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.
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.
"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.
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