Tropical rainforests contain the most species-rich plant assemblages on Earth, yet the processes that generate this diversity are poorly understood. The proposed research investigates genetic processes leading to speciation in 12 rainforest tree species found throughout tropical America and, for three species, West Africa. The study is focused on (1) the importance of tropical mountains as barriers to dispersal, (2) effects of regional forest histories on levels of genetic diversity within species, and (3) biotic interchange between Africa and the tropical America. This research will test the contrasting hypotheses that widespread tropical tree species are either genetically cohesive populations or species complexes with high levels of genetic divergence.
Tropical forests are under severe pressure from agricultural expansion and climate change. In order to conserve genetic diversity of tropical hardwoods it is essential to understand how genetic diversity is partitioned within and among species. The proposed research highlights the role of geography in shaping the distribution of genetic variation, and it will pinpoint biogeographic centers of genetic variation for important species such as kapok (Ceiba pentandra) and balsa (Ochroma pyramidale). The research involves collaboration with the Smithsonian Tropical Research Institute and Latin American and African institutions.
Many South American rainforest tree species range over thousands of kilometers and occur around major geographic barriers, such as the Andean mountains, yet very little is known about the ages of these species, their genetic variability, or the historical processes that led to their broad distributions. This study examined patterns of genetic variation in 12 common and widespread South American rain forest tree species. Among the findings, we discovered that two of the species had historically dispersed between Africa and South America via Atlantic ocean currents. This history of marine dispersal has led to many other floristic similarities between African and the South American rain forests. We also discovered that the majority of our study species were likely present before the modern Andes rose to its present elevation– 9 of the 12 species originated in the Miocene epoch when the Earth was much warmer than present. These species contain high levels of regional genetic divergence that is not expressed in their outward form (phenotype). The discovery of high levels of genetic variation has important implications for tropical silviculture, because several of the study species (example, the Kapok and Balsa trees) have global market value. The project provided research training for undergraduate and graduate students at the University of Michigan, along with an African post-doctoral researcher, who has since become the coordinator of African forest inventory plots at the Smithsonian Institution.
