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 ]