The Harsher The Environment, The More Likely One Species Will Diverge Into Many Subspecies

By Gabrielle Jonas on November 23, 2013 12:48 PM EST

California Ground Squirrel
This is one of many squirrels that branched off to adapt to the demands of its environment. (Photo: By Brocken Inaglory)

The number of subspecies — an evolutionary step in the process whereby one species becomes two — increases at higher latitudes and harsher climates, a study has confirmed. The findings suggest that there is more species turnover in uninviting environments, ecologists at the National Evolutionary Synthesis Center in Durham, N.C. said Friday.

The ecologists originally aimed to study why 66 percent of all mammals and birds are in the tropics, but they stumbled upon the unexpected finding along the way. They had assembled a data respository of the genetic data of 2,300 mammal species and 6,700 bird species worldwide, along with global climate and weather patterns.

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Lead author Dr. Carlos A. Botero and his team at North Carolina State University were surprised that the number of subspecies within each species (determined by the number of genetically distinct groups within each species) is greater in harsher environments of higher latitudes compared to the tropics near the equator. Investigators called this tendency the "latitudinal diversity gradient" and featured their findings in the November issue of the journal, Molecular Ecology.

"What is fascinating about this finding is that biologists often assume that the tropics are more diverse,because they are places where new species are more likely to arise," Dr. Botero told International Science News. "But we find that this is unlikely to be the case. Our study suggests that new species are likely to arise much more often in temperate regions, but since species are also more likely to go extinct in those regions the net result is that over time, the tropics are able to accumulate more biodiversity."

The genus Tamias — the one that contains chipmunks — is a case in point. The more northerly species, including Tamias minims, have nine to 18 subspecies, whereas the more southerly species tend to have two to six. A similar pattern reveals itself is the family of squirrels.

"As you go north, the number of subspecies per species tends to increase quite a bit," Dr. Botero said. His results are consistent with a 2007 University of British Columbia study suggesting that new species arise faster in temperate zones than in the tropics. Mammals and birds in temperate zones are more likely to freeze during frigid winters or die during severely hot summers.

A more grim way of looking at the phenomenon is what Dr. Botero terms, "higher potential for speciation counterbalanced by a higher potential for extinction." In other words, the potential for large numbers of a species to die off leads to more distinct groups within that species.

"If extreme weather events wipe out a population every now and then, but don't wipe out an entire species, the populations that survive will be geographically separated, and could start to diverge from one another," said Dr. Botero. "Species come and go more frequently in the temperate zones."

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