At the other end of the temperature scale
Life's Boiling Point
http://www.spaceref.com/news/viewpr.html?pid=26894
"Heat-loving organisms live where the water is hot but the gene pool is shallow.
Genetic analysis has shown that so-called thermophiles have fewer mutations in
their protein-coding genes than do their microbial cousins that live at room
temperature. This seems to imply that the opportunities to evolve decrease
as temperature increases."