Directional selection

Evolutionary BiologyDirectional Selection
Andrew MacColl
  • LAST REVIEWED: 10 July 2019
  • LAST MODIFIED: 13 January 2014
  • DOI: 10.1093/obo/9780199941728-0049


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.

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Directional selection

"Positive selection" redirects here. For positive selection of thymocytes during maturation, see Thymocyte.

For theories of goal-directed evolution, see Orthogenesis.

The different types of genetic selection: on each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the amount of organisms. Group A is the original population and Group B is the population after selection. Graph 1 shows directional selection, in which a single extreme phenotype is favored. Graph 2 depicts stabilizing selection, where the intermediate phenotype is favored over the extreme traits. Graph 3 shows disruptive selection, in which the extreme phenotypes are favored over the intermediate.

In population genetics, directional selection, or positive selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes. The increases are independent of the dominance of the allele, and even if the allele is recessive, it will eventually become fixed.[1]

Directional selection was first described by Charles Darwin in the book On the Origin of Species as a form of natural selection.[2] Other types of natural selection include stabilizing and disruptive selection. Each type of selection contains the same principles, but is slightly different. Disruptive selection favors both extreme phenotypes, different from one extreme in directional selection. Stabilizing selection favors the middle phenotype, causing the decline in variation in a population over time.[3]


Directional selection occurs most often under environmental changes and when populations migrate to new areas with different environmental pressures. Directional selection allows for fast changes in allele frequency, and plays a major role in speciation. Analysis on QTL effects has been used to examine the impact of directional selection in phenotypic diversification. This analysis showed that the genetic loci correlating to directional selection was higher than expected; meaning directional selection is a primary cause of phenotypic diversification, which leads to speciation.[4]

Detection methods[edit]

There are different statistical tests that can be run to test for the presence of directional selection in a population. A few of the tests include the QTL sign test, Ka/Ks ratio test and the relative rate test. The QTL sign test compares the number of antagonistic QTL to a neutral model, and allows for testing of directional selection against genetic drift.[5] The Ka/Ks ratio test compares the number of non-synonymous to synonymous substitutions, and a ratio that is greater than 1 indicates directional selection.[6] The relative ratio test looks at the accumulation of advantageous against a neutral model, but needs a phylogenetic tree for comparison.[7]


An example of directional selection is fossil records that show that the size of the black bears in Europe decreased during interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in a population of finches. Throughout the wet years, small seeds were more common and there was such a large supply of the small seeds that the finches rarely ate large seeds. During the dry years, none of the seeds were in great abundance, but the birds usually ate more large seeds. The change in diet of the finches affected the depth of the birds’ beaks in the future generations. Their beaks range from large and tough to small and smooth.[8]

African cichlids[edit]

African cichlids are known to be some of the most diverse fish and evolved extremely quickly. These fish evolved within the same habitat, but have a variety of morphologies, especially pertaining to the mouth and jaw. Albertson et al. 2003 tested this by crossing two species of African cichlids with very different mouth morphologies. The cross between Labeotropheus fuelleborni (subterminal mouth for biting algae off rocks) and Metriaclima zebra (terminal mouth for suction feed) allowed for mapping of QTLs affecting feeding morphology. Using the QTL sign test definitive evidence was shown to prove that directional selection was occurring in the oral jaw apparatus. However, this was not the case for the suspensorium or skull (suggesting genetic drift or stabilizing selection).[9]

Sockeye salmon[edit]

Sockeye salmon are one of the many species of fish that are anadromous. Individuals migrate to the same rivers in which they were born to reproduce. These migrations happen around the same time every year, but Quinn et al. 2007 shows that sockeye salmon found in the waters of the Bristol Bay in Alaska have recently undergone directional selection on the timing of migration. In this study two populations of sockeye salmon were observed (Egegik and Ugashik). Data from 1969-2003 provided by the Alaska Department of Fish and Game were divided into five sets of seven years and plotted for average arrival to the fishery. After analyzing the data it was determined that in both populations average migration date was earlier and was undergoing directional selection. The Egegik population experienced stronger selection and shifted 4 days. Water temperature is thought to cause earlier migration date, but in this study there was no statistically significant correlation. The paper suggests that fisheries can be a factor driving this selection because fishing occurs more in the later periods of migration (especially in the Egegik district), preventing those fish from reproducing.[10]

Ecological impact[edit]

Directional selection can quickly lead to vast changes in allele frequencies in a population. Because the main cause for directional selection is different and changing environmental pressures, rapidly changing environments, such as climate change, can cause drastic changes within populations.


