Hybridization as a stimulus for adaptation to a novel environment

Understanding processes contributing to the origin of novelty, including ecological transitions in resource or habitat use, is fundamental to evolutionary biology. Early geneticists speculated about the sudden appearance of new species via special macromutations, epitomized by Goldschmidt’s infamous “hopeful monster”. Transgressive segregation during hybridization is a more plausible mechanism for producing “monstrous” phenotypes beyond the range of parental populations. Transgressive hybrid phenotypes can be products of epistatic interactions or additive effects of multiple recombined loci. However, the importance of hybridization in the origin of novelty is contested because we do not know how often hybridization enhances the probability of an evolutionary transition. In Chapter 1 we compare several epistatic and additive models of transgressive segregation in hybrids and find that they are special cases of a general, classic quantitative genetic model. In Chapters 2 and 3 we take an empirical approach to determine whether hybridization consistently facilitates adaptation to a novel environment by selecting 36 different hybrid crosses among 12 distinct lineages of the red flour beetle (Tribolium castaneum) for performance on soy medium. In Chapter 2 we show that hybrid populations adapted to a challenging new environment more rapidly than non-recombinant populations. During 11 generations on soy medium, beetle populations evolved reduced density-dependence, resulting in greater population growth and steady state population size. Change occurred over several generations, and cannot be explained by simple F1 hybrid vigor. Instead, gradual (but rapid) evolutionary change in the ability to thrive in soy was manifested as altered population ecology. In Chapter 3 we show that the developmental rates of hybrid lines increased significantly while non-recombinant lines’ developmental rates increased only slightly. Evolution of accelerated developmental rate was not correlated with the evolution of decreased larval density-dependence described in Chapter 2. During the ecological transition to soy, hybridization facilitated adaptation along multiple dimensions, manifested separately at the population and individual levels.