Ecological niche differentiation mediates near complete premating reproductive isolation within the Gladiolus carneus...

Ecological niche differentiation is well associated with intraspecific divergence of functional traits, which may lead to the evolution of premating reproductive isolation. However, the link between the ecological niches, trait divergence and premating isolation remains poorly understood. This is particularly pertinent in hyperdiverse areas, such as the Cape Floristic Region of South Africa, where fine-scale ecological heterogeneity has been hypothesized as a major driver of speciation. Using the polymorphic geophyte Gladiolus carneus, endemic to the Cape Floristic Region, we test whether ecological niche differentiation mediates premating reproductive isolation.

We first tested whether putative ecotypes of G. carneus were distinct based on their floral and vegetative morphology. Next, we documented the abiotic niche, flowering phenology and pollination niche of each putative ecotype and tested whether any resulting niche differentiation causes premating reproductive isolation.

Seven distinct ecotypes were identified. Using niche modelling and multivariate analyses, we found that these ecotypes occupied distinct abiotic niches, resulting in strong ecogeographic isolation. They also had distinct flowering times, causing varying strengths of phenological isolation. For the pollinator niche, we found that all sampled populations were pollinated by one of three highly effective functional pollinators; however, at the ecotypic level there were no consistent trends, leading to varying strengths in pollinator-mediated isolation. Across all ecotypes, ecogeographic isolation was the strongest barrier to gene flow, which, combined with phenological and pollinator-mediated isolation, caused near complete premating reproductive isolation.

These results suggest that ecological niche differentiation between G. carneus ecotypes might be contributing to incipient speciation within the species complex and further suggest that ecological niche differentiation may be a major driver of speciation in the hyperdiverse Cape Floristic Region.