Populations of the common mussel (Mytilus edulis) from the North Sea area (Skakerrak-Kattegat) and those from the Baltic Sea are almost diagnostically differentiated at five out of 22 studied allozyme loci; at a further seven loci, alleles predominant or common in one area are nearly absent in the other. Genetic distance was estimated at 0.28; this is similar to the distances of these populations to the Mediterranean mussel M. galloprovincialis. The three mussel types obviously represent equal evolutionary divergence from one another, and should also be taxonomically equally separated; a semispecies rank within a more comprehensive M. edulis complex or superspecies is suggested. The age of the Baltic mussel type (‘M. trossulus’), as an independent evolutionary lineage, is probably far greater than that of the post-glacial Baltic Sea.
Allele frequencies change gradually and in parallel when entering from the Kattegat through the Sound into the Baltic. Only a slight Wahlund effect at the strongly diverged Gpi and Pgm loci was found in intermediate populations, indicating that extensive hybridization of the two taxa takes place in the area. However, strong interlocus genotypic associations suggest that selection against hybrids is intense in later generations; the c. 100 km wide hybrid zone is narrow relative to the dispersal distance. The genotypic structure of the Lap locus does not conform with those of the other loci studied in the hybrid zone; it cannot be viewed merely as a neutral marker of the process of hybridization.