Print PageUnderstanding the mechanisms contributing to the evolution of new species is a fundamental question in biology. The process in which reproductive isolation evolves as a consequence of divergent natural selection between environments may play an important role. A direct prediction of this process, termed ‘ecological speciation’, is that ecologically-divergent pairs of populations will exhibit greater levels of reproductive isolation than ecologically-similar pairs of populations. Using replicated comparisons of allopatric populations of the cynipid gall wasp Belonocnema treatae infesting two different host plant species, Quercus virginiana (Qv) and Q. geminata (Qg), we test the role that divergent natural selection plays in generating reproductive isolation. First, we test for differences in body size, gall size, and adult emergence phenology associated with B. treatae populations adapted to Q. virginiana or Q. geminata. Second, we examine the effect of two premating barriers, habitat isolation and sexual isolation, in a fully reciprocal mating experiment in which the native host plant of each individual was included in half of the trials. If premating reproductive isolation is generated by divergent selection to contrasting ecological niches, then the majority of matings should occur between B. treatae populations adapted to similar environments.
Results/Conclusions
Comparisons of multiple B. treatae populations on Q. virginiana and Q. geminata found consistent differences in body size (hind tibial length – males, Qg > Qv, P < 0.01; females, Qg > Qv, P < 0.001), gall size (chamber number - Qg > Qv, P < 0.01), and adult emergence phenology (median julian date: Qv = 101, Qg = 118, P < 0.05). Ecological divergence in these traits may represent important adaptations to each host plant itself or its distinct habitat. In the mating trials, when no host plant was present, partial assortative mating was observed (proportion of matings: Qg x Qg, Qv x Qv = 0.72, Qg x Qv = 0.59, P < 0.01). When host plants were present during mating trials, we found that females spent more time on their native host plant (time on native vs. non-native: Qg females = 0.67, P < 0.05; Qv females = 0.73, P < 0.01) and assortative mating increased (proportion of matings: Qg x Qg, Qv x Qv = 0.84; Qg x Qv = 0.47, P < 0.0001). This increase in assortative mating due to host plant is an explicit test of habitat isolation and its role in reproductive isolation between host-plant-associated populations of B. treatae.
