PROTEIN POLYMORPHISM OF THE HYBRIDIZING SEASTARSASTERIAS FORBESIANDASTERIAS VULGARISAND IMPLICATIONS FOR THEIR EVOLUTION

Abstract

The seastars Asterias forbesi and A. vulgaris share 67% of their genes in common (based on 27 loci). These species are normally easily characterized by 7 prominent phenotypic differences but naturally occurring hybrids are found in localities with typical adults of the two species. A. forebesi and A. vulgaris are thought to have evolved during the mid to late Pleistocene as a result of a restriction in the range of a more widely distributed Miocene or early Pleistocene form due to lowering of sea level and the coincident emergence of a disrupting land barrier (Cape Cod-Georges Bank). At least one local population of the ancestral species evolved into the present cold water form (A. vulgaris) during selection in an arctic-fed Gulf of Maine. Coincidently, at least one other local population of the ancestral species evolved with selection in warmer, southern waters into the present shallow water, temperate form (A. forbesi). If A. forbesi and A. vulgaris have been derived from a late Tertiary wide-ranging species which underwent geographic partitioning during the late Pleistocene, then both their present distribution and their lack of total reproductive isolation are understandable.