Divergent host-plant adaptation and the evolution of reproductive isolation

Understanding the process of speciation requires elucidating the processes driving and constraining the evolution of reproductive isolation. For example, reproductive isolation can evolve simply as a by-product of populations adapting to different ecological environments. This process of ‘ecological speciation’ predicts greater levels of reproductive isolation between ecologically-divergent pairs of populations than between ecologically-similar pairs of similar age. The evolution of reproductive isolation can also be promoted by selection against hybrids (reinforcement) and can be constrained by the homogenizing effects of gene flow. This the sis examines the role of selection and gene flow in the evolution of reproductive isolation among host-associated populations of Timema cristinae walking-stick insects. Populations living on different host-plant species (Ceanothus versus Adenostoma) exhibit genetically-based, adaptive divergence in a suite of traits, including color, color-pattern, body size, body shape and behavior. Multiple forms of reproductive isolation were greater between populations using different hosts than between similar-aged populations using the same host. This pattern was detected for habitat isolation, immigrant inviability, sexual isolation, and cryptic postmating isolation, indicating that divergent host-plant adaptation promoted the evolution of multiple reproductive barriers. Conversely, gene flow between populations tended to erode divergence, with the exception of sexual isolation where moderate levels of gene flow promoted reinforcement. Molecular and morphological evidence suggest that the host-associated forms of T. cristinae are unlikely to have achieved species status such that the host forms represent either an ongoing speciation event or population divergence that has reached equilibrium. Studies of more divergent taxa in the genus are required to build up a more complete understanding of how the process of speciation unfolds, from beginning to end.