Friday, March 16, 2007

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.


Sarda Sahney said...

Though I am familiar with many of the critisms surrounding calibrating molecular clock I don't know enough about the methods to comment myself. I did however, enjoy this quote:

"Trying to estimate the divergence times of fungal, algal or prokaryotic groups on the basis of a partial reptilian fossil and protein sequences from mice and
humans is like trying to decipher
Demotic Egyptian with the help of an odometer and the Oxford English
-- D. Graur & W. Martin.
2004. Trends in Genetics 20(2).

Sarda Sahney said...

As you stated, currently there is a greater diversity of life in the tropical regions. What I am wondering however, is if you are using molecular methods to determine the divergence of tropical species, then in calibrating your clock is it an issue that the tropics have not always been the most diverse areas? When the Earth is in an ‘icebox’ phase as we are now, the greatest diversity lies in the tropics but during a ‘greenhouse’ phase the greatest diversity lies in the temperate regions.

Malacoda said...

I hadn't really thought about it, but you make an important point. I didn't explicitly state it myself, but the divergence dates were based on molecular clocks, which themselves assume some constancy of rate of molecular evolution. And yet, for plants at least, this rate differs in different regions. Perhaps we should write a reply? ;)

Mambo-Bob said...

I just read a news brief on this today, but it was stated in it that places with high speciation rates (temperate zones in this case) will also have a high extinction rates. So collectively, there will be lower diversity in the temperate zones but higher diversity in tropical zones. I don't know what you guys think of this coz I'm not informed enough in this, but I just thought this was interesting.

Malacoda said...

Hi Mambo. You are right. The implication is that turnover is highest in the temperate regions. This is bad news for tropical species as it implies human-induced extinction there will be harder to bounce back from than in temperate regions.

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Currently I am founding member, president elect and entire membership of SWEMP (the Society of Wonky-Eyed Macroevolutionary Palaeobiologists). In my spare time I get paid to do research on very dead organisms and think about the really big questions in life, such as: What is the ultimate nature of reality? Why is there no room for free will in science? and What are the implications of having a wardrobe that consists entirely of hotpants?