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It seems like in any population subject to evolutionary pressure, sexual attractiveness will inversely correlate with other kinds of reproductive fitness. If you see a peacock with a small tail feather, this is information that it is likely above average in other areas, because otherwise its genes would already have been wiped out. So intuitively, you would think that it would benefit individuals to find less conventionally attractive mates.

So far, I have encountered two possible explanations for this. One idea was that the things that make an animal attractive correlate with evolutionary fitness. I'm not an expert here, but it feels like the correlation here will diminish over time as soon as there is direct pressure to increase attractiveness.

The other, more convincing, explanation was that it is advantageous to produce attractive offspring. So, there might be a feedback loop here where if members of a population care about sexual attractiveness more, then attractiveness would confer a greater advantage, making attractive offspring more important, increasing the evolutionary pressure for individuals to choose attractive mates.

Can someone give an intuition (or better, the relevant math) for why these two latter effects dominate my original intuition? Or are there other effects at work here?

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  • $\begingroup$ If your 3rd paragraph is true, isn't your 2nd paragraph also true? $\endgroup$ – Bryan Krause Sep 23 at 19:46
  • $\begingroup$ Good point, I guess I meant that sexually attractive features (e.g. big muscles) might correlate with advantages in individual (i.e., not sexual) selection. $\endgroup$ – Leo Ware Sep 23 at 20:06
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first off you are talking about runaway sexual selection, run away sexual selection works because attracting mates matters MORE than individual survival. An immortal survivally perfect creature that cannot mate is an evolutionary dead end and might as well not exist where its genes are concerned. A bright red neon sign on your back that draws the attention of potential mates is an advantage as long as it attracts mates more often than it attracts predators. Coincidentally this is why so many sexual displays are things you can turn off, even a peacock can fold its tail fan down. These displays often exploit existing attention factors, like eye shaped objects, bright contrasting colors, loud noises, ect, things organisms are already predisposed to pay attention to. Of course once neon sign displays exist your second idea comes into play, once neon signs are in play a female is best served if her own male offspring and descendants also have neon signs, which makes attraction to the neon signs (and thus the genes making them) an advantage.

For normal sexual selection both your ideas also come into play, likely more often. common attractive features are things like size, symmetry, health, or excess energy, direct signs of fitness.

Most of the time displays reaches a point of stability where the cost is balanced by the attractiveness, this is how most displays work. But sometimes runaway selection can drive it past the point of no return, where the cost of reducing it is so high (lack of mates) the only winning strategy is to invest more and more, on and on until the cost is so high you never get a chance to mate.

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You gave the example of a brightly colored male bird... and what about the females? The are well camouflaged and optimally fit to evade predation. So, even if 50% of males are caught and eaten due to theor bright colors, the demography stays the same.

The species as a whole is the same with or without the bright feathers. The sexual display amplifies the pressure for fitness selection, to obtain more food, evade predators better, and invest more energy in sexual lures, horns, feathers, song.

You are right, if a predated population places sexual selection on the females, it is counter productive. Fish, birds, insects, frogs, they nearly always have super robust camouflaged females and males that fight and display their high fitness, except parrots and reef fish.

Birds are so good at evading predators and flying to food, that even the females are colorful in some species i.e. parrots, and thats a less easy example, why do female parrots risk having no camouflage? Perhaps their climate doesnt provide enough selection.

www.google.com/search?q=sexual+selection+graph+feathers+demography&tbm=isch&ved=2ahUKEwjN77n5lobsAhUPWBoKHSgGAcMQ2-cCegQIABAC&oq=sexual+selection+graph+feathers+demography

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