Evolutionary time drives the inverse latitudinal diversity gradient of New World sparrows (Aves: Passerellidae)

Disentangling mechanisms shaping species richness is fundamental to understanding how biodiversity is structured across geography. The latitudinal diversity gradient (LDG) is a classic pattern in which the tropics harbour the greatest number of species. Patterns deviating from the classic LDG, such as an inverse gradient in which diversity is higher towards the poles, have been considered exceptions and attributed to taxon-specific causes rather than to general processes. Here, we evaluated the simultaneous contributions of various evolutionary and ecological factors to the geographical diversity patterns of New World sparrows (Aves: Passerellidae) at the family and lower clade levels. Three hypotheses were tested: geographical variation in diversification rates, time for species accumulation, and ecological limits. We also explored the influence of dispersal and colonization. A strong inverse diversity gradient was identified for the entire family and five of eight subclades, all of which were best explained by the time for species accumulation hypothesis. Conversely, diversification rates best explain clades exhibiting a classic LDG pattern. Our findings support an 'into the tropics' pattern of colonization, characterized by the origin of Passerellidae in temperate zones, the persistence in these regions and limited dispersal events leading to high diversification rates in the tropics.