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UBC Theses and Dissertations

The evolutionary ecology of hybridization Thompson, Ken A.

Abstract

Hybridization is the process that mediates gene flow between sexually reproducing lineages. As such, the factors that determine the fitness of hybrids are critically important for speciation. Although hybridization between natural populations occurs in an explicitly ecological context, where hybrids must compete for resources and attract mates under prevailing ecological conditions, little attention has been paid to the mechanisms that mediate the ecological performance of hybrids. In Chapter 2, I expand upon existing models of speciation and find that standing variation improves hybrid fitness when parents adapt to similar environments, but reduces hybrid fitness when parents adapt to different environments. Chapter 3 systematically reviews the literature to report that F1 hybrids are often quite mismatched for divergent parental traits, and also uses a field experiment with sunflowers to show that this mismatch reduces fitness via seed count. Chapter 4 also uses data from the literature to demonstrate that divergent adaptation proceeds via pleiotropic alleles. In Chapter 5, I measure morphological traits in the lab to illustrate that the magnitude of trait `mismatch' in hybrids increases with the phenotypic distance between cross parents. Chapter 6 uses a pond experiment hybridizing 'parallel' ecotypes and reports that hybridization after parallel evolution results both in heterosis and hybrid breakdown. Finally, Chapter 7 uses DNA sequence data to illustrate that the genetic signature of hybrid incompatibilities - excess heterozygosity - is greater when stickleback hybrids are raised in ponds than when they are raised in aquaria. In sum, my thesis demonstrates that ecological selection against hybrids can result from rapid adaptation from standing variation, from mismatched trait combinations that evolve somewhat predictably, and can have a detectable genetic signature. Hybridization after parallel evolution, at least in stickleback, leads to relatively equal measures of hybrid breakdown and heterosis. Ecologically-mediated natural and sexual selection can clearly play a large role in mediating the fitness of hybrids - future work should aim to establish the importance of these processes for speciation more broadly.

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