96. This geographic barrier merely permits divergence to occur; it does not guarantee it. Specification will only occur, for example, if the two divided subpopulations are subject to different selection pressures or at least one of them is small enough to accrue significant random changes in its gene pool ("genetic drift"). For good, non-technical descriptions of evolutionary theory, see Daniel C. Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life (1995); John Maynard-Smith, Did Darwin Get it Right? Essays on Games, Sex, and Evolution (1989); John Maynard-Smith, On Evolution (1972); George C. Williams, Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought (1966).