Timing the origin and expansion of the Mexican tropical dry forest.

Macroevolution examines the temporal patterns of biological diversity in deep time. When combined with biogeography, it can provide unique information about the historical changes in the distribution of communities and biomes. Here I document temporal and spatial changes of diversity in the genus Bursera and relate them to the origin and expansion of the tropical dry forests of Mexico. Bursera is very old, highly adapted to warm dry conditions, and a dominant member of the Mexican tropical dry forest. These characteristics make it a useful indicator of the history of this vegetation. I used a time-calibrated phylogeny to estimate Bursera's diversification rate at different times over the last 60 million years. I also reconstructed the geographic center and time of origin of all species and nodes from information on current distributions. Results show that between 30 and 20 million years ago, Bursera began a relatively rapid diversification. This suggests that conditions were favorable for its radiation and thus, very probably for the establishment of the dry forest as well. The oldest lineages diverged mostly in Western Mexico, whereas the more recent lineages diverged in the south-central part of the country. This suggests that the tropical dry forest probably first established in the west and then expanded south and east. The timing of the radiations in these areas corresponds to that suggested for formations of the mountainous systems in Western and Central Mexico, which have been previously recognized as critical for the persistence of the Mexican dry forest.