Herrera, Sara et al. 2018 suggest that there are at least 33 S-alleles in apricot.
I believe some species of Brassica have at least 60 S-alleles identified.
However, there are dominance relationships among these alleles, so I don't think you can directly extrapolate from the number alleles to to the rate at which non-self pollen will be excluded. The ...
I just want to report the finding of one paper here. I don't know much about the rest of the literature and can't comment on it.
Otto et al. (2015) investigate this question in sexually reproducing organisms. As I understand it, they assume that the amount of time spent in on phase of the life cycle is proportional to the amount of selection happening in ...
This is a hypothesis that I found in Desmond Morris book, The naked ape.
Basically, humans transit from a rear-penetration position to a frontal-penetration position. In this transition, the sexual clue performs by buttocks was perform now by breast.
In short, according to Morris: breasts are sexual clues because they look like buttocks.