Maintaining Genetic Variation in a One-Way, Two Island Model

Abstract

I used a stochastic simulation model to examine the loss of heterozygosity and allelic diversity in a bottlenecked population and a sink population subject to various migration rates from source populations of different sizes. In the bottlenecked population, the initial founder size influenced the level of allelic diversity more than heterozygosity, although the subsequent patterns of loss of the 2 measures were similar. The rescue effect, whereby migration from a source population offsets genetic drift in a sink population, was shown to have a detrimental counterpart termed the imperil effect, which may lead to the erosion of genetic variation. This is manifested when the source population has less genetic variation than the sink population. When genetic data about a source population are lacking or scant, to avoid the imperil effect, migration rates should be <1 migrant per generation. J. WILDL. MANAGE. 54(4):676-682 A suggested goal of captive breeding and genetic management is to maintain 90% of the heterozygosity of the source (wild) population over a period of 200 years (Soule et al. 1986). This goal becomes complicated when economic and spatial constraints dictate that population sizes of managed species remain small, making the populations susceptible to genetic drift. In addition to heterozygosity, which has been correlated with individual fitness (Beardmore 1983, Allendorf and Leary 1986), another populationlevel measure of genetic variance, allelic diversity, is crucial to the long-term adaptability of a population (Frankel and Soule 1981) and should also be considered for genetic management. When a large population is reduced in size to N individuals, the average heterozygosity per locus is expected to decrease by 1/(2N) (Nei et al. 1975), and the number of polymorphic alleles at a particular locus is expected to decrease by 2 (1 pj)2N (Denniston 1978), where p, is the -1ee frequency, and k is the number of alleles allele frequency, and k is the number of alleles at a locus. Both measures will continue to decline from generation to generation until a drift-mutation equilibrium is attained (Lacy 1987). If the population rebounds from the bottleneck, heterozygosity may show little or no reduction, but rare alleles have a high probability of being lost (Allendorf 1986, Fuerst and Maruyama 1986). Management techniques designed to maintain allelic diversity and heterozygosity are conflicting. Allelic diversity is promoted through the subdivision of populations (Chesser 1983), whereas heterozygosity is maintained through a large effective population size (N,) (Simberloff 1988). Limited migration among subpopulations or 1-way migration from an infinitely large population decreases loss of heterozygosity caused by genetic drift (Wright 1931, Lacy 1987). This rescue effect (Brown and KodricBrown 1977) provided by an impulse of new genetic material could reduce the extinction probability of an insular population by preventing the loss of genetic variation. Migration This content downloaded from 157.55.39.243 on Wed, 05 Oct 2016 04:47:51 UTC All use subject to http://about.jstor.org/terms J. Wildl. Manage. 54(4):1990 RESCUE EFFECT * Weishampel 677 rates of <1 (Foose et al. 1986, Off. Technol. Assessment 1987), 1 (Avery 1978, Allendorf 1983), 1-2 (Franklin 1980), and 1-5 migrants per generation (Frankel and Soul6 1981) have been proposed. In my study, I used a stochastic simulation model to examine how bottleneck severity, immigration rate, and source and sink population sizes affect heterozygosity and allelic diversity. This work was inspired by E. F. Connor, J. J. Murray, and H. H. Shugart. I wish to thank them, J. A. Yeakley, and 3 anonymous reviewers for their input. This research was supported in part by an NSF grant presented under Interagency Agreement No. BSR-8718168 with the Department of Energy and by the National Aeronautics and Space Administration Earth Sciences Division (UPN 677-80-06-05) to W. E.