Where is evolution fastest?

An incontrovertable fact of life is that species diversity is highest in the tropics and hence it has long been thought that rates of evolution are also highest there. Just last year this was put to the test by Shane Wright and colleagues from the University of Auckland in a paper published in PNAS last May. They examined rates of molecular evolution and showed that tropical plants did indeed seem to be evolving at a faster rate than species from the same genus found in temperate latitudes. Their preferred explanation was elevated rates of mutagenesis.

However, today in Science the whole notion of evolution being higher in the tropics was cast into doubt. Jason Weir and Dolph Schluter at the University of British Columbia took a different approach to examining speciation rates. They took sister-pairs (two species who are more closely related to each other than anything else) of birds and mammals and plotted out the time of divergence against the probable latitude of their common ancestor. Although most of the taxa had shared ancestors at low (tropical latitudes) those with a recent ancestor were concentrated at higher latitudes. When they fitted their data to a birth-death model using maximum likelihood they show a linear relationship between latitude and speciation rate (lineages per million years).

So, who's right? As a student of both evolutionary tempo and biogeography I find such links between the two interesting. Leaving the issue of taxa choice (plants versus birds/mammals) aside, I think it is crucial to point out that the two sets of authors are looking at two fundamentally different questions. Wright et al. were interested in the rate of molecular (DNA) change whereas Wier and Schluter were only examining the frequency of speciation (when one lineage splits into two). These two phenomena aren't necessarily interlinked. In fact Wright and colleagues even stated that, "greater rates of speciation in the tropics do not cause higher rates of molecular evolution."

Perhaps the most interesting conclusion from today's announcement is that tropical diversity was built up over an extended period, suggesting that it is the environmental stability of this region that has led to the huge build-up in species.