Genetic neighbourhood and effective population size for two endangered frogs

Populations with small effective sizes ( < 100) are prone to rapid divergence, loss of heterozygosity, inbreeding and random fixation of mutations. Estimating effective population size (Nₑ), and the comparison of Nₑ to census population size (N) is, therefore, important to understand the possible impacts of genetic processes on population survival. In this paper I report population sizes, estimate Are, and the size of genetic neighbourhoods of Geocrinia alba and Geocrinia vitellina, two endangered Myobatrachid frog species from south-western Australia. The diameters of genetic neighbourhoods were 37.9 m (G. alba) and 29.2 m (G. vitellina) with neighbourhood sizes of 2-137 for G. alba and 30-166 for G. vitellina. Most populations of G. alba (up to 89%) are very small (≤100 adults). The ratio of Nₑ to N was approximately one, in contrast to recent suggestions that Nₑ/N should be closer to 0.5 or 0.1 in wild populations. Previous studies of G. alba and G. vitellina indicate substantial genetic divergence among populations and low heterozygosity. The results of this study suggest that genetic drift is likely to be an important evolutionary process in both species and may account in part for the extreme genetic structuring.