This is clear, beautifully written, and funny, thanks for taking the time to create it.
Just to add a point to the section on Fisherian runaways, this is a wild guess, but I suspect that runaway sexual selection has several possible causes. One is the “overshoot effect” you describe, where sexual selection pushes the evolution of a beneficial trait and this continues past the optimum point. Another (this is where I’m guessing) is that while the fitness of mates is linked through their offspring, it’s not identical, and desirable traits in a mate are not necessarily the traits that would benefit the individual’s genes the most. For example, if an organism invests some energy into helping its kin, that benefits its alleles, but not those of its mate. Unlike the overshoot, this effect should persist even in equilibrium. Direct or kin selection pushes in one direction for the benefit of one’s own alleles, while sexual selection pushes in the other direction for the benefit of the alleles of your prospective mate. These forces settle at some equilibrium point.
For example, take a hypothetical bird species where males being large and strong is genuinely helpful for protecting their nest (increasing offspring fitness and thus the joint fitness of the male and their mate). However, having large and strong offspring is more of an energy investment for the parents. They need to feed their babies more worms while they’re growing, and therefore they’re limited to having fewer offspring if those offspring need to be big. Thus, individual / kin selection might want males to be smaller than sexual selection would want them to be. As a prospective mate, a female would not particularly care how many siblings a male has, but she would care about how well he can protect their nest. The equilibrium point should lie somewhere in between the point chosen by sexual selection and the point chosen by individual / kin selection. The males will tend to be a bit larger than they would be if females selected mates at random, and they’ll tend to be a bit smaller on average than the size that the females prefer the most.
This theory also generates the prediction that being ungenerous to one’s own kin should be attractive. (Generosity to the kin of one’s mate should be attractive, though.) This doesn’t seem true in humans, as far as I can tell. It’s important to take note of these contradicting bits of evidence.
This theory also generates the prediction that being ungenerous to one’s own kin should be attractive
Isn’t that what makes “Romeo and Juliet”-like stories “romantic”? When one forsakes one’s own genetic clan to elope with the mate, it signals extreme devotion to the mate and is therefore attractive.
This is clear, beautifully written, and funny, thanks for taking the time to create it.
Just to add a point to the section on Fisherian runaways, this is a wild guess, but I suspect that runaway sexual selection has several possible causes. One is the “overshoot effect” you describe, where sexual selection pushes the evolution of a beneficial trait and this continues past the optimum point. Another (this is where I’m guessing) is that while the fitness of mates is linked through their offspring, it’s not identical, and desirable traits in a mate are not necessarily the traits that would benefit the individual’s genes the most. For example, if an organism invests some energy into helping its kin, that benefits its alleles, but not those of its mate. Unlike the overshoot, this effect should persist even in equilibrium. Direct or kin selection pushes in one direction for the benefit of one’s own alleles, while sexual selection pushes in the other direction for the benefit of the alleles of your prospective mate. These forces settle at some equilibrium point.
For example, take a hypothetical bird species where males being large and strong is genuinely helpful for protecting their nest (increasing offspring fitness and thus the joint fitness of the male and their mate). However, having large and strong offspring is more of an energy investment for the parents. They need to feed their babies more worms while they’re growing, and therefore they’re limited to having fewer offspring if those offspring need to be big. Thus, individual / kin selection might want males to be smaller than sexual selection would want them to be. As a prospective mate, a female would not particularly care how many siblings a male has, but she would care about how well he can protect their nest. The equilibrium point should lie somewhere in between the point chosen by sexual selection and the point chosen by individual / kin selection. The males will tend to be a bit larger than they would be if females selected mates at random, and they’ll tend to be a bit smaller on average than the size that the females prefer the most.
This theory also generates the prediction that being ungenerous to one’s own kin should be attractive. (Generosity to the kin of one’s mate should be attractive, though.) This doesn’t seem true in humans, as far as I can tell. It’s important to take note of these contradicting bits of evidence.
Isn’t that what makes “Romeo and Juliet”-like stories “romantic”? When one forsakes one’s own genetic clan to elope with the mate, it signals extreme devotion to the mate and is therefore attractive.