Western Rattlesnake (Crotalus viridis)

Western rattlesnake (Crotalus viridis)

These rattlesnakes live throughout the western U.S. While there are diverse populations of western rattlesnakes, they can be divided into two major lineages, one on either side of the continental divide. Different populations may be on their way to becoming different species, which means that they will not interbreed. Some of these populations are separating because they are in competition with each other. For example, a group of western rattlesnakes in the Pacific Northwest has developed a more lethal venom than its neighbors. However, some diversification has resulted from changes in the geography of the West. The rise of the Colorado Plateau has separated eastern and western lineages.

Phylogeography is the analysis of population history and population divergence in relation to geography. By examining the genetic makeup of animals and plants in a region, and by dating geologic events in the same area, much can be learned about both geologic change and evolutionary response to that change.