Geographic isolation and reduced population sizes can lead to local extinction, low efficacy of selection and decreased speciation. However, population differentiation is an essential step of biological diversification. In allopatric speciation, geographically isolated populations differentiate and persist until the evolution of reproductive isolation and ecological divergence completes the speciation process. Pitcairnia flammea allows us to study the evolutionary consequences of habitat fragmentation on naturally disjoint rock-outcrop species from the Brazilian Atlantic Rainforest (BAF). Our main results showed low-to-moderate genetic diversity within populations, and deep population structuring caused by limited gene flow, low connectivity, genetic drift and inbreeding of long-term isolation and persistence of rock-outcrop populations throughout Quaternary climatic oscillations. Bayesian phylogenetic and model-based clustering analyses found no clear northern and southern phylogeographic structure commonly reported for many BAF organisms. Although we found two main lineages diverging by ~2 Mya during the early Pleistocene, species' delimitation analysis assigned most of the populations as independent evolving entities, suggesting an important role of disjoint rock outcrops in promoting high endemism in this rich biome. Lastly, we detected limited gene flow in sympatric populations although some hybridization and introgression were observed, suggesting a continuous speciation process in this species complex. Our data not only inform us about the extensive differentiation and limited gene flow found among Pitcairnia flammea species complex, but they also contain information about the mechanisms that shape the genetic architecture of small and fragmented populations of isolated rock outcrop of recently radiated plants.

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Dispersal ability is a key determinant of the realized species niche. Yet, whether dispersal ability influences environmental specialization and exerts a direct, indirect, or null effect on species’ tolerances is still unclear. Here, we ask whether and how dispersal ability can shape both the realized and fundamental niches. Focusing on plants, invertebrates, and vertebrates of the topographically complex Atlantic Rainforest, a top global biodiversity hotspot, we further evaluate how dispersal ability correlates with species range shifts in response to climate change. We find that high-dispersal species have broader thermal tolerances relative to low-dispersal taxa. When projected in geographic space, the data predict widespread upslope range shifts of the Atlantic Rainforest biodiversity with the intensity and direction depending on the species-specific trends depending on dispersal ability. These upslope movements, in turn, may negatively impact the native communities intrinsically associated with the Atlantic Forest mountaintops. Under the warmest climate scenario predicted for the end of the 21st century, the models project that those species with the lowest dispersal ability, particularly low-dispersible ectotherms, will be the most impacted by local extinctions. In turn, the wider thermal tolerance of high-dispersible species will reduce shifts in their geographical range due to climate change. Given the rapid rate of habitat conversion experienced by this and other landscapes worldwide, we argue that the smaller endurance of low-dispersible species to environmental changes deserves special attention, as dispersal ability appears relevant for biodiversity management in a warmer world, especially in threatened species-rich regions such as this.