Parapatric lake and stream sticklebacks on northern Vancouver Island: disjunct distribution or parallel evolution?

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Abstract
Morphological analysis of sticklebacks from Misty Lake and its inlet stream (Keogh River system, northern Vancouver Island) reveals two forms of Gasterosteus. Sticklebacks from the lake are large and slender bodied with a black back and sides, while sticklebacks from the inlet stream are smaller with a stockier body and mottled brown back and sides. Lake fish also have shorter jaws, longer spines, and more gill rakers than the stream fish. These differences in body shape, trophic structures, and colour are inherited, and the two forms breed true in the laboratory. This implies that they are independent gene pools rather than a single gene pool containing some complex polymorphism. The pair from Misty lake and stream is compared with ecologically similar pairs from Graham Island in the Queen Charlotte Archipelago. The lake–stream pairs on the two islands are strikingly similar. Two hypotheses are examined that might explain the origins and remarkable resemblance of these widely disjunct lake–stream pairs. One hypothesis is historically based and argues that the lake–stream pairs on northern Vancouver Island and the Queen Charlotte Islands share a common origin. The other hypothesis argues that the similarities between the lake–stream pairs on the two islands are the result of parallel evolution (i.e., independent evolution of similar phenotypes in response to similar selection regimes). The available data do not allow a distinction to be made between the hypotheses. The historical hypothesis, however, argues for a divergence that predates the Fraser glaciation, whereas the parallel-evolution hypothesis requires postglacial divergence. Since the two hypotheses differ in the amount of time available for divergence, it may be possible to distinguish between the hypotheses at the molecular level.

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