So reading this question sent me into a shallow-dive (one article deep) reading about animals that have this capability seems to suggest that actually selective breeding (as opposed to natural selection) might be the only way to create a species that could breathe both on land and underwater, as it seems like otherwise the tradeoff of creating two seperate breathing systems just wouldnt be worth the cost in the wild.

btw not a biologist, so everything i write is probably BS