Evolution, Natural and Sexual Selection, and Their Influences on the Sciences
Evolution, Natural and Sexual Selection, and Their Influences on the Sciences
Although evolution was not an altogether new topic, Charles Robert Darwin (1809-1882) brought it into the limelight in 1859 with the publication of his On the Origin of Species by Means of Natural Selection. His hypothesis that current species evolved from past species by the flexible and non-rigid pathway of natural selection brought him enormous praise, but also much criticism. Since then, scientists have learned about genetics and the role that genes and mutations play in natural selection, and have continued to provide confirmation of Darwin's ideas. Despite the considerable scientific evidence, evolution is still under fire from some present-day critics, who believe the theory flies in the face of religious beliefs.
Many scientists had a hand in developing what is now termed the theory of evolution. Evolution is essentially the transformation of organisms over time, where new species evolve from previously existing species. The scientifically accepted method of evolution, which was proposed simultaneously and independently by Darwin and fellow scientist Alfred Russel Wallace (1823-1913), is natural selection. Natural selection is the process that occurs when a heritable trait within a species has a favorable enough effect that those individuals with the trait have a better chance of surviving over time. In other words, those individuals with the trait are more likely to reproduce successfully, and they therefore pass the trait down through the generations. Eventually the species carries the trait as part of its general makeup.
Unlike many previous hypotheses, including the notion proposed by Jean Baptiste de Lamarck (1744-1929) that animals could control the course of their evolution, Darwin believed an animal's "will" had nothing to do with natural selection.
According to the theory of evolution by natural selection, new species can arise under various conditions. For example, a geographical barrier, such as the ocean separating two islands, may split a bird species into two separate populations. On separate islands, the birds may find different foods. Perhaps one island has an abundant population of insects that live in the bark of trees. The birds with the strongest, most chisellike beaks would have the greatest opportunity to find food by breaking through the bark and reaching the insects. They would also be more likely to survive to reproductive age. If that trait is heritable, descendants would become more likely to have that bill. If a strong bill continues to be beneficial, the birds generation-by-generation would continue to develop a more and more chisel-like bill. Likewise, birds on the second island might benefit from a bill more conducive to sipping nectar, and the population might eventually carry that trait. Eventually the two populations may become so different that they don't recognize individuals from the other population as potential mating partners. At that point, the two become different species by definition.
Sexual selection can also give rise to new species by reason of aesthetic preferences: individuals prefer mates with one trait over another. Perhaps females of a bird species prefer males with longer feathers. Those with the longer feathers mate more often, even to the exclusion of males with shorter feathers. Eventually the species will contain males with longer and longer feathers. Aesthetics, then, can also drive evolution.
Many other conditions and a wide variety of traits can occur to split one species into two.
Darwin developed his detailed hypothesis of evolution and natural selection over many years, beginning with his voyage on the HMS Beagle. The five-year trip (1831-36) was a surveying mission, and Darwin served as its naturalist. On the voyage, Darwin spent day after day observing, collecting specimens, and writing elaborately detailed notes on the flora and fauna. Drawing on his interest in geology, he also spent considerable time describing fossils and geological formations.
Once he returned, Darwin spent the next several decades poring over his notes, studying the sciences, particularly biology, and developing his ideas on "descent with modification." In 1859, when his manuscript for Origin of Species was well under way, Wallace sent Darwin an essay describing ideas very similar to those Darwin was also writing about. The two presented a joint paper, which received little notice, and the following year Darwin published his best-known work. The full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.
The publication of Origin of Species caused an uproar. Religious leaders, newspaper editorial writers, and many members of the general public exclaimed that the hypothesis suggesting that man is but an evolutionary end-product of previous organisms was preposterous and even blasphemous. In fact, Darwin had sidestepped the issue of the evolution of man in Origin of Species, but more than a decade later acknowledged man's place in evolution with The Descent of Man, and Selection in Relation to Sex.
