GENETIC EVIDENCE FOR NATURALLY OCCURRING HYBRIDS BETWEEN MYTILUS EDULIS AND MYTILUS GALLOPROVINCIALIS

An understanding of the process of speciation is a major goal in evolutionary genetics. Discussion has centered on the nature of the selective forces and the amount of genetic differentiation involved; and a long standing controversy has concerned the extent to which geographic isolation, and therefore prevention of gene flow between populations, is necessary in the early stages of speciation. A good way to assemble data bearing on these problems is to study closely related or incipient species where hybridization and possibly gene exchange is occurring between related but different gene pools. Among animals, hybridization is relatively rare in mammals, birds, and reptiles but more common in amphibians, and is most common in fishes with external fertilization (Hubbs, 1955, 1961; Lagler, Bardach and Miller, 1962). Introgression, involving the incorporation of genes from one species into the gene pool of another is even rarer; hybrids produced in nature are usually thought to have low fitness (Mayr, 1963). Much older work on hybridization was based on morphological or cytological analysis carried out before the development of gel electrophoresis. In the last decade electrophoretic evidence of the extent of variation at loci coding lor soluble enzyme proteins has accumulated, and it is now evident that closely related species may show allele frequency differences at up to 50% of these loci. Much recent work is reviewed by Ayala (1975) and Avise (1976).