Directional Selection

by

Andrew MacColl

• LAST REVIEWED: 13 January 2014
• LAST MODIFIED: 13 January 2014
• DOI: 10.1093/obo/9780199941728-0049

Introduction

Directional selection occurs when individuals with traits on one side of the mean in their population survive better or reproduce more than those on the other. It has been demonstrated many times in natural populations, using both observational and experimental approaches. Directional selection does the “heavy lifting” of evolution by tending to move the trait mean toward the optimum for the environment. It results in increased adaptedness of organisms. It is the principle process that Charles Darwin himself envisaged as driving adaptive evolution. Two of Darwin’s own examples were (1) faster wolves being more successful at hunting deer, and (2) flowers that produce more nectar being more successful in attracting pollinating insects. These both suggest directional rather than other forms (“modes”) of selection. Directional selection is the process that comes most easily to mind when thinking about natural selection, and it is the form of selection that has taken place in the best-known examples of evolution (e.g., the peppered moth, antibiotic resistance, finch beaks). However, directional selection does not always result in evolution, because it can be constrained in many ways. If directional selection acts in different directions in different populations or species, because of variation in environmental circumstances, then it is described as divergent. This results in populations becoming different, and it can contribute to speciation. Directional selection can also be artificially imposed, and it has commonly been used by animal and plant breeders to improve traits (such as yield) in domesticated organisms, as well as to better understand evolution. This bibliography first deals with natural directional selection, and then moves on to address artificial selection.

General Overviews

Most undergraduate textbooks on evolution, such as Futuyma 2009 and Barton, et al. 2007, contain basic definitions, descriptions, and examples of directional selection, as well as of natural selection more broadly. There are several excellent texts that deal with more advanced topics in natural selection, including directional selection. These include Williams 1966, a seminal book that emphasized that selection acts predominantly on individuals, and Endler 1986, Natural Selection in the Wild, a classic text that reviewed and synthesized evidence of the importance of natural selection in wild populations. More recent and thorough descriptions of the workings of natural selection are found in Mitton 1997, Selection in Natural Populations, and Bell 2008, Selection: The Mechanism of Evolution. No overview of directional selection would be complete without reference to the first book about natural selection, Darwin 1859, On the Origin of the Species by Means of Natural Selection. In addition to its historical interest, it remains an important book for the extent of its evidence and the depth of its insight.

• Barton, N. H., D. E. G. Briggs, J. A. Eisen, D. B. Goldstein, and N. H. Patel. 2007. Evolution. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory. 833.

  Well-illustrated undergraduate textbook with good basic introduction to directional selection. Lack of referencing makes it difficult to follow up on examples in detail. Very useful glossary.

• Bell, G. 2008. Selection: The mechanism of evolution. 2d ed. Oxford: Oxford Univ. Press.

  A very useful advanced textbook providing thorough and detailed consideration of the operation of selection in the evolution of adaptation. Examples are biased toward microbial systems. Concentrates on the evolutionary consequences, rather than the ecological context, of selection, although it includes interesting material on the latter.

• Darwin, C. 1859. On the origin of species by means of natural selection. London: John Murray.

  Without doubt the most famous book about natural selection, and in the whole of evolutionary biology. Still a very useful and entertaining read that is astonishing for the care with which its enormous evidence base was garnered and organized. Darwin understood the operation of directional selection better than most biologists that followed him for a century afterwards.

• Endler, J. A. 1986. Natural selection in the wild. Princeton, NJ: Princeton Univ. Press.

  The first review and synthesis of studies and concepts relating to natural selection in wild populations. It contains detailed consideration of the philosophy and methods for the study of natural selection. A seminal work.

• Futuyma, D. J. 2009. Evolution. Sunderland, MA: Sinauer Associates. 633.

  The latest edition of the standard undergraduate textbook for evolution. Contains introductory material on directional selection.

• Mitton, J. B. 1997. Selection in natural populations. Oxford: Oxford Univ. Press.

  A review and synthesis of studies of selection on genetic variation and protein polymorphisms in natural populations, and why this comes about. The approaches described have been outdated by the modern march to use genomic methods, but the book documents many classic and easily understood examples.

• Williams, G. C. 1966. Adaptation and natural selection: A critique of some current Evolutionary thought. Princeton, NJ: Princeton Univ. Press.

  An enormously influential book. Reacting to the group selectionist and teleological thinking that was common at the time of its writing, Williams was the first clearly and explicitly to advocate, in an accessible style, that the explanation for evolutionary adaptations should be sought mainly in the simple operation of natural selection at the level of the individual and the gene.