Take the 2-minute tour ×  I am currently reading 'The Selfish
Gene' by Richard Dawkins, which I am sure many here have read.
The topic are evolutionary stable strategies (ESS) regarding
cooperation.  I apologise for the long question. If you are
already familiar with the topic and Dawkins' model of Cheat,
Sucker and Grudger: my question is, how can Grudger be an ESS if
it could be invaded both by Suckers (because they have no
disadvantage against Grudger) and Cheats (because a Cheat
minority is unlikely to meet the same Grudger twice, turning
Grudger into Sucker effectively)?  More detailed:  The model
Near the end of chapter 10 (p 185 in my version), Dawkins uses a
model of birds who clean each other of parasites, therefore
helping in survival (as cleaning themselves they cannot reach
every spot of their body). He defines three different behaviours
for the model:    • Sucker - birds who indiscriminately help and
clean other birds   • Cheat - birds who let others help them but
never do so themselves   • Grud