Heterogeneity in effective population size and its implications in conservation genetics and animal breeding

Effective population size (N (e) ) is defined as the size of an idealized population undergoing the same rate of genetic drift as the population under study. It is a central concept in population genetics and used extensively in the design and monitoring of conservation and breeding programs. It is most often assumed that genetic drift effects are homogeneous across the genome and thus, so is N (e) . However, theory predicts that various processes can modify rates of genetic drift experienced by a locus in the genome. In particular, selection affecting linked neutral sites and rates of recombination can modulate the intensity of genetic drift throughout the genome. The increasing availability of sequence data has made it possible to test whether a single N (e) can account for the evolution of the genome. In this article, we discuss reasons why a heterogeneous N (e) can be expected theoretically and review what evidence is available from empirical studies. We finish by discussing potential implications for conservation and breeding practices. The evidence suggests that heterogeneity in N (e) can be just as important as heterogeneity in mutation rates in determining levels of genetic diversity throughout the genome.