Epidemics have a way of making one wonder about death. To put it plainly, in the raw form it takes as it first rises from our hearts: Why? Why on Earth does it have to be this way?
Say a certain man has a beneficial mutation. And say a certain woman has one of her own, only it’s different: It confers a different advantage, and it’s located somewhere else in the genome. And say, finally, this man and this woman get together and have kids. One of those kids might get both beneficial mutations, while another kid might get neither. When the doubly lucky kid survives and reproduces, while the doubly unlucky kid doesn’t, a single death becomes doubly useful: It pays for the simultaneous rise in frequency of two beneficial mutations.
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You can repeat the argument for parents who have two mutations, three, and so on. Thanks to sex, a single death can push many beneficial mutations toward their ultimate fixation. We can conclude, therefore, that although the price of life is death, sex improves the exchange rate.
