DISTORTION OF ALLELE FREQUENCY-DISTRIBUTIONS PROVIDES A TEST FOR RECENT POPULATION BOTTLENECKS

We use population genetics theory and computer simulations to demonstr ate that population bottlenecks cause a characteristic mode-shift dist ortion in the distribution of allele frequencies at selectively neutra l loci. Bottlenecks cause alleles at low frequency (<0.1) to become le ss abundant than alleles in one or more intermediate allele frequency class (e.g., 0.1-0.2), This distortion is transient and likely to be d etectable for only a few dozen generations, Consequently only recent b ottlenecks are likely to be detected by tests for distortions in distr ibutions of allele frequencies. We illustrate and evaluate a qualitati ve graphical method for detecting a bottleneck-induced distortion of a llele frequency distributions. The simple novel method requires no inf ormation on historical population sizes or levels of genetic variation ; it requires only samples of 5 to 20 polymorphic loci and approximate ly 30 individuals. The graphical method often differentiates between e mpirical datasets from bottlenecked and nonbottlenecked natural popula tions. Computer simulations show that the graphical method is likely ( P > .80) to detect an allele frequency distortion after a bottleneck o f less than or equal to 20 breeding individuals when 8 to 10 polymorph ic microsatellite loci are analyzed.