Altitudinal differentiation between adjacent populations of a ground beetle, Elaphrus americanus Dejean (Coleoptera: Carabidae: Elaphrini)

Samples of Elaphrus americanus from five mountains in the Pacific Northwest were compared for variation in two enzymes (an esterase and an aldehyde oxidase) and a body colour polymorphism. There were significant differences in all three characters between subalpine parkland and temperate forest samples from Mount Rainier and Mount Baker (Washington) and Tusk Mountain (British Columbia). There were no differences between subalpine and temperate forest samples from Mount Adams (Washington) and Mount Hood (Oregon). Subalpine parkland samples from Baker differed from Tusk parkland samples in one character and from Rainier parkland samples in two characters. Differences between forest samples were slight. Large differences over short distances were not expected because dispersal between sites in forest areas is frequent. Parkland populations on Rainier, Baker, and Tusk mountains may be reproductively isolated from adjacent forest populations and could constitute disjunct populations of a sibling species that was widespread in the Pleistocene. Alternatively, subalpine parkland and forest populations may not be reproductively isolated from one another and differences between them are being maintained by selection. Parkland populations on one or more mountains may have diverged independently of the others. Beetles found in parkland areas on Hood and Adams mountains probably dispersed from forest sites.