Ecological Character Displacement and Speciation in Sticklebacks

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Abstract
We present evidence of ecological character displacement among species of threespined sticklebacks that inhabit small lakes of coastal British Columbia. Geological data suggest that the populations resulted from multiple divergence and speciation events over the past 13,000 yr. In lakes with two species, one is invariably "limnetic" in morphology and habitat, and the other is "benthic." Other lakes contain a single form morphologically intermediate between sympatric species and exploiting both habitat's. Within solitary populations individuals exploit one habitat or the other according to their morphology, which underscores the divergent selection pressures operating on either side of the habitat boundary. Our results satisfy at least four of the six major criteria for demonstrating the occurrence of character displacement: (1) a statistical null model of no displacement is easily rejected; (2) variation between and within populations is genetically based; (3) differences in sympatry reflect evolutionary shifts, not merely the biased extinction of similar forms; and (4) morphology is closely linked to resource use. Minimal resource differences between one- and two-species lakes and the presence of competition between similar phenotypes are still to be established. Our data suggest that competition for food has played a critical role in the divergence between species. We consider the possibility that it also helped in species formation, which would expand its potential role in adaptive radiation.

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