Parallel variation at an enzyme locus in sibling species of field crickets

Abstract

ELECTROPHORETIC surveys of levels of heterozygosity in animal species have established that populations often contain large amounts of genetic variation at loci coding for soluble proteins^1–3. This has led to an extended debate about the causes of such variation. In particular, two opposing views have emerged, one emphasising the importance of balancing selection in maintaining the protein variation (selectionist) and the other emphasising the importance of stochastic processes (neutralist)^4–6. Here I examine spatial patterns of variation at a single enzyme locus in two closely related insect species. I argue that since these sympatric species not only have similar mean frequencies of particular electrophoretic variants, but also exhibit a common pattern of geographic variation, parallel selection provides the only satisfactory explanation.