DIFFERENTIATION OF POPULATIONS

Evidence is presented from a variety of sources which indicates that species should not be thought of as evolutionary units held together by the cohesive force of gene flow. Gene flow in nature is much more restricted than commonly thought and experimental evidence is badly needed to document the extent to which it does occur. Selection itself is both the primary cohesive and disruptive force in evolution; the selective regime determines what influence gene flow has on observed patterns of differentiation. Populations will differentiate if they are subjected to different selective forces and will tend to remain similar if they are not. For sexual organisms it is the local interbreeding population and not the species that is clearly the evolutionary unit of importance.