The religious battle against evolution continued to roil in the United States and came to a head with the 1925 Scopes trial, when a Tennessee high school teacher was convicted for teaching evolution in his class. The U.S. Supreme Court overturned the decision in 1968, finally removing any legal barriers to teaching what had become a fundamental scientific view and the basis for biological sciences. About a decade later, other states tried again to temper the theory of evolution by requiring educators to teach creationism along with evolution. The Supreme Court stepped in again in 1987 to deny those stipulations.
As late as 1999, creationists in the United States had again succeeded in turning back the clock by taking another tack. This time, the Kansas Board of Education forbade teachers from testing students on the subject of evolution. Scientists immediately decried the action, with some even calling for colleges and universities to deny admission to students from Kansas.
In response to the board's decision, acclaimed biologist and author Stephen Jay Gould wrote in Time: "No scientific theory, including evolution, can pose any threat to religion—for these two great tools of human understanding operate in complementary (not contrary) fashion in their totally separate realms: science as an inquiry about the factual state of the natural world, religion as a search for spiritual meaning and ethical values." He added, "The major argument advanced by the school board—that large-scale evolution must be dubious because the process has not been directly observed—smacks of absurdity and only reveals ignorance about the nature of science."
On the scientific front, Darwinian evolution took on a revolutionary air in the late nineteenth century. Collections of fossils were now views into the family history of life on Earth. The relationship of animals and plants gained an added depth with the understanding that one may have given rise to another, or that two separate species had a common ancestor. Scientists had yet to understand the role of genes in heredity, and a new scientific field began to form as questions arose about the mechanisms behind natural selection.
When Gregor Mendel's experiments on heredity in pea plants was rediscovered at the turn of the twentieth century, and the field of genetics slowly began to develop, evolution had already become a mainstay in the biological sciences. Genes became the previously unknown gears that turned the wheels of natural selection and that, in turn, drove evolution. Late in the twentieth century, the field of developmental biology became entwined with genetics and evolution when scientists found that essentially the same genes control specific stages of development. These control genes not only occur in different animal species but even cross the barrier between the plant and animal kingdoms.
Studies also began on the impact of evolution on animal behavior. Scientists used the ideas put forth by the theory of evolution to begin to understand kin selection, in which an individual in an animal population looks out for a relative, even at that individual's own expense. For example, some animals take on the role of look-out for a group and will issue a predatorwarning call even though that call will likely draw the attention of the predator to the calling individual. The idea that kin selection would ultimately favor the related individuals' ability to reproduce and thus the continuation of their genetic composition, had its essence in evolution and natural selection.
Sexual selection provided explanations particularly about aesthetic changes that evolved in animals. For example, the presence of large, cumbersome antlers on male white-tailed deer or the persistence of colorful plumage on some male birds, even when such plumage makes them noticeable to predators. Sexual selection explains that if a female preferentially mates with males carrying the heritable trait, that trait will continue and possibly even become enhanced in future generations. Sexual selection can work both ways. Certain male frogs, for example, appear to prefer larger females.
Scientists have also raised questions regarding the potential battles between sexual and natural selection. For example, when does the benefit of larger and larger antlers become a big enough detriment to the health and survival of the male deer that the latter outweighs the former? In other words, under what conditions does sexual selection bow to natural selection?
This research has spurred studies in other areas as well. Work on the evolution of animal behavior became a paradigm for understanding human behavior. During the twentieth century, scientists in many fields began to consider the influence of evolution on many aspects of human behavior. Evolution has become ingrained in psychology, the social sciences, and even political science. Studies are not only considering how human behavior has evolved to its current state, but how humans are continuing to adapt to changing social and political conditions.
In summary, the theory of evolution by natural selection has not only changed the way biological scientists view the world, but has become a pillar of scientific research in general.
LESLIE A. MERTZ
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Darwin, Charles R. The Descent of Man and Selection in Relation to Sex. London: John Murray, 1871.
Greene, John C. The Death of Adam: Evolution and Its Impact on Western Thought. Ames, Iowa: Iowa State University Press, 1959.
Gould, Stephen J. "Dorothy, It's Really Oz: A Pro-creationist Decision in Kansas is More Than a Blow Against Darwin," Time, 154 (23 August 1999): 59.