Evolution of Sex
According to Plato's romantic myth, the world was at some remote time populated by perfect beings who were female on one side of the body and male on the other. Then angry gods sundered the two sides, and made the detached halves, females and males, forever seek to restore the lost wholeness in love. Until the beginning of the current century, Plato's myth was about as good an elucidation of the origin and meaning of sex as could be had. Aristotle, Plato's more realistic-minded disciple and rival, declared that the female supplies the matter and the male the motion of the future life. He thought that the female was "cold" and the male "hot." Consequently, the conceptions which occur when warm winds are blowing give more males, and cold winds bring more females. The conjectures and speculations concerning sex which were being solemnly discussed in Darwin's day were not much above the level reached by Aristotle more than two thousand years earlier.
Two discoveries changed the situation at the turn of the twentieth century. First, the microscope revealed the existence of sex chromosomes (see Chapter 3). In organisms with separate sexes every cell of a female body differs in the chromosome complement from every male cell. The behavior of the sex chromosomes at meiosis explains very simply how the sex of an individual is decided at the moment when the egg is fertilized. Second, an insight into the biological meaning of sex was obtained by deduction from Mendel's discoveries. Mendel's work showed that gene recombination in sexually produced progenies creates an immense amount of genetic variability and of raw materials for evolution (Chapter 2). Sex arose in organic evolution as a master adaptation which makes all other evolutionary adaptations more readily accessible.