15 MARCH 2007
Ask any conservation biologist, and you'll get the same message: The tropics have an outsize number of species. And you'll probably hear the same explanation: Species form more quickly in the tropics, and they may also go extinct more slowly in the relatively stable tropical climate.
This week, Science carries a surprising statistical analysis of the issue. "We wanted to address why there are more species in the tropics," says first author Jason Weir, a PhD student in the department of zoology at the University of British Columbia. "In almost every taxonomic group you look at, there is significantly more diversity in the tropics. The prevailing view for the past 50 years is that there is a faster rate of speciation in the tropics. We wanted to test that."
Image courtesy Jason Weir
To document evolutionary relationships, the researchers studied DNA from 618 mammal and bird species, grouped into pairs of closely related "sister species." Specifically, they looked at DNA from the mitochondria, sub-cellular structures that are inherited through the mother and whose DNA mutates at a steady rate. To take a fictional example, if you compare mitochondrial DNA from the barred owl-catcher and its nearest relative, the besotted brow-snatcher, you may notice, say, 20 differences. Knowing that the mitochondrial DNA in each bird mutates, say 10 times in a million years, you calculate that these birds last shared a common ancestor one million years ago.
the dating game
This number amounts to the "age" of the species. As you might expect, Weir's method is rather more complex, but the chief finding was this: Species form faster near the poles. The average tropical species was 3 to 4 million years old, while the average Arctic species was less than 1 million years old.
The result, says Weir, "was highly unexpected. If speciation rates are [as conventional wisdom holds] faster in tropics, we would expect the sister species to be younger, but that is exactly the opposite of what we found."
By crunching numbers in the same database, Weir found that species also go extinct more quickly near the poles. If so, the paucity of biodiversity near the poles arises because rapid extinction more than offsets faster speciation. The overall number of species in a location, after all, reflects the rate of speciation minus the rate of extinction.
The study is "great science," says Donald Waller, professor of botany at the University of Wisconsin-Madison. The genetic data show that "you don't have a lot of ancient history for a lot of temperate birds and mammals. That makes sense. If you have ice one mile thick [during the ice ages], you are not going to have a lot of birds and mammals around. This is one of those papers where you say, 'Yeah, of course, that makes sense,'" but only after you read the study.
Extinction: Changing the equation
Waller agrees that the study helps explain the comparative biodiversity of tropical and temperate zones. "We have known for a long time that tropical species tend to be far more endemic, to have limited ranges. This could result from recent speciation, slow spread, and gradual extinction."
Photo: National Zoo
But if species form, expand and go extinct more slowly in the tropics, he says, that "immediately explains that pattern of endemism." The range of a species tends to "collapse" as it heads toward extinction, Waller says. The shrinkage is slower in the tropics, where the environment was historically stable, and that favors the presence of many small patches of species, which translates into a higher degree of biodiversity.
Now, rapid human alteration has destabilized the tropics, undermining the "slow-extinction" leg of the equation, setting the stage for an explosion of extinction, Waller adds.
Having fueled the effort to explain biodiversity around the globe, Weir now plans to examine pairs of same-aged species from different latitudes. Do a pair of tropical birds that diverged two million years ago look more alike than an equal-aged pair from Canada or Siberia? Finding that the bodies of polar animals are actually changing faster would tend confirm the surprising finding of high-latitude evolutionary overdrive.
— David Tenenbaum
The Latitudinal Gradient in Recent Speciation and Extinction Rates of Birds and Mammals, Jason T. Weir* and Dolph Schluter, Science, 16 March 2007.