Desert pupfish live in springs of the american southwest. today there are about 30 species of pupfish, but they all evolved from a common pleistocene ancestor. the southwestern united states was once much wetter than it is now, and the pleistocene pupfish flourished over a wide geographic area. over thousands of years, however, the sierra nevada mountain range was pushed upward by geological forces, blocking rainfall from the pacific ocean. as the large lakes dried up, small groups of pupfish remained in springs and pools fed by groundwater seepage. now, although many of these small springs still have pupfish, each population, through evolution, has become very different from populations of pupfish in other springs. the variation in gene pools among the 30 pupfish populations occurred through an evolutionary mechanism called: