Northwest Fisheries Science Center

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Document Type: Chapter or Section
Center: NWFSC
Document ID: 1822
Type of Book: Technical
Section or Chapter Title: Effective population number
Book Title: Brenner's encyclopedia of genetics (second edition)
Author: Robin S. Waples
Editor: Stanley Maloy, Kelly Hughes (Eds.)
Publication Year: 2013
Publisher: Elsevier. New York
Pages: 453-455
Keywords: census size, genetic drift, ideal population, inbreeding, metapopulation, parentage analysis, poisson variance, reproductive success, sex ratio

Effective population number (or more commonly, effective population size, Ne) is the evolutionary analogue to the census size, N. Ne, rather than N, determines the rate of random genetic drift and hence the rates of allele frequency change, loss of genetic variability, and increase in inbreeding. Ne=N only in the special case of an ‘ideal’ population, in which variation among individuals in reproductive success is completely random. In most real populations, variation in reproductive success exceeds the random expectation, and this reduces Ne compared to N. Unless the sex ratio is exactly 1:1, members of the less numerous sex must on average produce more offspring, and this reduces Ne; however, the effect is not large unless the sex ratio is strongly skewed. Long-term Ne depends on the harmonic mean of single-generation values. Because the harmonic mean is dominated by small values, bottlenecks (generations with small Ne) can have a profound effect on genetic diversity. Recent decades have seen a great deal of interest in using genetic methods to estimate Ne, to better understand evolutionary processes and to aid conservation efforts. These studies have provided empirical evidence for wide variation in the Ne/N ratio among species with different life histories and ecologies.

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