Consequences of fragmentation for the ability to adapt to novel environments in experimental Drosophila metapopulations

We used experimental populations of Drosophila melanogaster, which had either been subdivided (metapopulations) or kept undivided for 40 generations, to study the consequences of population subdivision for the tolerance and adaptive response after six generations of exposure to novel environmental factors (high temperature, medium with ethanol or salt added) for traits with different genetic architectures. In this setup, we attempted to separate the effects of the loss of fitness due to inbreeding (i.e., the survival upon first exposure to stress) from the loss of adaptive potential due to the lack of genetic variation. To place our experimental results in a more general perspective, we used individual-based simulations combining different options of levels of gene flow, intensity of selection and genetic architecture to derive quantitative hypotheses of the effects of these factors on the adaptive response to stress. We observed that population subdivision resulted in substantial inter-deme variation in tolerance due to redistribution of genetic variation from within demes to among demes. In line with the simulation results, the adaptive response was generally lower in the subdivided than in the undivided populations, particularly so for high temperature. We observed pronounced differences between stress factors that are likely related to the different genetic architectures involved in resistance to these factors. From a conservation genetics viewpoint, our results have two important implications: (i) Long-term fragmentation in combination with restricted gene flow will limit the adaptive potential of individual subpopulations because adaptive variation will become distributed among populations rather than within populations. (ii) The genetic architecture of the trait(s) under selection is of great significance to understand the possible responses to novel stresses that may be expected.