Drivers of phenotypic differentiation across rock outcrop sky islands in Impatiens plants

Abstract

Habitat specialization can lead to naturally fragmented populations separated by unoccupied habitats that comprise potential barriers to dispersal. Habitat fragmentation can result in phenotypic divergence between populations, and eventual speciation. We investigate trait divergence in a habitat specialist plant species - Impatiens lawii - that is specialized on rock outcrops in the Western Ghats mountains of India. We compare trait divergence in this species with that in a closely related species, I. oppositifolia, which is a continuously distributed habitat generalist that co-occurs with the former species on rock outcrops in addition to being found in intervening regions. As predicted, we found that floral traits were more strongly structured across populations in I. lawii than in I. oppositifolia, supporting the role of habitat specialization. In both species, floral traits were more strongly structured across populations than were vegetative traits. Interestingly, the population pairs that were significantly differentiated for floral traits differed from those differentiated for vegetative traits. Furthermore, some population pairs differed in vegetative traits but not in floral traits. The incongruence in patterns of floral and vegetative trait differentiation suggests the role of selection. There was no association between floral trait variation and geographic distance in either species. Surprisingly, there was a significant correlation between geographic distance and vegetative trait variation in I. oppositifolia, but not in I. lawii (Fig 4D; Table 2), possibly due to genetic drift. Overall, a combination of habitat specificity, drift and selection best explains trait variation in I. lawii.