Genetic diversity in experimental metapopulations

The biological diversity of our planet is rapidly declining. Because of habitat deterioration and fragmentation, many species have been reduced to small, more or less isolated populations with an increased extinction risk. Such small populations suffering from temporary extinctions, but connected through (limited) migration constitute a metapopulation. The genetic diversity in a metapopulation is mostly decided by “genetic drift” removing genetic variation from small populations, and by “gene flow”, for example through migration between populations, that mitigates such a loss of variation. Estimates of the “effective” population size and migration rate of a metapopulation are therefore crucial for the development of nature conservation measures. Such estimates are commonly based on highly abstract, unrealistic theoretical models that are rarely validated in natural systems. In this study, I have evaluated the practical value of such methods using experimental metapopulations of fruit flies (Drosophila) and realistic, individual-based computer simulations. The average dynamics of genetic variation in the experimental metapopulations was predicted fairly well by general theory but the replicates often deviated strongly, hence conclusions based on a single metapopulation can be quite misleading. Experimental details affected the results substantially: factors such as the method and timing of migration, and whether migrants are mostly females or males, are important for the structure of genetic variation. Summarizing, I conclude that general theory can reliably predict average trends, but not the dynamics of a single metapopulation. Since the dynamics tend to be sensitive to parameters that are often insufficiently known in practice, wide safety margins are advisable for practical applications, such as developing management strategies for nature conservation.