We have a population whose males are undergoing evolution of some characteristic such as tail length in widow birds, under the influence of female preference tending to make the tails longer and utilitarian selection tending to make the tails shorter. The reason there is any momentum in the evolution towards longer tails is that, whenever a female chooses a male of the type she ‘likes’, she is, because of the non-random association of genes, choosing copies of the very genes that made her do the choosing. So, in the next generation, not only will the males tend to have longer tails, but the
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