Because of the promiscuous mating system of fruit flies, each fly will mate with many other flies over its lifetime. Neither partner has any interest in increasing the fitness of any particular mate, only in increasing its own fitness:
i Male fruit flies have toxic chemicals in their seminal fluid that inhibit the sperm of other males (this is an example of sperm competition, explained in the earlier section "Sperm competition" in this chapter). This trait is advantageous for the male because it increases the chance that his sperm will be the ones that fertilize the female's eggs. But the same trait is bad for the female because the semen is toxic to her; she's being poisoned.
i The very act of mating seems to decrease the survival of female flies, because repeated mating with males is physically damaging — a situation the female fly tries to avoid.
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