Natural Selection

Natural selection is treated as the primary— and by some scientists the sole—mechanism producing evolutionary change. In essence it involves the process whereby some aspect of an organism's environment (either climate, competition, predation, or parasitism) interacts with one or more traits of the organism, such that the survival or reproduction of that organism is enhanced (or diminished) relative to organisms that lack those traits. Natural selection is intimately tied to the phenomenon of adaptation: adaptations are honed by natural selection; over time the proportion of organisms with these adaptations will increase in a population because of natural selection.

Natural selection is associated with the idea that nature is red in tooth and claw, the implication being that there is a fierce competitive struggle among all organisms, with natural selection weeding out those less fit organisms while favoring those that are more fit. Charles Darwin is usually credited with developing the concept of natural selection, but this viewpoint is, however, incomplete. For instance, Augustin de Candolle wrote in the early 1820s that all nature is at war with itself—implying an intuitive grasp of the prin ciple of natural selection; Darwin was familiar with de Candolle's work. The philosopher Herbert Spencer also anticipated aspects of Darwinian natural selection in 1852, particularly through his use of the term "survival of the fittest." Darwin later championed this term as a synonym for natural selection.

More important, Alfred Russel Wallace, while recovering from a tropical fever in the Malay Archipelago in 1858, discovered the principle of natural selection, which he described in his manuscript "On the Tendency of Varieties to Depart from the Original Type." In this manuscript Wallace discussed how in nature there is a struggle for existence such that organisms become adapted. He also emphasized how human manipulation of domesticated crop and animals—specifically, the artificial selection of desired types and the removal of other types by humans—was prima facie evidence for the importance of natural selection to evolution in the wild. This was a theme that Darwin also emphasized.

Wallace subsequently mailed his manuscript to Darwin, along with a letter asking him to present it to the Linnean Society in London. Darwin did present the paper, but he added his name as first author and changed the title, and it was published in 1858 in the Jour nal of the Proceedings of the Linnean Society. Darwin subsequently published his book On the Origin of Species in 1859. It is true that Darwin had been thinking about issues relating to natural selection prior to 1858, but these thoughts were only mentioned to a few close friends or transcribed in his private diaries or notebooks. Thus, based on the principle of priority, Wallace deserves significant credit for his theoretical contributions and insight.

It has been asserted by some that natural selection is a mechanism that over time will produce organisms that are more complex and progressive. That is not certain, though, and Darwin was ambivalent on this point. At times Darwin (1859) argued that organisms only become adapted to local environments, and that this need not lead to advances toward what he referred to as a higher type of organism (one more complex or progressive). Furthermore, Darwin articulated the point that supposedly simple, parasitic organisms actually have very complex life cycles, such that it is often difficult to determine how complex and advanced organisms are. Still, Darwin (1859) also argued that on the whole organisms do become more fit as they evolve, and he predicted that modern organisms would be able to outcompete extinct organisms. More recently, scientists, especially Stephen Jay Gould, have convincingly argued that there is little if anything about natural selection or evolution that is progressive. This can be partly attributed to the fact that environments oscillate through time, such that traits that were once adapted may not be for long. Moreover, there are major mass extinction events in the history of life that seem to play little favorites among organisms that prior to extinction were well adapted.

Although Darwin may not have unequivocally pioneered thoughts on natural selection, he can likely be credited with developing and pio neering ideas on sexual selection. This idea differs in an important way from natural selection, because sexual selection is a mechanism that involves competition for mates, yet those structures that may aid in getting mates may reduce the probability that an organism survives. Thus natural and sexual selection are different mechanisms that can at times act against each other.

One thing that is potentially confusing to those working outside of evolutionary biology is that some scientists have incorrectly equated natural selection and evolution. As mentioned above, natural selection is a mechanism that produces evolution or descent from a common ancestor. The synonymy between natural selection and evolution is valid only if natural selection is the sole mechanism that produces evolutionary change. It is clear, however, that there are many other factors that have caused life to descend or evolve from a common ancestor. Thus, challenges to aspects of the theory of natural selection do not represent challenges to evolution per se, just as a change from Newtonian to Einsteinian mechanics did not obviate the significance of gravity. Our understanding of evolution has changed much from the time of Darwin. For example, we now know that chance factors contribute in an important way to evolutionary divergence. These factors include mass extinction events as well as effects associated with how evolution occurs in small populations. Other important challenges to the relative role of natural selection include the neutral theory of evolution, developed by Kimura, which suggests that many or even most genetic and evolutionary changes that occur are neither adaptive nor inadaptive but merely selectively neutral.

