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.
Adaptation; Biological Evolution; Darwin, Charles.
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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