Define the terms "population," "deme," and "species." What is the ultimate criterion in determining whether a population is its own species; why can anatomical, physiological, and behavioral differences among populations only act as guides in deciding whether a population is a separate species.
A group of individuals that share a common gene pool is called a population. Small, local populations of individuals that share a gene pool are called demes. Subject to similar selection pressures and more likely to mate with each other than with individuals outside the deme, members of a deme resemble each other more than they do members of other demes. Yet, demes are temporary units of population that intergrade with other surrounding demes. The larger units of population that contain all the demes of a certain organism are both more distinct from each other than demes are from one another and are longer-lasting. The larger units are called species. A species is the largest unit of population within which exchange of genetic material does occur or has the potential to occur. If two populations are almost identical but are completely and permanently reproductively isolated from each other, they must be considered separate species. In contrast, if two populations have great outward differences but the exchange of genetic material takes place between them, the groups must be considered to belong to the same species. Thus, the ultimate criterion in
