When closely related species overlap geographically, a change of character state may occur in one species which is directly attributable to the presence in the same environment of an ecologically similar species1,2. Most reports of this phenomenon—called character displacement—have been concerned with divergence, rather than convergence, in morphologial characters between species in the zone of sympatry. There has been only one report on genetic displacement at the allelic level3. In this, Murphy observed a divergence in allele frequency at a leucine aminopeptidase locus in sympatric species of marine limpets which he interpreted as allelic accommodation in response to interspecific competition3. I report here a study of two closely related species of cockles, Cerastoderma edule and Cerastoderma glaucum, in which C. glaucum exhibits a convergence in allele frequency at the phosphoglucomutase (Pgm) locus when it is found intermixed with C. edule. |
