ECOLOGY AND EVOLUTION OF SYMPATRIC STICKLEBACKS (GASTEROSTEUS) - ORIGIN OF THE SPECIES PAIRS

Pairs of plankton-feeding and benthos-foraging sticklebacks (Gasterosteus aculeatus) (limnetics and benthics) coexist in six small lakes in southwestern British Columbia. In spite of a persistent low level of hybridization the pairs maintain themselves as distinct genetic and ecological entities; because of this they are viewed as biological species. They appear to be restricted to the central Strait of Georgia region, and the geological history of the area argues that they are of postglacial origin. The pairs occur on three islands (Vancouver, Texada, and Lasqueti), but even on these islands they are not found in all available lakes. Both lake morphometry (size and depth) and altitude appear to influence local distribution. Hypotheses that might explain the origin of the pairs are examined. Neither theory nor data support a sympatric origin, but an allopatric origin through two invasions of marine sticklebacks is consistent with both the geological history of the area and the details of local distribution. An independent but parallel evolution of the pairs on at least two islands (Vancouver and Texada) is implicit in this hypothesis. The hypothesis also assumes that although the initial divergence started in allopatry, competitive interactions in sympatry played a major role in the evolution of reproductive isolation and resource partitioning.