First phylogeographic study of a rainforest tree

In species-rich rainforests of the New World tropics most trees have broad geographic distributions–from Mexico to Bolivia and sometimes to the West Indies. Either they have excellent dispersal abilities, or they established broad ranges prior to the formation of present geographic barriers. In a study featured in American Naturalist, Christopher Dick, Kobinah Abdul-Salim and Eldredge Bermingham address these questions in the first phylogeographic study of a rainforest tree.

The morphology of the study species, Symphonia globulifera (Clusiaceae) is uniform across a natural range that includes the New World tropics and Africa. Symphonia globulifera also has a detailed fossil pollen record, which the authors used to calibrate a molecular clock for DNA sequences obtained from African and Neotropical populations and to estimate when these populations were separated. The study revealed that, although trees from different populations look the same, the evolutionary history of these populations is probably quite distinct.

Although marine dispersal of S. globulifera is considered improbable because it has salt-intolerant seeds, the authors demonstrate that Symphonia expanded into Mesoamerica, the Amazon basin and the West Indies via oceanic currents at least three times. The three major New World clades – found in Mesoamerica, the Amazon basin, and the West Indies – diverged over 15 million years ago, and appear to have been genetically isolated ever since, giving Symphonia globulifera the status of “living fossil”.

Panama contained the largest number of genetically distinctive populations, in contrast to Amazon basin Symphonia, which displayed no genetic differentiation across 2500 km of lowland forest. The authors suggest that strong phylogeographic structure in the Panamanian Isthmus stems from its history as a land bridge between Central America, South America, and the proto-Isthmian archipelago of islands, which were connected approximately 3 million years ago.

The authors suggest that same history may explain the high level of beta diversity (changes in species composition among sites) of rainforest trees in Panama, in contrast to western Amazon (Condit et al. Science 2002 v292, 666-669). While ecologists attribute the beta diversity in Panama to local adaptations to varied habitats, the diversity may also reflect the incomplete mixing of independently evolved floras during the Great American Biotic Interchange across the Isthmus of Panama.

Ref. Molecular systematics reveals tertiary diversification of a widespread tropical rainforest tree. Christopher Dick, Kobinah Abdul-Salim, and Eldredge Bermingham. American Naturalist, December, 2003.