Size and genetic composition of the colonizing propagules in a butterfly metapopulation

The amount and spatial distribution of genetic variation that is maintained in a metapopulation depends critically on the colonization process. Here, we use molecular markers to determine the number and genetic relatedness of individuals establishing new local populations in a large metapopulation of the Glanville fritillary butterfly Melitaea cinxia. The empirical results are compared with the predictions of a dispersal model based on a diffusion approximation of correlated random walk, which serves as a base-line hypothesis about the rate and pattern of colonization. The results show that half of the new local populations consisted of a single larval group of full sibs and hence necessarily of the off spring of a single female. If the colonization involved two or more larval groups, these were usually oviposited by two different females that were unrelated to each other. The pattern of colonizations is thus intermediate between the propagule pool and the migrant pool models. These results elucidate the generation of genetic stochasticity, which may influence the dynamics of small populations. The dispersal model predicted well the pattern of habitat occupancy and the pattern of colonizations in relation to landscape structure, though which particular habitat patches became colonized was influenced also by measures of habitat quality not included in the model.