The Mammalian Sex Chromosomes

Abstract

Twenty-five years is the reasonable estimate of our own species‘ generation time. It follows then that since the birth of Jesus Christ, man has gone through about 80 generations. Any practicing geneticist knows that a period covered by 80 generations is just too short a period for a large population to undergo any sort of striking genetic changes. Meanwhile, populations of viruses, bacteria, and even small rodents must have undergone considerable adaptive changes, for they have gone through many times greater numbers of generations. This observation, illustrates the problem of evolutionary slow-down faced by all large mammalian species. The practice of polygynous matings adapted by many of these species can be viewed as a reasonable attempt at compensation. The polygynous mating system exploits the one-to-one sex ratio that invariably produces a large surplus of adult males. That being the case, each adult male’s aggression should be channeled strictly toward other adult males of the same species. In order to do so, adult males should wear tell-tale biological symbols of masculine supremacy so that they cannot be confused with adult females and juveniles of both sexes: hence the development of extreme sexual dimorphism, as already noted. Yet, those heritable traits that made certain males victorious in male-to-male combat must have necessarily benefited females as well as males of the next generation. Accordingly, it is essential to keep a genetic difference between the sexes to the necessary minimum. Once again, we encounter yet another evolutionary paradox: Extreme sexual dimorphism should be manifested on the basis of a minimal sex difference in the genetic constitution.