Recently C. W. Dick, E. Bermingham, M. R. Lemes, and R. Gribel published the interesting article “Extreme long-distance of the lowland tropical rainforest tree Ceiba pentandra L. (Malvaceae) in Africa and the Neotropics” in Molecular Ecology 16 (2007):3039-3049.

Ceiba pentandra is a rainforest tree commonly called the cotton tree or kapok tree, and is widely distributed in the tropics of Africa and the New World. More than one hundred species of flowering plants are shared between the two continents. The presence of similar plants in both Africa and South America has often been explained as the result of vicariance, that is, a splitting of an ancestral range that occurred when African and South American plates diverged. An alternative explanation is dispersal of seeds across the Atlantic Ocean. This article reports the results of a molecular test to distinguish vicariance from dispersal in the case of the cotton tree.

If the cotton trees of Africa and South America have been separated for 100 million years as in the vicariance explanation, there should be a large difference in the DNA sequences of trees from the two regions. A small difference would indicate a more recent separation, which would imply dispersal. The molecular test showed only a small difference between trees from Africa and South America, ruling out the vicariance explanation and indicating that the cotton tree has somehow been able to disperse across the Atlantic Ocean, probably from Africa to South America.

Comment: Floral and faunal similarities among the southern continents have been interpreted as evidence for vicariance caused by a long history of the movement of tectonic plates. However, few of the proposed examples have withstood testing based either on molecular comparisons or on fossil evidence. Present understanding indicates that dispersal played the major role in distributions of organisms restricted to the southern hemisphere, especially among plants, marine organisms, and invertebrates. These results are consistent with creationist expectations.