I personally do not see the problems with "result" and "relative", which just seem simpler and less ambiguous words to me than "function" and "differential". And I do not see why you simply reverted all the attempts to make the word order better. But in any case we have to find some way to make this sentence better, surely? It is too long and complex. Behold: Natural selection is the gradual natural process by which biological traits become either more or less common in a population as a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment. Can we split this up for example?