Population

=== Genetics ===
{{main|Population genetics}}

In genetics, a ''population'' is often defined as a set of organisms in which any pair of members can [[Breeding in the wild|breed]] together. They can thus routinely exchange gametes in order to have usually fertile progeny, and such a [[Sexual reproduction|breeding]] group is also known therefore as a gamodeme. This also implies that all members belong to the same species.<ref>{{cite book | last=Hartl | first=Daniel | title=Principles of Population Genetics | publisher=[[Sinauer Associates]] | year=2007 | isbn=978-0-87893-308-2 | page=95}}</ref>
If the gamodeme is very large (theoretically, approaching infinity), and all gene alleles are uniformly distributed by the gametes within it, the gamodeme is said to be panmictic. Under this state, [[allele]] ([[gamete]]) frequencies can be converted to genotype ([[zygote]]) frequencies by expanding an appropriate [[quadratic equation]], as shown by Sir [[Ronald Fisher]] in his establishment of quantitative genetics.<ref>{{cite book | last=Fisher | first=R. A. | title=The Genetical Theory of Natural Selection | publisher=[[Oxford University Press]] (OUP) | year=1999 | isbn=978-0-19-850440-5}}</ref>

This seldom occurs in nature: localization of gamete exchange – through dispersal limitations, preferential mating, cataclysm, or other cause – may lead to small actual gamodemes which exchange gametes reasonably uniformly within themselves but are virtually separated from their neighboring gamodemes. However, there may be low frequencies of exchange with these neighbors. This may be viewed as the breaking up of a large sexual population (panmictic) into smaller overlapping sexual populations. This failure of [[panmixia]] leads to two important changes in overall population structure: (1) the component gamodemes vary (through gamete sampling) in their allele frequencies when compared with each other and with the theoretical panmictic original (this is known as dispersion, and its details can be estimated using expansion of an appropriate [[binomial equation]]); and (2) the level of homozygosity rises in the entire collection of gamodemes. The overall rise in homozygosity is quantified by the inbreeding coefficient (f or φ). All homozygotes are increased in frequency – both the deleterious and the desirable. The mean phenotype of the gamodemes collection is lower than that of the panmictic original – which is known as inbreeding depression. It is most important to note, however, that some dispersion lines will be superior to the panmictic original, while some will be about the same, and some will be inferior. The probabilities of each can be estimated from those binomial equations. In [[plant breeding|plant]] and [[animal breeding]], procedures have been developed which deliberately utilize the effects of dispersion (such as line breeding, pure-line breeding, backcrossing). It can be shown that dispersion-assisted selection leads to the greatest genetic advance (ΔG=change in the phenotypic mean), and is much more powerful than selection acting without attendant dispersion. This is so for both allogamous (random fertilization)<ref>{{cite journal | last=Gordon  | first=Ian L. | title=Quantitative genetics of allogamous F2 : an origin of randomly fertilized populations | journal=[[Heredity (journal)|Heredity]]| year=2000  | volume=85  | pages=43–52  | doi=10.1046/j.1365-2540.2000.00716.x  | pmid=10971690| doi-access=free  }}</ref> and autogamous (self-fertilization) gamodemes.<ref>{{cite journal | last=Gordon  | first=Ian L. | title=Quantitative genetics of autogamous F2 | journal=[[Hereditas]] | year=2001 | volume=134 | pages=255–262 | doi=10.1111/j.1601-5223.2001.00255.x | pmid=11833289 | issue=3| doi-access=free }}</ref>