Originally Posted by Happy Monkey

Well, piercehawkeye45 is correct that individuals don't mutate in response to situations- the gene has no way to know in advance what sort of mistranscribing will be useful. But the selection of beneficial mutations that "mutates" the species in general is in response to situations.

Biologist PZ Meyers has an interesting article on these moths, tying them into the comic book "Y the Last Man".