Effective Population Size
Effective population size is a term that defines the individuals within a population that contribute next offspring generation. In terms of ecology, the population size is measured by counting adults in a given locality. However, in the case of population genetics theory, what is important is the possibility that 2 gene copies are sampled to produce the next generation which is affected by the structure of the breeding population.
American geneticist, Sewall Wright is the one who introduced the term and he described it as “ the number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration”
From a general perspective, effective population is described as figure of individuals within an ideal population which has the value of that given population with genetic quantity equal to the quantity value of that population that is of interest. Wright identified 2 population genetic quantities which included:
- 1 generation increase that varies across populations that are replicate (effective variance population size)
- 1 generation change in inbreeding coefficient (effective inbreeding population size).
Note that while these two are linked closely and though they result from F-statistics, they aren’t identical. Currently, the estimation of effective population size is empirically made in regard to coalescence time or sojourn which is estimated as genetic diversity divided by the rate of mutation. Regardless of the definition used, effective population size is often less compared to census population size. In the population size N, there are 2N genes and the appropriate interpretation of N for theoretical equations can be described as N which is appropriately measured with the drawing of 2 copies of the matching gene which is (1/2N)2.
If these 2 genes are drawn from that population at a location, it is likely that for varying reasons there will be two copied of an identical gene which is implied by the ecological naïve measure of the size of population. In the ideal population size of the Wright-Fisher model there is conditional allele frequency variance ρ’ in which the allele frequency ρ in prior generation is:
Var (ρ’/ p)= p(1-p)
In populations of Drosophila of census size sixteen, the effective size population is measured as one that is equal to 11.5. The measurement is attained through the study of frequency changes of neutral allele one generation to the next in more than one hundred replicate population. For the size of the population to be constant, every individual is supposed to contribute 2 gametes on average to next generation. An ideal population makes the assumption that this is in line with Poisson distribution such that the variance in the gametes number contributed is equal to mean number contributed.
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