Natural populations of organisms often occur in isolated patches with limited exchange of migrants among them. The potential for evolution to local environmental conditions is enhanced when gene flow among subpopulations is reduced.
Geographic barriers can restrict genetic exchange among populations even further.
I have measured differentiation among populations using polymorphic enzymes. I study populations of the montane leaf beetle Chrysomela aeneicollis in the Sierra Nevada mountains of California. These populations occur in three drainages at the southernmost tip of the insect's range. The abundance of the beetles has fluctuated dramatically over the last 15 years. These fluctuations suggest that non-equilibrium processes may strongly influence the genetic structure of these populations.