The history of genetic diversity and effective population size of an isolated Microtus oeconomus population on Kis Balaton

The distribution of many species is fragmented due to human activities and isolated populations are often affected by genetic changes. Small populations are endangered by inbreeding depression, often leading to a low mean fitness of small populations and to an increased extinction risk. Using museum samples of protected Microtus oeconomus mehelyi from the years 1950, 1964 and 2000, we studied the population which inhabits the Kis Balaton area that has considerably changed by human activity in the last centuries. The detected effective population size, differences in allele frequencies, associations between alleles and a high genetic differentiation may be explained by 50 years of random genetic drift in this population in the significantly altered Kis Balaton area. We have detected a genetic difference between Kis Balaton populations and published populations from Hungary, Austria and Slovakia. Using linkage disequilibrium and temporal approaches, the estimates of the effective population size are below 100/1000, thresholds to limit inbreeding depression and to retain evolutionary potential. Taking the fragmented occurrence of Microtus oeconomus mehelyi in central Europe into account, it is possible that also other populations may also undergo the same processes after significant isolation.