The Species Problem within Gasterosteus aculeatus on the Pacific Coast of North America

Two kinds of Gasterosteus aculeatus, leiurus and trachurus, are considered species in several streams and lakes in British Columbia. The two sticklebacks may act as species over a much wider range, but at present there is insufficient evidence for firm conclusions. The data of Miller and Hubbs (Copeia 1969: 52–69, 1969) given as evidence for introgression between leiurus and trachurus on the Pacific coast cannot be taken as well established. Should conclusive data for introgression be forthcoming, it would then seem that the two sticklebacks act as species in places and as subspecies in others. At least along the Pacific coast the pattern of variation found within Gasterosteus is exceedingly complex and cannot be accounted for wholly upon any simple explanation in terms of hybridization between leiurus and trachurus. There is evidence that much of the interpopulation variation is a result of adaptation and natural selection — not introgression. It will in fact require much more investigation before we can proclaim the causes of the variation we see so strikingly in Gasterosteus.