“Effective Population Size and Patterns of Molecular Evolution and Variation”, 2009-03 (; backlinks; similar):
The effective size of a population, Ne, determines the rate of change in the composition of a population caused by genetic drift, which is the random sampling of genetic variants in a finite population. Ne is crucial in determining the level of variability in a population, and the effectiveness of selection relative to drift.
There are 3 major ways by which the rate of genetic drift can be modelled in the simplest type of population: increase in variance of allele frequencies; approach to identity by descent of all alleles; and coalescence of a sample of alleles into an ancestral allele.
A general method for calculating Ne has been developed using coalescent theory for populations in which there are several ‘compartments’ (for example, ages or sexes) in the population from which alleles can be sampled. This involves the fast timescale approximation, in which the flow of alleles between compartments is faster than the rate of coalescence.
Formulae are presented for the effects on Ne of differences in numbers of breeding males and females, differences in variance of offspring number between males and females, levels of inbreeding, changes in population size and modes of inheritance.
Methods for estimating Ne for natural and artificial populations are described, using both demographic and genetic approaches. Examples of the results of the application of these methods are presented.
The theory of how selection operates in a finite population is outlined, based on the formula for the probabilities of fixation of favourable and deleterious mutations. These depend on the product of the selection coefficient and Ne.
There are data that show that the level of molecular sequence adaptation is reduced when Ne is low, as predicted from the product of the selection coefficient and Ne.
The problem of describing neutral variability and the outcome of selection in a spatially or genetically structured population is discussed. A great simplification occurs when there is a large number of local populations in a structured metapopulation.
Selection at one or more sites in the genome can influence the value of Ne at other, genetically linked sites by several mechanisms. Balancing selection can elevate Ne, whereas positive selection and purifying selection reduce it.
Data on molecular variation and evolution are described, which are consistent with these theoretical predictions.
The effective size of a population, Ne, determines the rate of change in the composition of a population caused by genetic drift, which is the random sampling of genetic variants in a finite population. Ne is crucial in determining the level of variability in a population, and the effectiveness of selection relative to drift. This article reviews the properties of Ne in a variety of different situations of biological interest, and the factors that influence it. In particular, the action of selection means that Ne varies across the genome, and advances in genomic techniques are giving new insights into how selection shapes Ne.