So, having accepted the assumption that females do the choosing, we next take the crucial step that Fisher took in confounding Darwin's critics. Instead of simply agreeing that females have whims, we regard female preference as a genetically influenced variable lust like any other. Female preference is a quantitative variable, and we can assume that it is under the control of polygenes in just the same kind of way as male tail length itself. These polygenes may act on any of a wide variety of parts of the female's brain, or even on her eyes; on anything that has the effect of altering the female's preference. Female preference doubtless takes account of many parts of a male, the colour of his shoulder patch, the shape of his beak, and so on; but we happen to be interested, here, in the evolution of male tail length, and hence we are interested in female preferences for male tails of different length. We can therefore measure female preference in exactly the same units as we measure male tail length - inches. Polygenes will see to it that there are some females with a liking for longer than average male tails, others with a liking for shorter than average male tails, and others with a liking for tails of about average length.  

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3 Tails in waiting