Abstract
The Tropical Niche Conservatism hypothesis is one of the most relevant theories to explain why tropical diversity is high, although the mechanisms underlying this hypothesis require further clarification. A possible research avenue to address the underlying mechanisms includes determining population-level processes associated with such a hypothesis, in particular by trying to identify how adaptation may occur in extreme niche conditions at the edges of species ranges. However, the determinants of molecular diversity at the edges of geographical distributions of tropical taxa are still poorly known. Here we assessed which environmental variables determine diversity in nuclear and plastid genetic markers for populations of four Passiflora species in the southern limit of their geographical distributions. Climatic factors can drive genetic diversity, and their importance varies according to the marker. The primary predictors are variables representing higher temperatures during cold periods of the year and higher precipitation during dry periods. We concluded that, although these species are present in colder areas at the edge of their range, Tropical Niche Conservatism acts as a restraining force on genetic diversity in southern populations of Passiflora.