The role of selection and gene flow in the evolution of sexual isolation in Timema walking sticks and other Orthopteroids

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The formation of new species involves the evolution of barriers to gene exchange. One such barrier is sexual isolation, where divergent mate preferences prevent copulation between taxa. Sexual isolation can evolve via a number of processes, including natural selection, sexual selection, genetic drift, and reinforcing selection to avoid maladaptive hybridization. Conversely, gene flow between populations generally erodes the evolution of sexual isolation. In Timema cristinae walking stick insects, some sexual isolation between populations evolved through ecological divergence in host-plant use, implicating a role for natural selection. However, reinforcement and gene flow also contribute, such that sexual isolation is strongest when migration between hosts is high enough to result in selection to avoid hybridization, but low enough to prevent gene flow from eroding adaptive divergence in mate choice. Both parallel and contrasting patterns can be seen in other Orthopteroids. This variation among groups might reflect 1) the degree to which different groups are intimately associated with their food resources, 2) the types of traits used in mate choice, and 3) how the geographic arrangement of populations affects the opportunity for reinforcement.