Editing Natural selection (section)

===Directionality of selection===<!-- This section is linked from [[Race and intelligence]] -->
{{Main|Directional selection}}

When some component of a trait is heritable, selection alters the frequencies of the different alleles, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies: [[directional selection|directional]], [[stabilizing selection|stabilizing]], and [[disruptive selection]].<ref name="Rice">{{harvnb|Rice|2004|loc=See especially chapters 5 and 6 for a quantitative treatment}}</ref> Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population. This process can continue until the allele is [[fixation (population genetics)|fixed]] and the entire population shares the fitter phenotype.<ref>{{cite journal |author1=Rieseberg, L.H. |author2=Widmer, A. |author3=Arntz, A.M. |author4=Burke, J.M. |date=2002 |title=Directional selection is the primary cause of phenotypic diversification |journal=PNAS |volume=99 |issue=19 |pages=12242–12245 |doi=10.1073/pnas.192360899 |pmid=12221290 |pmc=129429|bibcode=2002PNAS...9912242R |doi-access=free }}</ref> Far more common is stabilizing selection, which lowers the frequency of alleles that have a deleterious effect on the phenotype—that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Stabilizing selection [[Conserved sequence|conserves]] functional genetic features, such as [[protein biosynthesis|protein-coding genes]] or [[regulatory sequence]]s, over time by selective pressure against deleterious variants.<ref>{{cite journal |vauthors=Charlesworth B, Lande R, Slatkin M |date=1982 |title=A neo-Darwinian commentary on macroevolution |journal=Evolution |volume=36 |issue=3 |doi=10.1111/j.1558-5646.1982.tb05068.x |pmid=28568049 |pages=474–498|jstor=2408095 |s2cid=27361293 |doi-access=free }}</ref> Disruptive (or diversifying) selection is selection favouring extreme trait values over intermediate trait values. Disruptive selection may cause [[sympatric speciation]] through [[niche partitioning]].

Some forms of [[balancing selection]] do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in [[diploid]] species (with pairs of chromosomes) when [[Zygosity#Heterozygous|heterozygous]] individuals (with just one copy of the allele) have a higher fitness than homozygous individuals (with two copies). This is called heterozygote advantage or over-dominance, of which the best-known example is the resistance to malaria in humans heterozygous for [[sickle-cell anaemia]]. Maintenance of allelic variation can also occur through [[disruptive selection|disruptive or diversifying selection]], which favours genotypes that depart from the average in either direction (that is, the opposite of over-dominance), and can result in a [[Multimodal distribution|bimodal distribution]] of trait values. Finally, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of [[game theory]] have been applied to understand the fitness distributions in these situations, particularly in the study of kin selection and the evolution of [[reciprocal altruism]].<ref name="Hamilton"/><ref name="Trivers">{{cite journal |last=Trivers |first=Robert L. |author-link=Robert Trivers |date=March 1971 |title=The Evolution of Reciprocal Altruism |journal=The Quarterly Review of Biology |volume=46 |issue=1 |pages=35–57 |doi=10.1086/406755 |jstor=2822435|s2cid=19027999 }}</ref>