Loss of Genetic Diversity from Managed Populations: Interacting Effects of Drift, Mutation, Immigration, Selection, and Population Subdivision

Abstract

A computer simulation program was used to examine interacting effects of genetic drift, mutation, immigration from outside populations, directional and balancing selection, and population subdivision on the loss of genetic variability from small, managed populations. Stochastic events were simulated with a pseudo-random number generator, and the genetic variation (expected heterozygosity) within and between populations was monitored in 25 populations for 100 generations. Genetic drift was the overriding factor controlling the loss of genetic variation Mutation has no noticeable effect on populations of the size typically managed in zoos and nature preserves Immigration from a large source population can strikingly slow, halt, or even reverse the loss of genetic variation, even with only one or a few migrants per generation. Unless selection is stronger than commonly observed in natural populations, it is inefficient in countering drift when population sizes are on the order of 100 or fewer. Subdivided populations rapidly lose variability from within each sub-population but retain variation across the sub-populations better than does a panmictic population. These results suggest that population managers should be concerned with the variation-depleting effects of genetic drift, perhaps almost to the exclusion of consideration of selection and mutation Drift can be countered by the introduction of very occasional immigrants or, less effectively, by division of the managed population into smaller breeding groups that interchange enough migrants to prevent unacceptably deleterious inbreeding within each sub-population.