A population is a summation of all the organisms of the same group or species, which live in a particular geographical area, and have the capability of interbreeding.〔(【引用サイトリンク】url=http://www.biology-online.org/dictionary/Population )〕
In ecology, the population of a certain species in a certain area is estimated using the Lincoln Index. The area that is used to define a sexual population is defined as the area where inter-breeding is potentially possible between any pair within the area. The probability of interbreeding is greater than the probability of cross-breeding with individuals from other areas. Under normal conditions, breeding is substantially more common within the area than across the border.
In sociology, population refers to a collection of humans. Demography is a social science which entails the statistical study of human populations. This article refers mainly to human population.
==Population genetics (ecology)==
In population genetics a sexual population is a set of organisms in which any pair of members can breed together. This means that they can regularly exchange gametes to produce normally-fertile offspring, and such a breeding group is also known therefore as a ''gamodeme''. This also implies that all members belong to the same of species, such as humans.
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.
This seldom occurs in nature : localisation of gamete exchange – through dispersal limitations, or preferential mating, or cataclysm, or other cause – may lead to small actual gamodemes which exchange gametes reasonably uniformly within themselves, but are virtually separated from their neighbouring gamodemes. However, there may be low frequencies of exchange with these neighbours. 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''). Note that ''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 and animal breeding, procedures have been developed which deliberately utilise the effects of dispersion (such as line breeding, pure-line breeding, back-crossing). 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)
and autogamous (self-fertilization) gamodemes
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