Evolution faster in temperate zones than tropics

new evidence suggests that evolution of birds and mammals in the temperate zone is faster than in the tropics, contrary to an earlier theory that said tropics were more favourable. It has been believed that since the tropics have more number of species, the rate of evolution is more in this region. Scientists at the University of British Columbia now say that tropics have more number of species because the rate of extinction of species is less there.
Faster temperateIn a paper published on March 15 in Science, they proved through dna analysis of closely related species that speciation--the process by which one species splits into two--takes place faster in temperate zones than in the tropics. Researchers have charted the genetic family tree of 309 mammal and bird species in the Americas. The rate of speciation at the equator was found to be around 0.18 new species per species per million years. The extinction rate here was around 0.04. At the transition between tropical and temperate (23 latitude) regions, the speciation rate was higher at around 0.38 and extinction rate at 0.25. At the transition between temperate and the arctic (65 latitude), the speciation rate was lower at around 0.8 while the extinction rate was higher at 0.67.

Jason Weir of the University of British Columbia, one of the authors of the study, said these measurements included studies fromNorth, Central and South America but excluded oceanic islands like the Caribbean. The reason behind difference in evolution of species was most likely intense climatic fluctuation at high latitudes as opposed to much more stable climate in the tropics.

"Climatic fluctuations most likely exposed species to the pressures of selection, increasing the rate of evolution. Also, they pushed habitats southwards during cooling episodes. That's how species ranges might have become more fragmented than in the tropics. Range fragmentation is necessary for speciation to occur in birds and mammals. On the other hand, climatic instability seems to be the cause of a high extinction rate," Weir added.
No generalisationScientists working on speciation acknowledge that the work highlights a new direction in the variation of speciation rate with latitude. At the same time they point out that the theory cannot be generalised for all species.

"There have been two major hypotheses for greater species diversity in the tropics--greater speciation rate and lower extinction rate. However, the calculation of speciation rate on the basis of dna analysis is relatively recent," says Kartik Shanker of the Indian Institute of Science, Bangalore.

dna data showing species age was not available till about 15 years ago and the data for this kind of analysis was not available till three years ago, he said. "This is one of the first studies on the differences in speciation rates between temperate and tropical regions on the basis of dna analysis. It is wide-ranged. However, what is true for the species represented in this study may not be true for all the species," he added.