That the human and chimp sperm genes were so radically different suggests exceedingly rapid evolution. When he compared the genes in humans and gorillas, which are more distantly related, he noticed that the rate of evolution of maleness genes was still accelerated, but at a much slower rate in gorillas. The researchers also surveyed 50 genes from humans and old world monkeys, of which 18 pertained to male reproduction and the rest did not. They found that 11 of the 18 male genes evolved much faster than the average rate of evolution of the non-sexual genes. Wu thinks that the rapid rate of evolution of male genes is indicative of the sexual behavior of the species.
Chimps exhibit very promiscuous behavior.
A female may mate with numerous males during her receptive phase. The competition to attract a mate, as well as internal sperm competition, puts intense pressure on chimp male genes to evolve rapidly. Conversely, gorillas employ a harem system where one high-ranking male has access to several females who mate only with him. In this case, the male can be relatively sure that he is the genetic father of any offspring produced by his harem. This takes some of the pressure off sperm-producing genes to mutate rapidly because they don't compete directly with other sperm in the female, said Wu.