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When the habitat occupied by a specialist species is patchily distributed, limited gene flow between the fragmented populations may allow population differentiation and eventual speciation. 'Sky islands'-montane habitats that form terrestrial islands-have been shown to promote diversification in many taxa through this mechanism. We investigate floral variation in Impatiens lawii, a plant specialized on laterite rich rocky plateaus that form sky islands in the northern Western Ghats mountains of India. We focus on three plateaus separated from each other by ca. 7 to 17 km, and show that floral traits have diverged strongly between these populations. In contrast, floral traits have not diverged in the congeneric I. oppositifolia, which co-occurs with I. lawii in the plateaus, but is a habitat generalist that is also found in the intervening valleys. We conducted common garden experiments to test whether the differences in I. lawii are due to genetic differentiation or phenotypic plasticity. There were strong differences in floral morphology between experimental plants sourced from the three populations, and the relative divergences between population pairs mirrored that seen in the wild, indicating that the populations are genetically differentiated. Common garden experiments confirmed that there was no differentiation in I. oppositifolia. Field floral visitation surveys indicated that the observed differences in floral traits have consequences for I. lawii populations, by reducing the number of visitors and changing the relative abundance of different floral visitor groups. Our results highlight the role of habitat specialization in diversification, and corroborates the importance of sky islands as centres of diversification., Competing Interests: The authors have declared that no competing interests exist.

The northern section of the Western Ghats is a biodiversity hotspot with a predominance of rock outcrops. Although these outcrops are characterized by environmental extremes, they support a large diversity of species, many of which are endemics. High-elevation rock outcrops of the Western Ghats exhibit two lithotypes of distinct geological origins, namely basalt mesa (BM) and high-level ferricrete (HLF). We examined the interrelation between environmental factors, lithotypes and functional type abundance. We recorded 265,447 individuals belonging to 127 taxa categorized under seven plant functional types in 128 quadrats from sixteen high-elevation rock outcrops. Cluster analysis separated the sites based on the lithotypes. Graminoids and Therophytes were the dominant functional types across both the lithotypes. Latitude, seasonality, soil carbon and nitrogen were the influential environmental factors. Multivariate analysis of variance (MANOVA) showed that based on environmental characteristics, the two lithotypes are significantly different from each other. Different plant functional types were associated with different environmental variables, as shown by canonical correspondence analysis (CCA). Lithotype and levels of soil carbon and nitrogen together seem to shape the functional type abundance. Overall, we observed that there is a marginal difference in the vegetation across the two lithotypes of outcrops. The abundance of each plant functional type was deferentially associated with the seasonal environment and soil nutrients. [ABSTRACT FROM AUTHOR]