“The rarity of the royal lines is actually an evolutionary strategy by the cheats to escape suppression by the altruistic masses that they exploit.”

That part doesn’t make sense to me, so I wonder if I’m missing something.

“The rarity of royal cheats is best explained as an evolutionary strategy to avoid suppression by cooperative genotypes, the efficiency of which is frequency-dependent.”

The sentence is a little unclear. What’s frequency-dependent – the efficieny of the suppression, or the efficiency of the strategy to avoid suppression, or both?

Strategy may not be the best word. Presumably the royal alleles are not directly sensing and limiting their own frequency, which is what “rarity as a strategy” means to me.

Presumably there is a Nash equilibrium here, where

1. non-cheaters suffer a net fitness loss by further intensifying supression, as long as the cheater allele frequency is (at most) what it is now, and

2. the allele, at any higher frequency, would be a net liability to its bearers, as long as the intensity of supression is (at least) what it is now…

…so neither cheaters nor cooperators can unilaterally improve their position.

It’s easy to see why intensifying suppression could have diminishing returns for non-cheaters – kind of like getting more impact out of the first 20 police you add to the force than you get out of the next 20.

A cheater allele could become detrimental at high frequencies because more suppression is provoked. If the cheater allele has some intrinsic fitness costs, non-cheaters can simply partially block its benefits, to the point where the allele is barely net-profitable. If the cheater allele has no intrinsic fitness costs, non-cheaters may have to actually impose costs themselves – that is, attack and harm the cheaters.