Genetic variation at the alcohol dehydrogenase locus in natural populations of the seaweed fly, Coelopa frigida.

Samples of the seaweed fly, Coelopa frigida, from six populations widely spaced around the English coast were collected during 1974-76 and again in 1979-80. Gene frequencies at the Alcohol dehydrogenase (Adh) locus were found not to vary significantly between sites, nor over the five year period of their study. Genotype distributions are compared with Hardy-Weinberg expectations and show consistent, mostly significant, differences. These results are interpreted in the light of an association between the Adh locus and a chromosomal inversion polymorphism. It is argued that the remarkable geographical uniformity must indicate the presence of strong selection, probably taking the form of heterokaryotypic advantage.