Another challenge to the exclusive dominance of the mechanism of natural selection involves the recognition that although natural selection was defined in relation to the dynamics of survival and the reproduction of organisms within populations, other types of entities can be selected. For example, populations or groups can be selected. This occurs when there are traits emergent at the group level that make groups more likely to survive or give rise to new groups. Genes can also be selected. The best example of this involves selfish DNA. Some genes have been found consisting of thousands of copies. These genes have no effect on an organism's adaptedness, but the number of copies of these genes appears to have increased over time, simply because these genes are more likely to undergo duplication events than other genes. Thus these genes are better adapted than other genes because, in effect, their ability to produce similar descendants is enhanced. Species are other units that potentially may be selected, though the existence of species selection has neither been refuted nor corroborated. Species selection is a mechanism which requires that certain species have species-level characteristics that would make them more likely to speciate (or less likely to go extinct). If species selection is a valid mechanism, it could have an important effect on evolution, because certain groups might become more diverse over time— not because they were more adapted through the action of natural selection but because they were more likely to speciate. Thus traditional, organism-based natural selection is not the only selection process that occurs and influences evolutionary dynamics. In the elaboration of ideas on group and species selection, Elisabeth Vrba and other scientists have recognized that it is important to distinguish between evolution produced by group or species selection and evolution that is just an effect of selection acting at the level of organisms.

Natural selection has also been challenged by some on philosophical grounds. These arguments have been based on the premise that natural selection is tautologous and therefore not scientific. In particular, it has been argued by some that with survival of the fittest only the fittest survive—and the only way to determine the fittest organisms is to see who survives. This reasoning is in fact fallacious. Natural selection is best thought of as a statistical law. On average, those organisms that are most fit are the most likely to survive or give rise to more offspring. Organisms may die for many reasons, including chance. Fitness is not equivalent to survival or the number of offspring an organism produces, but in the long run individuals that are more fit are more likely to survive and produce more offspring.

In spite of challenges to the idea of natural selection, it is clear that it is a potent evolutionary force. Studies conducted in the lab and in the wild on time scales ranging from weeks to decades indicate that selection can lead to evolutionary divergence that, when extrapolated over time, is sufficient to produce new species in a few hundred years. In fact, the ability of natural selection to produce evolutionary divergence stands at odds with much of the data from the fossil record, which shows that species often change little over many millions of years. Moreover, the minor changes that occurred typically require that natural selection have operated at only an almost infinitesimal level.

Some of the best examples of the power of natural selection involve activities related to our own species. Humans have become powerful agents of selection, both purposely and inadvertently; the development of antibiotic resistance by bacteria is a frequently cited and unfortunate example.

—Bruce S. Lieberman

See also: Adaptation; Darwin, Charles; Evolution Bibliography

Brooks, John L. 1984. Just before the Origin: Alfred Rus-sel Wallace's Theory of Evolution. New York: Colum bia University Press; Darwin, Charles. 1859. On the Origin of Species by Means of Natural Selection or the Preservation of Favored Races in the Struggle for Life. Cambridge: Harvard University Press; Darwin, Charles. 1871. The Descent of Man. New York: P. F. Collier and Son; Eldredge, Niles. 1989. Macroevolu-tionary Dynamics. New York: McGraw Hill; Gould, Stephen J. 1996. Full House. New York: Harmony; Lewontin, Richard C. 1974. The Genetic Basis of Evolutionary Change. New York: Columbia University Press; Mayr, Ernst. 1982. The Growth of Biological Thought. Cambridge: Harvard University Press; Vrba, Elisabeth S. 1989. "Levels of Selection and Sorting with Special Reference to the Species Level." Oxford Surveys in Evolutionary Biology 6:111-168.

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