Effects of intense versus diffuse population bottlenecks on microsatellite genetic diversity and evolutionary potential

Population bottlenecks occur frequently in threatened species and result in loss of genetic diversity and evolutionary potential. These may range in severity between short intense bottlenecks, and more diffuse bottlenecks over many generations. However, there is little information on the impacts of different types of bottlenecks and disagreement as to their likely impacts. To resolve this issue, we subjected replicate Drosophila populations to intense bottlenecks, consisting of one pair over a single generation, versus diffuse bottlenecks consisting of an effective size of 100 over 57 generations. The intense and diffuse bottlenecks were designed to induce identical losses of heterozygosity. However, computer simulations showed that the probability of retaining alleles is lower in the intense than the diffuse bottleneck treatment. The effects of these bottlenecks on genetic diversity at nine microsatellite loci in Drosophila were evaluated. Bottlenecks substantially reduced allelic diversity, heterozygosity and proportion of loci polymorphic, changed allele frequency distributions and resulted in large differences among replicate populations. Allelic diversity, scaled by heterozygosity, was lower in the intense than the diffuse treatments. Short-term evolutionary potential, measured as the ability of bottlenecked populations to cope with increasing concentrations of NaCl, did not differ between the intense and diffuse bottlenecked populations. The effects of bottlenecks on short-term evolutionary potential relate to loss of heterozygosity, rather than allelic diversity.