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Ecological mechanisms in species origins : divergent natural selection and the evolution of reproductive isolation between sympatric sticklebacks Rundle, Howard Douglas

Abstract

Under the ecological model of speciation, reproductive isolation arises ultimately as a result of divergent natural selection. Laboratory experiments have demonstrated the feasibility of this mechanism but evidence from nature is lacking. This thesis investigates the role of ecological mechanisms in the origin of morphologically and ecologically distinct sympatric species-pairs of threespine sticklebacks (Gasterosteus aculeatus complex) that inhabit a few post-glacial lakes in southwestern British Columbia. Ecological speciation predicts that the fitness of hybrids should depend on their ecological context. This can be tested using reciprocal transplant experiments, provided the contribution of any genetic incompatibilities is controlled. I develop a quantitative genetic model to determine whether this is possible. Analysis of the model reveals that it is, and the experimental design involves the use of both hybrid backcrosses. I employ this design in a reciprocal transplant experiment in nature using the stickleback species-pairs. Results reveal a striking pattern of ecological-dependence: in each habitat the backcross more similar to the native parent species grew at approximately twice the rate of the other. Thus ecological mechanisms have been central to the evolution of postmating isolation between these stickleback species. Ecological speciation also predicts that reproductive isolation among populations should be correlated with environmental differences. To test this prediction I conducted laboratory mating trials using the stickleback species-pairs from three lakes. Results support the prediction, demonstrating that populations that evolved under different ecological conditions show strong premating isolation, whereas populations that independently evolved under similar ecological conditions lack isolation. Such parallel evolution of premating isolation, in correlation with environmental differences, strongly implicates divergent selection in the evolution of these species-pairs. Finally, I address the mechanisms responsible for divergent natural selection in the evolution of these species-pairs, focussing on the role of ecological interactions between populations. I use a pond experiment to test the combined effect of predation and competition on the strength of divergent selection.on a target population. Results suggest a strong interaction: the competition treatment generated divergent selection on the target population only under high predation conditions. This suggests that the ecological mechanisms responsible for the evolution of the species-pairs include both competition and predation.

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