Effective population size and patterns of molecular evolution and variation

The effective size of a population, N-e, 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. N-e 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 N-e in a variety of different situations of biological interest, and the factors that influence it. In particular, the action of selection means that N-e varies across the genome, and advances in genomic techniques are giving new insights into how selection shapes N-e.