MITOCHONDRIAL DNA IN GASTEROSTEUS AND PLEISTOCENE GLACIAL REFUGIUM ON THE QUEEN CHARLOTTE ISLANDS, BRITISH COLUMBIA

Freshwater populations of threespine stickleback (Gasterosteus aculeatus) along the coast of western North America exhibit extensive morphological divergence from marine conspecifics, the presumed ancestors to freshwater populations (Hagen and McPhail, 1970; Bell, 1976). Marine sticklebacks are characterized by a complete series of bony lateral plates (30 to 35 per side) and prominent dorsal and pelvic spines. Typically, freshwater sticklebacks resemble the marine form, but can be distinguished by a deeper, slightly smaller body, and fewer lateral plates (usually three to eight per side). On the Queen Charlotte Islands (QCI), an archipelago 80 kilometers off the coast of British Columbia, Canada, freshwater populations of stickleback have been found that have highly derived morphological characters. These include gigantism in body size, complete loss of lateral plates and loss of dorsal and pelvic spines (Moodie, 1972; Moodie and Reimchen, 1976a; Reimchen, 1980, 1984; Reimchen et al., 1985). Populations with modest divergence in one or more of these traits occur elsewhere in the circumboreal distribution of the species including California (Miller and Hubbs, 1969), Alaska (Francis et al., 1986) and the Outer Hebrides (Campbell, 1979). Geological evidence suggests that much of the Pacific coast of North America, north of the coterminous United States, was under the Cordilleran Ice Sheet during the last (Fraser) glaciation (Flint, 1971; Clague, 1989a). Since ice sheets extended well beyond present day shorelines, almost to the edge of the continental shelf (Dyke and Prest, 1987; Blaise et al., 1990), coastal islands, including the QCI, were presumed to be devoid of significant Pleistocene biological refugia (Sutherland-Brown, 1968; Sutherland-Brown and Nasmith, 1962). Morphological differentiation between marine sticklebacks and some of the highly distinctive freshwater forms was therefore ascribed to rapid post-glacial evolutionary divergence (Moodie and Reimchen, 1976b). A suite of unique biological features on the QCI have led to suggestions that parts of the islands were ice free during the last glacial advance (Calder and Taylor, 1968). Endemism occurs in QCI birds and mammals (McCabe and Cowan, 1945; Foster, 1965), crustaceans