In July 1999, 49 three-spined sticklebacks were sampled from Enos Lake, Canada. Here a benthic/limnetic species pair is known to exist. In the sample, six individuals (12) were of intermediate ...phenotype. These represent 17 of the 35 males in the sample. This large number of intermediates contrasts with the 1 in previous samples reported in 1984 and 1992 and may signal the collapse of the species pair. Copyright 2001 The Fisheries Society of the British Isles
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Adaptive radiations are a major source of evolutionary diversity in nature, and understanding how they originate and how organisms diversify during the early stages of adaptive radiation is a major ...problem in evolutionary biology. The relationship between habitat type and body shape variation was investigated in a postglacial radiation of threespine stickleback in the upper Fish Creek drainage of Cook Inlet, Alaska. Although small, the upper Fish Creek drainage includes ecologically diverse lakes and streams in close proximity to one another that harbour abundant stickleback. Specimens from ancestral anadromous and derived resident freshwater populations differed substantially and could be distinguished by body shape alone, suggesting that the initial stages of adaptation contribute disproportionately to evolutionary divergence. Body shape divergence among resident freshwater populations was also considerable, and phenotypic distances among samples from freshwater populations were associated with habitat type but not geographical distance. As expected, stream stickleback from slow-moving, structurally complex environments tended to have the deepest bodies, stickleback from lakes with a mostly benthic habitat were similar but less extreme, and stickleback from lakes with a mostly limnetic habitat were the most shallow-bodied, elongate fish. Beyond adapting rapidly to conditions in freshwater environments, stickleback can diversify rapidly over small geographical scales in freshwater systems despite opportunities for gene flow. This study highlights the importance of ecological heterogeneity over small geographical scales for evolutionary diversification during the early stages of adaptive radiation, and lays the foundation for future research on this ecologically diverse, postglacial system.
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