bright spots of pigmentation that attract females. Males that are more brightly colored mate with more females.

There are three forms of selection: directional, stabilizing, and diversifying. Directional selection changes the average value of a trait in some populations . For example, female guppies that prefer to mate with male guppies that have more orange spots will increase the average number of orange spots on males in the next generation. Stabilizing selection reduces variation in a population by selecting against the extreme individuals. In a similar example, females liked males that had only five spots of orange, but disliked males with more or less than five spots. Diversifying selection increases the variation in a population by favoring the extreme individuals, for example males with lots of orange spots or with no orange spots, and disfavoring males with average amounts of orange spots.

see also Adaptation; Biological Evolution; Darwin, Charles.

Laura A. Higgins

Bibliography

Andersson, Malte. Sexual Selection. Princeton, NJ: Princeton University Press, 1994.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Endler, John A. Natural Selection in the Wild. Princeton, NJ: Princeton University Press, 1986.

Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

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natural selection

natural selection The process that, according to Darwinism, brings about the evolution of new species of animals and plants. Darwin noted that the size of any population tends to remain constant despite the fact that more offspring are produced than are needed to maintain it. He also saw that variations existed between individuals of the population and concluded that disease, competition, and other forces acting on the population eliminated those individuals less well adapted to their environment. The survivors would pass on any heritable advantageous characteristics (i.e. characteristics with survival value) to their offspring and in time the composition of the population would change in adaptation to a changing environment. Over a long period of time this process could give rise to organisms so different from the original population that new species are formed. See also adaptive radiation. Compare punctuated equilibrium.

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natural selection

natural selection In evolution, theory that advantageous change in an organism tends to be passed on to successive generations. Changes arise out of natural genetic variation, especially mutation. Those that give an individual organism a greater capacity for survival and reproduction in a particular environment help it to produce more offspring bearing the same beneficial characteristic or trait. Thus white rabbits will survive better than brown ones in a wintry environment. This theory was proposed by English naturalist Charles Darwin in his book The Origin of Species (1859). It is still regarded as the key mechanism of evolution. When humans breed plants and animals to perpetuate desired traits, this is called artificial selection.

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natural selection

nat·u·ral se·lec·tion • n. Biol. the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution. Compare with survival of the fittest (see survival).

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natural selection

natural selection(‘survival of the fittest’) A complex process in which the total environment determines which members of a species survive to reproduce and so pass on their genes to the next generation. This need not involve a struggle between organisms. Natural selection is not necessarily the only mechanism for evolution (see neutrality theory of evolution).

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natural selection

natural selection (‘survival of the fittest’) A complex process in which the total environment determines which members of a species survive to reproduce and so pass on their genes to the next generation. This need not necessarily involve a struggle between organisms.

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natural selection

natural selection (‘survival of the fittest’) A complex process in which the total environment determines which members of a species survive to reproduce and so pass on their genes to the next generation. This need not necessarily involve a struggle between organisms.

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natural selection

natural selection (‘survival of the fittest’) Complex process in which the total environment determines which members of a species survive to reproduce and so pass on their genes to the next generation. This need not necessarily involve a struggle between organisms.

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Natural Selection

Natural Selection

See Evolution, Biological; Selection. Levels of

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natural selection

natural selection: see selection.

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natural selection

natural selection See DARWINISM.

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Natural Selection

Natural Selection


Natural selection is the process of survival and reproduction of organisms that are best suited to their environment. It is a unifying idea for all of biology, and it also explains how the theory of evolution (the process by which living things change over generations) actually works.

The theory of evolution was first suggested by the English naturalist Charles Robert Darwin (1809–1882) in 1858. As a major theory, it attempted to account for the amazing diversity in the living world and to explain how present-day organisms came to be. It stated that all life progressed from simple to more complex organisms, and that gradual genetic changes occurred over a long period of time. Darwin's idea of natural selection was the key to explaining how this is accomplished. Natural selection has been described as the mechanism of evolution or even as the cause of evolution. The end result of natural selection is that organisms are able to adapt to their environment and change over time.

Natural selection has been described as the "survival of the fittest" because it is an unforgiving natural process that weeds out those traits that are less fit. This was explained by Darwin himself who said that natural selection rested on a few obvious facts. The first of these is that in the natural world, reproduction has the potential to produce more individual organisms than can survive. For example, a single leopard frog has the capability to produce 3,000 frogs a year. Since every species' living space or habitat has limited resources, population increases result in competition between individuals. Darwin said that at any one time, each organism is competing with others as well as members of its own species for such things as food, shelter, and members of the opposite sex with

CHARLES ROBERT DARWIN

English naturalist Charles Darwin (1809–1882) is without doubt one of the greatest and most influential life scientists who ever lived. His theory of evolution became the dominant concept for all of biology. Darwin's explanation that individual species can change or adapt over time, that humans have evolved from earlier "less human" forms, and that ultimately all life on Earth is connected and passed from simple to more complex organisms, is the "big idea" of biology.

Born in Shrewsbury, England, to a well-to-do family, the young Darwin had a physician father, and his two grandfathers were both wealthy and influential individuals. Expected to become a doctor like his father, Darwin could not tolerate watching surgery while in medical school, so his father suggested a career in the church. When the young man proved unwilling to pursue that profession, his father lost hope and declared he would grow up as a disgrace to the family. Throughout all this, however, the young man was starting to cultivate an interest in the natural world, and he eventually made natural history his hobby. His first real exposure to science was a field trip he took that was led by the English geologist (a person specializing in the origin, history, and structure of Earth) Adam Sedgwick (1785–1873). It was Sedgwick who recognized that there was something special about Darwin. This also enabled him to meet an English botany (the study of plants) professor, John Stevens Henslow (1796–1861), who helped Darwin obtain a position on a scientific expedition that would be making a five-year voyage to South America and the South Pacific Islands. Darwin's father eventually allowed him to go, and the twenty-two-year-old Darwin became an unpaid naturalist on the government survey ship, the H.M.S. Beagle, as it set sail in December 27, 1831. This grand voyage by sea not only transformed Darwin into a real naturalist (what is now called a biologist), but it proved to be one of the most important scientific voyages ever undertaken.

