Sexy son hypothesis

The sexy son hypothesis in evolutionary biology and sexual selection—proposed by Ronald Fisher in 1930—states that a female's ideal mate choice among potential mates is one whose genes will produce male offspring with the best chance of reproductive success and implies that a potential mate's capacity as a parental caregiver or any other direct benefits the father can offer the mother such as nuptial gifts, or good territory are irrelevant to his value as the potential father of the female's offspring. Fisher's principle means that the sex ratio (except in certain eusocial insects) is always 1:1 between males and females, yet what matters most are her "sexy sons"' future breeding successes, more likely if they have a promiscuous father, in creating large numbers of offspring carrying copies of her genes. This sexual selection hypothesis has been researched in species such as the European pied flycatcher.

Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sex characteristics. In 1976, prior to Weatherhead and Robertson's paper,Richard Dawkins had written in his book The Selfish Gene:

In a society where males compete with each other to be chosen as he-men by females, one of the best things a mother can do for her genes is to make a son who will turn out in his turn to be an attractive he-man. If she can ensure that her son is one of the fortunate few males who wins most of the copulations in the society when he grows up, she will have an enormous number of grandchildren. The result of this is that one of the most desirable qualities a male can have in the eyes of a female is, quite simply, sexual attractiveness itself.

...
Wikipedia