Ecology and evolution of sympatric sticklebacks (Gasterosteus): evidence for a species-pair in Paxton Lake, Texada Island, British Columbia

Abstract

Two forms of Gasterosteus coexist in Paxton Lake on Texada Island, British Columbia. One form (benthic) is predominantly a littoral, benthic forager, whereas the other form (limnetic) is an open-water plankton feeder. The benthic form is large (up to 90 mm in standard length), with a stout body, a wide mouth, relatively few short gill rakers, and a greatly reduced number of lateral plates and dorsal spines. The entire pelvic girdle is absent in about 80% of adults. In contrast, the limnetic form is slim bodied, with a narrow mouth, numerous long gill rakers, and normal lateral plates, dorsal spines, and pelvic girdle. In laboratory crosses, Paxton Lake benthic and limnetic sticklebacks breed true and this suggests that they represent separate gene pools rather than a foraging polymorphism. The differences in morphology and allele frequencies between the two forms have remained stable for over two decades (about 20 generations), yet discriminant function analysis indicates that about 1% of the adult sticklebacks in Paxton Lake are hybrids. This estimated level of hybridization has remained constant since 1968. The stable differences in allele frequencies and morphology suggest that there is no gene flow between the two forms. There is, however, some evidence for gene flow in the past, and although the two forms now behave as biological species, they probably retain the capacity to exchange genes. Both the genetic distance between the two forms, estimated from 25 presumptive loci, and the postglacial history of the area suggest that the divergence is recent.