POPULATION STRUCTURE OF FRESHWATER FISHES I. GENETIC VARIATION OF BLUEGILL (LEPOMIS MACROCHIRUS) POPULATIONS IN MAN-MADE RESERVOIRS.

Abstract

A recent resurgence of interest in the of animal and plant populations is attributable to the strong theoretical foundations earlier laid by Wright, Malecot, Nei, Kimura, and others, coupled with the advent of empirical techniques (primarily electrophoresis) for monitoring structure through time and space. Any subdivision due to departures from panmixia may be termed population structuring. Deviations from panmixia may result from inbreeding, selection, or assortative mating due to behaviorally or environmentally imposed restrictions on migration or mating success. Significant structuring of local populations resulting from one or more of these factors has been demonstrated in herbaceous plants (Hamrick and Allard, 1972; Levin, 1975; Schaal, 1975; Schaal and Levin, 1976) and in a variety of animals ranging from snails (Selander and Kaufman, 1975) to insects (McKechnie et al., 1975; Taylor and Powell, 1977) to mammals (Kidd and CavalliSforza, 1974; Selander, 1970), including extensive works on man (Neel and Ward, 1972; Nei and Imaizumi, 1966; Workman and Niswander, 1970). Nonetheless, the relative importance of potential extrinsic barriers to migration in determining structure, particularly in species with large numbers of mobile individuals, remains debatable (Ehrlich and Raven, 1969; Mayr, 1970). It seems ironic that a particularly suitable setting for analysis of structure, the freshwater environment, has been almost totally ignored. Darlington (1957) and other zoogeographers clearly recognize analogies between drainage basins and terrestrial islands in effects on dispersal of their respective biotas. Even within drainage basins, well-defined habitats such as lakes are usually interspaced with alternative habitats which may be less favorable to particular species. The patchwork nature of freshwater habitats may have important consequences for the genetic structure and evolution of many species. Retreating Pleistocene glaciers left a pockmarked northern landscape and thousands of natural lakes. In the southern and central United States, beyond the glacial advance, the great majority of standing bodies of water are of man-made origin. Virtually every major river draining into the Atlantic Ocean or Gulf of Mexico has been dammed, often repeatedly, to form reservoirs primarily for purposes of flood control, power production, and recreation. This practice has profoundly influenced the faunal compositions of southern drainages by direct and indirect effects of alterations in the relative proportions of standing and running waters. Furthermore, the dams themselves may be formidable barriers to dispersal of many freshwater organisms. In this study we analyze the microgeographic genetic structure of bluegill sunfish (Le-