Skeptic - Our Neandertal Brethren; August 2010; Scientific American Magazine; by Michael Shermer; 1 Page(s)

According to the late Harvard University biologist Ernst W. Mayr, the greatest evolutionary theorist since Charles Darwin, “species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”

Reproductive isolation is the key to understanding how new species form, and many types of barriers can divide a population and split it into two different groups: geographic (such as a mountain range, desert, ocean or river), morphological (a change in coloration, body type or reproductive organs), behavioral (a change in breeding season, mating calls or courtship actions), and others. After isolation, if members of the split populations encounter one another and cannot produce viable offspring that can themselves later successfully interbreed and produce viable offspring (hybrids such as mules are infertile), then these two populations constitute two different species.