Directional selection

In population genetics, directional selection is a mode of natural selection in which individuals with a trait (for example, beak size) at one extreme of a phenotypic distribution have better fitness than individuals with intermediate or opposite extreme phenotypes. Over time, the allele frequencies, and consequently the population mean for the trait, shift consistently in the direction of the extreme phenotype with greater fitness. An example is the evolution of antibiotic resistance in bacteria – the introduction of a strong selective pressure (the antibiotic) selects resistant strains of bacteria, thereby shifting allele frequencies toward phenotypes with strong resistance to the antibiotic. This type of selection plays an important role in the emergence of complex and diversifying traits and is also a primary force in speciation. Natural phenomena that might promote strong directional selection include: * Sudden environmental changes (biotic or abiotic) favouring one phenotype over a previously dominant phenotype; * Colonization of a new habitat with novel selection pressures (as was the case with Darwin’s finches migrating to the Galápagos Islands two million years ago); * The genetic context offers sufficient heritable variation and involves relatively minor interactions or correlations among genes (pleiotropy or epistasis ) and trade-offs (antagonistic pleiotropy).…

In population genetics, directional selection is a mode of natural selection in which individuals with a trait (for example, beak size) at one extreme of a phenotypic distribution have better fitness than individuals with intermediate or opposite extreme phenotypes. Over time, the allele frequencies, and consequently the population mean for the trait, shift consistently in the direction of the extreme phenotype with greater fitness. An example is the evolution of antibiotic resistance in bacteria – the introduction of a strong selective pressure (the antibiotic) selects resistant strains of bacteria, thereby shifting allele frequencies toward phenotypes with strong resistance to the antibiotic. This type of selection plays an important role in the emergence of complex and diversifying traits and is also a primary force in speciation. Natural phenomena that might promote strong directional selection include: Sudden environmental changes (biotic or abiotic) favouring one phenotype over a previously dominant phenotype; Colonization of a new habitat with novel selection pressures (as was the case with Darwin’s finches migrating to the Galápagos Islands two million years ago); The genetic context offers sufficient heritable variation and involves relatively minor interactions or correlations among genes (pleiotropy or epistasis ) and trade-offs (antagonistic pleiotropy). Such interactions would not necessarily preclude directional selection, but would make it more complicated.

Examples

African cichlids

African cichlids are known to be a diverse fish species, with evidence indicating that they evolved extremely quickly. These fish evolved within the same habitat, but have a variety of morphologies, especially pertaining to the mouth and jaw. Experiments pertaining the cichlid jaw phenotypes were done by Albertson and others in 2003 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 feeding) allowed for mapping of QTLs affecting feeding morphology. Using the QTL sign test, definitive evidence was used to support the existence of directional selection in the oral jaw apparatus in African cichlids. However, this was not the case for the suspensorium or skull QTLs, suggesting genetic drift or stabilizing selection as mechanisms for the speciation.

Sockeye salmon

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 a 2007 study 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, Egegik and Ugashik, were observed. 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 the average migration date was earlier and the populations were undergoing directional selection as a result of changing ecological conditions. The Egegik population experienced stronger selection and the migration date shifted four days. The paper suggests that fisheries can be a factor driving this selection because fishing occurs more often in the later periods of migration (especially in the Egegik district), preventing those fish from reproducing.

See also

Adaptive evolution in the human genome Balancing selection Disruptive selection Frequency-dependent foraging by pollinators Negative selection (natural selection) Stabilizing selection Peppered moth evolution Fluctuating selection

References

Further reading

Types of Selection Natural Selection Modern Theories of Evolution

Text derived from https://en.wikipedia.org/w/index.php?oldid=1378513846 under CC BY-SA 4.0; modifications have been made.