There has never been an agreed set of rules on possessive apostrophes. 'Survival of the fittest' was used by Darwin as a metaphor for "better designed for an immediate, local environment", it's been commonly used out of context since Darwin used the term. I watch far too much QI... I'm guilty of grammatical errors on this website as much as the next person but I don't really care. As long as a post can be deciphered easily enough I'm happy. Checking every post I make for spelling and grammar?