Imagine a case where the children of a bird each have a 50% chance of being born blind.
But the bird always has on average 4 chicks. So on average 2 will be blind and 2 sighted.
(Perhaps this could evolve if the energy needed to create a chick with 100% probability of being sighted is more than the energy needed to create two chicks each with 50% probability of being sighted. The eye being a very complicated organ).
Now, the mother brings food to the chicks and only the sighted children eat the food. The blind chicks (the 'runts') are thrown out of the nest. (It is common there being a 'runt' of the litter. So this scenario has some truth in it.)
Thus all adult birds of this species are sighted. (The 'fittest' birds).
(This is a kind of surival of the fittest but in a family setting.)
Is there a name for this kind of process? This could happen in any animal that has on average more than 2 children. Where it might be cost effective not to make sure that 100% of the children are fit, because the weakest will die anyway.
Secondly, imagine a scenario, where there is a surplus of food one year, so that even the 'runts' make it to adulthood. Now, 50% of the adult birds will be blind. And people might say "There is an epidemic of blind birds". Can we make comparrisons with the theory that there is "an epidemic of myopia" in modern human populations, possibly due to modern medicine decreasing childhood mortality?