Darwin's job on the trip was to make geological and biological observations, keep records, and collect specimens. With each day, he learned a little more about the incredible variety in the natural world, and after awhile, he began to question why he found species that were closely related but still had noticeably different characteristics. Four years into his journey, Darwin landed on the Galapagos Islands far off the coast of Ecuador. There he noticed that there were about fourteen different types of finches on these different islands, with each bird apparently having adapted perfectly to its particular island environment. He also found that the natives could tell just by looking which island a giant tortoise had come from because of its distinctive features. All the while, Darwin searched for a pattern of meaning in this, and after some thought, he began to realize that it might be that species could actually change. It would make very good sense if one type of land finch colonized these islands and then each adapted, or changed, slightly to better fit its particular island. However, Darwin could not explain how this might occur, and he eventually returned home with no real answers.

After publishing his very popular book, A Naturalist's Voyage on the Beagle, Darwin began to seriously work out an explanation for the ideas he was considering. Those ideas were influenced not only by the writings of his good friend, the Scottish geologist Charles Lyell (1797–1875), but by those of the English economist, Thomas Robert Malthus (1766–1834). Malthus had written that a population always grows faster than does its food supply, and when Darwin applied this notion to the natural world, he realized that it might explain how species change. Since each individual is slightly different from every other, those that possess a certain trait that gives them some sort of advantage in competing for food would have a better chance of surviving and passing that trait on. A new species, therefore, is developed that is better able to survive in its environment.

In 1844, Darwin started a book that would explain his theory, but by 1858 he still did not have it completed. So when the English naturalist Alfred Russel Wallace (1823–1913) sent Darwin a draft of his own paper on this very same subject, the two decided to issue their papers together. Darwin, however, went on to elaborate much more fully in his 1859 book, On the Origin of Species by Means of Natural Selection. Although his book was immediately controversial, most of the scientific community was persuaded, and every copy sold out on its first day. With the post-1900 discovery of Austrian botanist Gregor Mendel's laws of inheritance (characteristics are not inherited in a random way but instead follow predictable, mathematical patterns), and the later discovery that genes are the basic units of heredity, Darwin's theory of evolution at last had a mechanism that explained exactly how it could take place.

Although Darwin lived a fairly long life, he was always troubled by a variety of physical problems, and while it was thought by many that he was probably a hypochondriac (someone who believes that they are always ill), many now think that he ruined his health during the Beagle trip by contracting a tropical disease. No book has been as important or as controversial as Darwin's, given that it goes against the Biblical view of creation. Although some today still disagree with its conclusions and implications, most life scientists agree that his theory remains the only viable scientific explanation for the amazing variety and diversity of life on Earth.

whom to mate. The organism's habitat or environment is the key to natural selection since it is the standard against which an individual's "fitness" is measured. Simply, if one organism possesses traits that make it more fit for its particular environment than does another (whether it is a longer neck, different color, larger antlers, or aggressive temperament), it is more likely that the "fitter" one will have a better chance to survive, to reproduce, and to pass on those advantageous traits. Since the environment is the measure, natural selection becomes an essentially random process. This means that nature has no master plan to favor one trait over another. Rather, a variety of traits usually exist in a given population, and whichever one happens to give its owner an edge over others becomes the trait that the environment favors.

GENETIC VARIETY

This leads to the second fact stated by Darwin: that close examination of any population of the same organisms reveals that not all individuals are exactly alike. In every group of like animals, there is always variety in both form and function. Simply, no two animals of the same species are identical because of the factor Darwin called "genetic variety." Genetic variety states that the individuals making up a population have inherited characteristics that make them slightly different from one another. This becomes obvious when we realize that if each individual were identical genetically, then it would make no difference which one survived to reproduce. But when genetic differences do exist, who gets to pass on what trait makes a very big difference.

Genetic variety comes about in two ways. The first is the physical result of sexual reproduction in which a unique individual is created who possesses a mixture of genes from both parents. This is called genetic recombination. The other way that genetic variety occurs is by mutations or accidental changes in a gene. A mutation is not necessarily something bad, and sometimes a chance change in a gene can result in a trait that gives an individual an advantage over others.

Finally, natural selection is always tied to reproduction, since it does no good for an organism to live a very long time if it does not reproduce. The way natural selection works is that those best suited to their environment (the fittest) survive better and get to produce more offspring, thus passing on their genes to future generations. The end result of natural selection is a process called "adaptation." Through natural selection, which favors organisms that fit their environment best and which weeds out those badly fitted, living things become better suited, fit, or "adapted" to their local environment or habitat. As this process continues over millions of years, new species evolve which are better adapted to their habitat or ecological niche (a specific job, or role, in a community that relates to feeding).

Although the theory of natural selection is popularly identified with Darwin, his contemporary, the English naturalist Alfred Russel Wallace (1823–1913), came up with the same theory in the same year. Darwin was at first amazed when he received Wallace's not-yet-published ideas in 1858, but the two men became allies and published their ideas together in a scientific journal that year. Later, Darwin went on to produce a fuller and more complete theory with his book, On the Origin of Species, and as a result received much more recognition than Wallace.

[See alsoEvolution; Evolution, Evidence of; Evolutionary Theory; Human Evolution ]

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