New research suggests much more subtle interactions with the environment can play a role.
One of the puzzles of evolution is how, exactly, one species becomes two. Molecular biologists routinely compare the relatedness of species by methods such as comparing the similarity of DNA sequences. But exactly how one species splits in the first place is not well understood. For example, scientists know that the life span of various species of mammals can be quite long. As a result, researchers are lucky to be able to observe even a few generations of reproduction in any one species.