Reinforcement between incipient outcrossing and selfing Clarkia species
Studies of reproductive isolation often find that prezygotic barriers evolve more rapidly than postzygotic barriers between incipient species. However, it has been challenging to determine whether selection has directly caused elevated isolation (reinforcement) or whether is has occurred as a by-product of adaptation to alternative environments. In Clarkia xantiana, there is a pronounced pattern of reproductive character displacement (RCD) between incipient plant species that remain cross-compatible, a key signature of reinforcement. The self-fertilizing taxon is recently derived from the primarily outcrossing taxon (~65,000 yrs) and phylogeographic studies show that they have come into secondary sympatry in a narrow zone. Here, we test whether reinforcement selection has directly caused the evolution of RCD in floral traits using a series of large field experiments.
Our results indicate that hybridization between incipient species is strongly reduced between sympatric genotypes but is approximately twice as likely when allopatric genotypes of the two taxa are paired, consistent with the reinforcement hypothesis. By contrast, we found no evidence that floral evolution has occurred as by-product of adaptation to contrasting pollination environments in the allopatric and sympatric regions. These results are novel in demonstrating a contribution of both reinforcement selection and mating system divergence to the speciation process.