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Dispersal, vicariance and press releases

Ceiba pentandra (the silkcotton, kapok or ceiba tree) has a broad distribution including across the Neotropics and Africa. A cultivated form is grown across a wider range for kapok fibres. The trans-Atlantic distribution of C. pentandra has attracted attention for a long time – based on the fact that there are nine species in the Neotropics and only one in Africa, it has generally been assumed that it is a Neotropical species that dispersed to Africa. How it got there has long been a question.

EurekaAlert! published a press release attributed to the National Science Foundation about a paper published in Molecular Ecology concerning the trans-Altantic distribution of Ceiba pentandra, the silk cotton or kapok tree. The paper* considers three mechanisms by which the species could have come to be present on both sides of the Atlantic: Gondwana vicariance, Boreotropical dispersal (and subsequent vicariance) and long-distance dispersal.

In the case on Gondwana vicariance, the distribution would reflect the fact that South America and Africa were connected (as parts of Gondwana) until 96 million years ago.  In the case of Boreotropical dispersal, the species would have colonised Africa about 35 million years ago through a then warmer North America and Europe, only to be lost from those areas as the climate warmed.  Both of these would be expected to produce deep divergence between African and South American C. pentandra populations.  On the other hand, if long-distance dispersal across the Atlantic took place more recently, the divergence would be smaller.

The paper found evidence for recent dispersal.  The authors noted that:

Ceiba pentandra has the weakest phylogeographical structure yet reported for a widespread rainforest tree species. Apart from the cluster of sites in Western Ecuador having a variant psbB-psbF haplotype, there was no cpDNA variation across Mesoamerica, the Amazon basin, and West Africa. In fact, this study found less cpDNA variation across three continents than some rainforest tree species exhibit within putative breeding populations in French Guiana and Brazil (Hamilton 1999b; Dutech et al. 2000; Latouche-Halle et al. 2003) or among sample sites at regional scales in French Guiana and across Mesoamerica (Caron et al. 2000; Cavers et al. 2003).

In this regard, C. pentandra is by no means unique – several other species have been found that have recently crossed the Atlantic, including Symphonia globulifera, which apparently colonised the Americas from Africa.

One thing that bothers me is the press release.  It opens with

Celebrated in Buddhist temples and cultivated for its wood and cottony fibers, the kapok tree now is upsetting an idea that biologists have clung to for decades: the notion that African and South American rainforests are similar because the continents were connected 96 million years ago.

It’s obvious that whoever wrote the press release didn’t spend much time looking at the paper.  While the issue of Gondwanan roots is considered, it’s by no means a new idea.  At the same time, it’s a fairly limited suite of trees that are shared between the two areas, and evidence of recent trans-Atlantic dispersal by a few species does not “[upset] an idea that biologists have clung to for decades”.  That’s nonsense.

* Christopher W. Dick, Eldredge Bermingham, Maristerra R. Lemes and Rogerio Gribel. Extreme long-distance dispersal of the lowland tropical rainforest tree Ceiba pentandra L. (Malvaceae) in Africa and the Neotropics. Molecular Ecology (OnlineEarly Articles). doi:10.1111/j.1365-294X.2007.03341.x

H/T Agricultural Biodiversity Weblog.


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