Beyond looking good as a couple, how does picking a male with a showy tail increase the female's fitness, particularly if the showy tail makes survival more difficult? You'd think that if she wants to get her genes into the future, the female would be better off choosing males with less-showy tails so that her offspring have a better chance of survival.
As it turns out, that's not quite how it works. Instead, specific cases have been found in which female fitness is reduced when females mate with less-showy males because the mating success of their sons is lower. In other words, having a sexy son can provide a fitness advantage for a female who chooses to mate with an elaborate male.
Imagine a mutation that makes a female less likely to choose a male with a big tail. Now consider how that choice may affect her fitness. True, her sons wouldn't be hobbled with enormous tails, but they wouldn't be prime mating material either. If these poor sons can't get mates, they can't pass on their (and their mom's) genes.
To see whether this hypothesis were true, evolutionary biologists came up with a testable prediction: The sons of less-flashy fathers should have lower mating success than the sons of more-flashy fathers. After identifying species in the wild for which they could measure the attractiveness of males, who mates with whom, and the mating success of the sons of more- and less-flashy fathers, scientists discovered that less-flashy sons actually do have less success in the mating department — and that decreases both their fitness and their mothers'.
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