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Genetic Variation for Habitat Preference: Evidence and Explanations

Because adaptive shifts may often be initiated by evolutionary changes in behavior, it is of interest to determine the extent to which natural populations harbor genetic variation for ecologically important behaviors. Habitat preference is an especially significant behavior, because it determines the regime of natural selection acting on loci that affect adaptation to the environment. A survey of the literature reveals that genetic variation for habitat selection is common, especially in arthropods and mollusks, the groups that have been studied most frequently. Possible adaptive mechanisms by which this variation could be maintained within populations include a genetic correlation between density-independent fitness in a habitat and a preference for it; and soft selection, whereby density-dependent population regulation occurs independently in separate habitats. Several studies have documented a phenotypic correlation between preference and performance, but as yet, no such genetic correlations have been unequivocally demonstrated. We show theoretically that under hard selection, optimal habitat selection may often lower the probability of maintaining a polymorphism at a locus that affects adaptation to different habitats. Soft selection appears much more likely to promote variation for habitat preference. Mechanisms including resource competition and natural enemies whose numbers build up in a habitat-specific manner in response to host or prey density have the capacity to bring about selection favoring alleles whose carriers prefer relatively underused habitats. We believe that more progress in understanding the evolution of habitat preference will come from studies of these ecological mechanisms than from further demonstrations of the mere existence of genetic variation for such preferences.