Polly, P. D. 1999. Selection. In: R. Springer (ed.), Encyclopedia of Paleontology. Fitzroy Dearborn Publishers, Chicago, pp. 1084-1086.

Theories of Selection

It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. (Darwin, 1859: 133).

Selection is the process by which many evolutionary phenomena—extinction, the origin of new species, morphological change, and adaptation—are explained in modern evolutionary theory. While it is generally agreed that selection is a major cause of evolutionary change, many details of its role are not agreed upon. Arguments rage about which features of an organism are subject to selection; about whether selection produces gradual, continuous evolutionary change or whether its effects are sudden and sporadic; about whether selection acts on genes, individuals, populations, or species; and even about whether selection plays an important role in evolution.

Selection works like a filter, removing unfit individuals from a population and leaving those that are more fit (Sober, 1984). Amid the variety of individual differences, some organisms are better able to survive and have offspring. The ones who do pass on to the next generation those features (or combinations of features) that made them more fit (Darwin, 1859). Over time, detrimental features are systematically removed from an evolving population, while advantageous features remain. Variation within a population is not lost, however, because mutation adds new features and sexual reproduction creates new combinations of old features. Every generation has a new spectrum of traits for selection to act upon.

Selection can both cause and prevent evolutionary change. Directional selection causes evolutionary change by favoring traits that are rare in a population over those that are the norm, while stabilizing selection prevents change by favoring those traits that are the norm in a population over those that are rare (Schmalhausen, 1946). Directional selection usually causes either anagenesis (directional change in an evolving lineage) or extinction (if the population does not contain individuals who can survive the rigors of the selective process). Stabilizing selection prevents evolutionary change by removing extreme variants from a population. The result is stasis. The Red Queen’s Hypothesis—which states that there are so many factors causing directional selection, each of which favors different traits, that the end result is constant evolution around a non-changing norm—can be viewed as another sort of stabilizing selection (Van Valen, 1973).

Selection often results in adaptation, which is both the evolutionary process of becoming more fit and the features that are produced by that process. It is easy to imagine that just about any feature of an organism is an adaptation that makes it more fit in its environment—the long horns of Triceratops might be an adaptation for protection against Tyrannosaurus and the upright stance of Australopithecus might be an adaptation for moving around the Savannah carrying

tools. However, the scientific identification of adaptation is rarely straightforward. In its classic form, selection causes change in a feature that makes an organism better able to survive and reproduce in its environment. However, some features may be selected for because they make an organism more desirable as a mate—this is known as sexual selection. Often several features of an organism are functionally and developmentally interlinked so that selection on one trait has effects on others. In this case, only those traits on which selection acted can properly be called adaptations. Features may also evolve for reasons that have nothing to do with selection. Some features may exist because the founding members of that species just happened to have them (founder effect), while some new features may be fixed in a species simply because there was no selection against them (drift). In some cases, a feature may evolve in response to selection pressure at a given time, but might later be co-opted for a different use (exaptation; Gould and Vrba, 1982).

An example of the difficulty inherent in identifying adaptations are the enormous horns of the extinct Irish elk, which sometimes spread more than twelve feet. As with all members of the Deer family, the horns were grown and shed annually, a process that required a tremendous metabolic expenditure. The evolution of these antlers has been variously explained as the direct result of selection because they would have allowed their owners to better fend off rivals during the mating process; as the result of sexual selection because females might have preferred males with larger racks; and as an allometric coincidence that was the indirect result of selection for larger body-size.

Usually selection is thought of as acting on individual organisms within a population—those organisms that are more fit manage to reproduce. However, there are also arguments that selection acts on genes (Dawkins, 1976), demes (Wright, 1932), species (Stanley, 1979), and clades (Vrba, 1989). Dawkins argued that genes themselves are the objects of selection—the organism merely functions as a convenient housing for self-replicating DNA. Wright, Stanley, Vrba, and others have argued that groups of individuals—whether they be demes, species, or clades—have defining properties that make them more or less able to survive than other such groups. Through the process known as species sorting, groups with more favorable features persist and speciate, while those with less favorable features become extinct. As a result, a large-scale evolutionary trend

can be seen that is not present within any individual species. All of these ideas have been thoroughly reviewed and critiqued by Sober (1984).

How important is selection in evolution? This is one of the main debates among evolutionary theorists today. In the decades following Darwin’s Origin of Species, evolutionists—notably E. D. Cope and A. Hyatt among paleontologists—rejected natural selection as a primary mechanism of evolution, preferring development-based explanations instead (Bowler, 1988). Early 20th Century scientists of the Modern Synthesis—including the paleontologist G. G. Simpson—revived natural selection as the predominant factor. Evolution was viewed as changing gene frequencies resulting from selection on individual organisms. Some theorists since then have questioned the importance of organismal selection in evolution. Kimura (1983) has argued that most evolution at the molecular level is neutral and not the result of selection at all. Gould and Lewontin (1979) have popularized the search for other causal factors of evolutionary change. Dawkins (1976) has reduced selection to the level of the gene, while Stanley (1979) and Vrba (1989) have urged paleontologists to look for evolutionary patterns and mechanisms above the species level. Clearly selection remains a timely and vibrant topic in paleontology today.

References Bowler, P. J. 1988. The Non-Darwinian Revolution:

Reinterpreting a Historical Myth. Johns Hopkins: Baltimore.

Darwin, C. R. 1859. The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. John Murray, London.

Dawkins, R. 1976. The Selfish Gene. Oxford University Press, Oxford.

Gould, S. J. and R. C. Lewontin. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society London, Series B, 205: 581-598.

Gould, S. J. and E. S. Vrba. 1982. Exaptation—a missing term in the science of form. Paleobiology, 11: 2-12.

Kimura, M. 1983. The Neutral Theory of Molecular Evolution. Cambridge University Press, Cambridge.

Schmalhausen, I. I. 1946. Faktory evolyutsii. Teoria stabiliziruyushchego otbora (Factors of Evolution: The Theory of Stabilizing Selection). Academy of Sciences USSR, Moscow. English editions: 1949, Blakiston Co., Philadelphia; 1986, Chicago University Press, Chicago.

Sober, E. 1984. The Nature of Selection: Evolutionary Theory in Philosophical Focus. University of Chicago Press, Chicago.

Stanley, S. M. 1979. Macroevolution: Pattern and Process. W. H. Freeman, San Francisco.

Van Valen, L. 1973. A new evolutionary law. Evolutionary Theory ,1: 1-30.

Vrba, E. S. 1989. Levels of selection and sorting with special reference to the species level. Oxford Surveys in Evolutionary Biology, 6: 111-168.

Wright, S. 1932. The roles of mutation, inbreeding, crossbreeding, and selection in evolution. Proceedings of the 6th International Congress on Genetics, 1: 356-366.

Further Reading Darwin, C. R. 1859. The Origin of Species by Means of

Natural Selection or the Preservation of Favoured Races in the Struggle for Life. John Murray, London.

Gould, S. J. and R. C. Lewontin. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society London, Series B, 205: 581-598.

Johnson, C. 1976. Introduction to Natural Selection. University Park Press, Baltimore.

Sober, E. 1984. The Nature of Selection: Evolutionary Theory in Philosophical Focus. University of Chicago Press, Chicago.

Williams, G. C. 1992. Natural Selection: Domains, Levels, and Challenges. Oxford University Press, New York and Oxford.