Multiple reproductive barriers maintain species boundaries in stone plants of the genus Argyroderma

Measuring the strength of different reproductive barriers across species pairs is key to reveal the mechanisms that have led to evolutionary radiations. Here we study a genus of miniature plants, Argyroderma, which comprises 11 species restricted to a single plain of the Southern African desert. We measure different reproductive barriers to understand how species boundaries are maintained in this genus. Our results show that reproductive isolation is almost complete between all species pairs and relies on three pre-mating barriers: geographic isolation operating at spatial scales of c. 10 km, phenological isolation in flowering time, and habitat isolation operating at spatial scales of just a few metres, which is thought to be due to contrasting edaphic preferences between species. In comparison, post-mating isolation arising before seed formation is weak and does not restrict gene flow much between species. Interestingly, the high levels of both geographic and habitat isolation that we have measured between Argyroderma species might be due to their miniature size, which leads to restricted gene flow across space and to adaptation to spatially restricted micro-habitats.