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I was just reading a book yesterday by Steven Pinker in which he says that the reason sexual reproduction exists rather than females just making clones of themselves is because of pathogens - viruses and bacteria and so on. Genetic varitaion throws them off and is required because they evolve so much faster.
He also said that the size of testicles in primates is in part determined by how promiscuous the females are. The reasoning goes like this: females that have sex with many males create competition inside their vaginas (sperm can last several days wandering about), so the more semen a male can produce, the better chance his sperm will win. In that way, males evolve bigger and bigger balls to ejecaulate more and more semen. This is why men have larger testicles than gorillas (relative to body size). Penis size is determined by similar factors - any longer than a certain optimal length and there is no advantage, but shorter than that and there is.
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