Typically directional selection acts strongly for short bursts and is not sustained over long periods of time.[11] If it did, a population might hit biological constraints such that it no longer responds to selection. However, it is possible for directional selection to take a very long time to find even a local optimum on a fitness landscape.[12] A possible example of long-term directional selection is the tendency of proteins to become more hydrophobic over time,[13] and to have their hydrophobic amino acids more interspersed along the sequence.[14]

See also[edit]


  1. ^Molles, MC (2010). Ecology Concepts and Applications. McGraw-Hill Higher Learning.
  2. ^Darwin, C (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray.
  3. ^Mitchell-Olds, Thomas; Willis, John H.; Goldstein, David B. (2007). "Which evolutionary processes influence natural genetic variation for phenotypic traits?". Nature Reviews Genetics. Springer Nature. 8 (11): 845–856. doi:10.1038/nrg2207. ISSN 1471-0056. PMID 17943192. S2CID 14914998.
  4. ^Rieseberg, Loren H.; Widmer, Alex; Arntz, A. Michele; Burke, John M. (2002-09-17). "Directional selection is the primary cause of phenotypic diversification". Proceedings of the National Academy of Sciences of the United States of America. 99 (19): 12242–5. Bibcode:2002PNAS...9912242R. doi:10.1073/pnas.192360899. PMC 129429. PMID 12221290.
  5. ^Orr, H.A. (1998). "Testing Natural Selection vs. Genetic Drift in Phenotypic Evolution Using Quantitative Trait Locus Data". Genetics. 149 (4): 2099–2104. doi:10.1093/genetics/149.4.2099. PMC 1460271. PMID 9691061.
  6. ^Hurst, Laurence D (2002). "The Ka/Ks ratio: diagnosing the form of sequence evolution". Trends in Genetics. Elsevier BV. 18 (9): 486–487. doi:10.1016/s0168-9525(02)02722-1. ISSN 0168-9525. PMID 12175810.
  7. ^Creevey, Christopher J.; McInerney, James O. (2002). "An algorithm for detecting directional and non-directional positive selection, neutrality and negative selection in protein coding DNA sequences". Gene. Elsevier BV. 300 (1–2): 43–51. doi:10.1016/s0378-1119(02)01039-9. ISSN 0378-1119. PMID 12468084.
  8. ^Campbell, Neil A.; Reece, Jane B. (2002). Biology (6th ed.). Benjamin Cummings. pp. 450–451. ISBN .
  9. ^Albertson, R. C.; Streelman, J. T.; Kocher, T. D. (2003-04-18). "Directional selection has shaped the oral jaws of Lake Malawi cichlid fishes". Proceedings of the National Academy of Sciences. 100 (9): 5252–5257. Bibcode:2003PNAS..100.5252A. doi:10.1073/pnas.0930235100. ISSN 0027-8424. PMC 154331. PMID 12704237.
  10. ^Quinn, Thomas P.; Hodgson, Sayre; Flynn, Lucy; Hilborn, Ray; Rogers, Donald E. (2007). "Directional Selection by Fisheries and the Timing of Sockeye Salmon (Oncorhynchus Nerka) Migrations". Ecological Applications. Wiley. 17 (3): 731–739. doi:10.1890/06-0771. ISSN 1051-0761. PMID 17494392.
  11. ^Hoekstra, H. E.; Hoekstra, J. M.; Berrigan, D.; Vignieri, S. N.; Hoang, A.; Hill, C. E.; Beerli, P.; Kingsolver, J. G. (2001-07-24). "Strength and tempo of directional selection in the wild". Proceedings of the National Academy of Sciences. 98 (16): 9157–9160. Bibcode:2001PNAS...98.9157H. doi:10.1073/pnas.161281098. ISSN 0027-8424. PMC 55389. PMID 11470913.
  12. ^Kaznatcheev, Artem (May 2019). "Computational Complexity as an Ultimate Constraint on Evolution". Genetics. 212 (1): 245–265. doi:10.1534/genetics.119.302000. PMC 6499524. PMID 30833289.
  13. ^Wilson, Benjamin A.; Foy, Scott G.; Neme, Rafik; Masel, Joanna (24 April 2017). "Young genes are highly disordered as predicted by the preadaptation hypothesis of de novo gene birth"(PDF). Nature Ecology & Evolution. 1 (6): 0146–146. doi:10.1038/s41559-017-0146. hdl:10150/627822. PMC 5476217. PMID 28642936.
  14. ^Foy, Scott G.; Wilson, Benjamin A.; Bertram, Jason; Cordes, Matthew H. J.; Masel, Joanna (April 2019). "A Shift in Aggregation Avoidance Strategy Marks a Long-Term Direction to Protein Evolution". Genetics. 211 (4): 1345–1355. doi:10.1534/genetics.118.301719. PMC 6456324. PMID 30692195.

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19.3B: Stabilizing, Directional, and Diversifying Selection

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Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.

Learning Objectives

  • Contrast stabilizing selection, directional selection, and diversifying selection.

Key Points

  • Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favors an average phenotype and selects against extreme variations.
  • In directional selection, a population’s genetic variance shifts toward a new phenotype when exposed to environmental changes.
  • Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
  • In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.

Key Terms

  • directional selection: a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction
  • disruptive selection: (or diversifying selection) a mode of natural selection in which extreme values for a trait are favored over intermediate values
  • stabilizing selection: a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

Stabilizing Selection

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most-closely matched to that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population’s genetic variance will decrease.


Stabilizing selection: Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.


Directional selection: Directional selection occurs when a single phenotype is favored, causing the allele frequency to continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution. Similarly, the hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.


Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts. Known as diversifying or disruptive selection, this is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.


Diversifying (or disruptive) selection: Diversifying selection occurs when extreme values for a trait are favored over the intermediate values.This type of selection often drives speciation.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Comparing Types of Natural Selection



Selection directional



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