In two out of the three lakes with sympatric dwarf and normal smelt populations the life-history types were characterized by distinct mtDNA composite genotype frequency distributions (fig.
Although the frequency distributions of mtDNA genotypes were highly distinct between dwarf and normal smelt in Lake Utopia, the life-history types clustered together in terms of net sequence divergence (fig.
Although morphological variation among smelt populations has been studied previously (Copeman 1977; Frechet et al.
Although we have no direct evidence of a genetic basis for the morphological and meristic differences that we have described, gill raker number remains constant in smelt populations transplanted between contrasting lake environments (Copeman and McAllister 1978; Lanteigne and McAllister 1983).
3) and their associated ecological distinctions led a few taxonomists to designate some dwarf smelt populations with high gill raker count, including the Green Lake and Lake Utopia dwarf populations of our study, as the pygmy smelt, Osmerus spectrum, distinct from the more widespread rainbow smelt, O.
Taken together, our data strongly suggest that smelt life-history types are polyphyletic and that there have been multiple, independent episodes of divergence of dwarf and normal lake smelt from anadromous fish and from one another throughout our study area (cf.
In two of our study lakes, Lake Utopia and Lochaber Lake, sympatric dwarf and normal smelt had distinct mtDNA genotype frequency distributions (fig.
In Lake Utopia, however, normal smelt apparently are more similar genetically to populations classified as O.
The smelt populations of Lake Utopia and Lochaber Lake are part of a growing list of sympatric reproductively isolated populations in several north temperate freshwater fishes, for example, in Gasterosteus (McPhail 1984), Coregonus (Fenderson 1964; Kirkpatrick and Selander 1979; Bernatchez and Dodson 1990a), Salmo (brown trout, Ferguson and Taggart 1991; Ryman et al.
Genetically distinct sympatric smelt populations may have arisen by allopatric divergence and secondary contact, or they may have diverged sympatrically within each lake.
Second, although the identification of two phylogenetic groups of mtDNA in our study may suggest that smelt survived the Wisconsinian glaciation in two refugia (cf.
The patterns of mtDNA variation, differences in spawning time and place, and divergence in morphological traits important in feeding ecology and competition between sympatric smelt populations that we have documented are consistent with aspects of some proposed models of sympatric speciation (Rosenzweig 1978; Harrison 1991) and, in particular, with models of intralacustrine speciation in "species flocks" of Laurentian Great Lake coregonids and African Great Lake cichlids (Smith and Todd 1984; Dominey 1984; Meyer et al.
(1991) recently examined mtDNA variation in rainbow smelt from Quebec and the northwestern Gulf of St.