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Reproductive isolation between Dolly Varden (salvelinus malma) and bull trout (s. confluentus) in sympatry : the role of ecological factors Hagen, John


Experimental evidence and theoretical arguments support the notion that ecological factors can be important in divergence and speciation. Empirical evidence suggests that ecology plays a role in divergence, but field studies addressing the importance of ecology in the evolution of reproductive isolation are few. Here I suggest that divergent natural selection is contributing to reproductive isolation between the closely related salmonid fish species Dolly Varden {Salvelinus malma) and bull trout (S. confluentus) in areas of their sympatry. In the Thutade watershed of northcentral British Columbia, hybridization occurs between these species naturally and produces viable, fertile offspring, suggesting the sympatry is not being maintained by genetic incompatibility. Both forms are found in the tributary streams, but resource use differences are subtle and insufficient to reduce the intensity of interspecific competition, suggesting no obvious basis, in this habitat, for natural selection against phenotypically intermediate hybrids. Life history differences, however, are obvious. Bull trout undergo a niche shift that the obligatorily streamresident Dolly Varden do not, migrating to Thutade Lake and becoming piscivorous. The substantial level of pre-mating isolation is probably caused by the large differences in body size (and correlated spawning behaviours) that result from the life history differences, while selection against intermediate, hybrid phenotypes can also be reasonably expected due to the completely different physical and biological environments in lake and stream habitats. Two potential ecological mechanisms for the life history differences are supported: i) divergent natural selection for alternative resource environments, which is suggested by morphological adaptations by each species to different-sized prey, and ii) divergent natural selection to reduce inter-specific competition, which is suggested by evidence for character displacement in life history.

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