Evolutionary Responses of a Butterfly Metapopulation to Human- and Climate-Caused Environmental Variation

Abstract

We studied effects of spatiotemporally variable natural selection on local adaptation. A butterfly (Euphydryas editha) metapopulation contained patches of two types: undisturbed outcrops and logged clearings (where the insects' traditional host had been killed). Butterflies retained their traditional host in the outcrops, but in the clearings they used a novel host that was abundant in both patch types. We estimated the strength of natural selection on host use within and among these patch types. We also repeatedly measured spatial variation of oviposition preference, a heritable univariate trait. The causes of this variation were different at three scales. First, a genetic difference in preference between insects in an adjacent clearing and outcrop (200 m apart) was due principally to nonrandom gene flow. Insects actively assorted themselves between the patches according to their preference genotypes. Second, preferences in outcrop patches 1-4 km apart were significantly associated with their degree of isolation from clearings. Emigration from clearings had diluted local adaptation in outcrops. Finally, differences in preference between disturbed and undisturbed metapopulations 8.5 km apart were due to differences between them in natural selection. Within the disturbed metapopulation, the preference difference between a clearing and an adjacent outcrop remained constant over time even while massive temporal changes in mean preference occurred across both patches in response to fluctuating natural selection on host use. A frost in 1992 extinguished all the clearing populations, and use of the novel host ceased. However, preferences in an outcrop patch continued to differ from those in the undisturbed metapopulation with the same diet, a legacy of past gene flow from clearing to outcrop.