Note: This is interesting. I wonder if it has anything to do with
thermoregulation of external testes. (Testes are thought to be external, in
many mammalian species, so they can be cooler than the core body temperature.
This differential is probably less, in a warmer environment, and it makes sense
to me, that mutation rates could be higher, then the germ-line cells are exposed
to higher temperatures.)

Dan


http://news.bbc.co.uk/2/hi/science/nature/8115464.stm


Evolution faster when it's warmer

Wednesday, 24 June 2009
By Victoria Gill
Science reporter, BBC News


Climate could have a direct effect on the speed of "molecular evolution" in
mammals, according to a study.

[Photo captioned, "The results could help explain why the warm tropics are so
species-rich"]

Researchers have found that, among pairs of mammals of the same species, the DNA
of those living in warmer climates changes at a faster rate.

These mutations - where one letter of the DNA code is substituted for another -
are a first step in evolution.

The study, reported in Proceedings of the Royal Society B, could help explain
why the tropics are so species-rich.

DNA can mutate and change imperceptibly every time a cell divides and makes a
copy of itself.

But when one of these mutations causes a change that is advantageous for the
animal - for example, rendering it resistant to a particular disease - it is
often "selected for", or passed down to the next few generations of that same
species.

Such changes, which create differences within a population but do not give rise
to new species, are known as "microevolution".

The idea that microevolution happens faster in warmer environments is not new.
But this is the first time the effect has been shown in mammals, which regulate
their own body temperature.

"The result was unexpected," said Len Gillman from Auckland University of
Technology, who led the study.

"We have previously found a similar result for plant species and other groups
have seen it in marine animals. But since these are 'ectotherms' - their body
temperature is controlled directly by the environment - everyone assumed that
the effect was caused by climate altering their metabolic rate."

Scientists believe that this link between temperature and metabolic rate means
that, in warmer climates, the germ cells that eventually develop into sperm and
eggs divide more frequently.

"An increase in cell division provides more opportunities for mutations in the
population over a given time," explained Dr Gillman.

"This increases the probability of advantageous mutations that are selected for
within the species."

'Sister species'

"We suspected the same effect might be happening in mammals, because seasonal
changes affect the animals' activity," Dr Gillman told BBC News.

He and his team compared the DNA of 130 pairs of mammals, looking at genetically
similar "sister species" - where each of the pair lived at a different latitude
or elevation.

They tracked changes in one gene that codes for a protein known as cytochrome b,
comparing the same gene in each of the pair of mammals to a "reference" gene in
a common ancestor.

[Photo captioned, "Lemurs were one of the 130 pairs of closely related mammals
investigated"]

By looking for mutations in the DNA code for this gene - each point where one
letter in the code was substituted for another - the researchers were able to
see which of the two mammals had "microevolved" faster.

Animals living in environments where the climate was warmer, had about 1.5 times
more of these substitutions than the animals living in cooler environments.

Dr Gillman explained that, at higher latitudes where environments are colder and
less productive, animals often conserve their energy - hibernating or resting to
reduce their metabolic activity.

"In warmer climates annual metabolic activity is likely to be greater, so this
will lead to more total cell divisions per year in the germline."

These results support the idea that high tropical biodiversity is caused by
faster rates of evolution in warmer climates.