The greater diversity of life in the tropics compared with the poles is so obvious it barely seems worth thinking about. But explaining it is probably the oldest problem in biology — over the past 200 years (starting with Alexander von Humboldt) there have been more than 100 explanations advanced for what is known as the latitudinal diversity gradient. All these are some variation on: either species are more likely to evolve in the tropics, or they are less likely to go extinct, or both. (In case you hadn't already guessed, there's a chapter on all this in my book.)
Or, as Ledyard Stebbins put it in 1974, are the tropics the cradle of biodiversity, or its museum? 'Both', say David Jablonski and his colleagues in the 6 October issue of Science.
The fossil record shows that biodiversity has been concentrated in the tropics for at least 270 million years. But to answer the cradle/museum question, you need estimates of both origination and extinction rates, which are hard to come by.
Jablobski and co looked at marine bivalves (i.e. clams), one of the few groups with a good enough fossil record to address this question. Going back about 11 million years, into the Miocene, they found that about twice as many groups make their first appearance in the tropics as outside them.
So that's the cradle. In fact, Jablonski had already shown this by looking at marine fossils in what I think is a very good Nature paper from 1993. The new study builds on this by finding that bivalves are also more likely to go extinct outside the tropics: "only 30 exclusively tropical genera go extinct as compared to 107 extratropical and cosmopolitan ones". So that's the museum.
The researchers advocate what they call the 'out of the tropics' (OTT) model: "lineages not only preferentially originate in the tropics, but also persist there as they expand poleward…most extratropical species belong to lineages that originated in the tropics." This includes us, of course.
This seems to me a persuasive general explanation for the underlying driver of the diversity gradient. It certainly seems more general than the various explanations based on ecological factors — there are more ecological niches in the tropics, or competition is fiercer — or on climatic stability, although arguments such as this may be needed to explain why tropical species seem less likely to go extinct.
And, we have a mechanism for why evolution runs faster in the tropics — the warmer temperatures there increase metabolic rates, and so mutation rates, and so (probably) evolutionary rates. This is the 'energetic theory of speciation' that came up the other week at Santa Fe (see previous post); the person doing most work on it is Drew Allen, who is studying how speciation rates in foraminifera relate to temperature, another group with a fossil record to die for.