Humans can threaten species with extinction in many ways, including overfishing, pollution and deforestation. Now a pair of studies points to a new danger to the world's biodiversity: humans may be blocking new species from evolving.

New species evolve when old species split apart. Animals living on a peninsula might become cut off from the mainland by rising sea levels, for example.

They would adapt to life on the island and acquire mutations not shared by the rest of their species. If the sea level should drop and the animals could mix together again, the mutations might make it less likely that the two populations could interbreed. They would be on their way to becoming separate species.

A few places provide scientists with a good view of these forces at work. In western Canada, many lakes formed when the glaciers retreated at the end of the last ice age 11,000 years ago.