Imperfect offspring are still useful for a society, they just are maybe suited at different things. Both a strong but dumb person and a frail but smart person are useful in their own ways. :)

In a society, non productive members do not contribute.

Birds: if in a nest, there are healthy fledglings and a runt, the weakest will be pushed out of the nest by the others because it is a drain on resources.

Big Cats: as big cats age and become frail, they will leave the group and stay back alone to die hidden. This is so they don't slow down the rest of the group. Also..their death will attract predators and scavengers endangering the rest.

And yet, altruism shows up in many ways in nature. Yet, altruism is weaker with human beings and when resources are scarce, there seems to be no will to optimize it.

Why are humans different..even to our detriment?