What are the evolutionary mechanisms explaining the similar species richness patterns in tropical mosses? Insights from the phylogeny of the pantropical genus Pelekium.

As opposed to angiosperms, moss species richness is similar among tropical regions of the world, in line with the hypothesis that tropical bryophytes are extremely good dispersers. Here, we reconstructed the phylogeny of the pantropical moss genus Pelekium to test the hypothesis that high migration rates erase any difference in species richness among tropical regions. In contrast with this hypothesis, several species considered to have a pantropical range were resolved as a complex of species with a strong geographic structure. Consequently, a significant phylogeographical signal was found in the data, evidencing that cladogenetic diversification within regions takes place at a faster rate than intercontinental migration. The shape of the Pelekium phylogeny, along with the selection of a constant-rate model of diversification among species in the genus, suggests, however, that the cladogenetic speciation patterns observed in Pelekium are not comparable to some of the spectacular examples of tropical radiations reported in angiosperms. Rather, the results presented here point to the constant accumulation of diversity through time in Pelekium. This, combined with evidence for long-distance dispersal limitations in the genus, suggests that the similar patterns of species richness among tropical areas are better explained in terms of comparable rates of diversification across tropical regions than by the homogenization of species richness by recurrent migrations.