How the widowbird got its absurdly long tail

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One glaring difficulty for early evolutionary biologists was the evolution of exaggerated male secondary sexual traits, which should hinder the survival of males.  Darwin was the first to suggest that sexual selection favoring an exaggerated trait could 'override' natural selection opposing the same trait, as long as the reproductive benefits outweighed the survival costs associated with it.  During the modern synthesis, Ron Fisher proposed a mechanism of ornament evolution, wherein female preferences for an initially advantageous male trait drive that trait to an extreme value such that it no longer confers a survival advantage.  Despite these theoretical advances, little empirical evidence of sexual selection on elaborate traits existed even in the early '80s.  Anderrson's work on Long-tailed Widowbirds provided the first experimental evidence of female preference for a male ornament.  This beautifully designed manipulation study was able to control for the confounding influence of variation in male territory quality, the experimental manipulation, and male behavior.
And a later paper summarizing subsequent work on widowbirds

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