The advocate derives ought from is without any compelling (and reasonable) link.
An example of the Naturalistic Fallacy is the most basic argument in favour of Social Darwinism - a theory of societal ethics which claims its basis is in nature (evolution by natural selection - though it has a closer resemblance to selective breeding). Social Darwinists argue that if nature is this way (only the 'fittest' survive), then it ought to be this way with various features of society. (It has been mostly used as a justification for laissez-faire economics and eugenics.) But this is an unfounded logical leap. As far as an ethical theory goes, an argument needs a better claim than, because something is done this way, it ought to be. This has nothing to do with whether it is right or wrong. It is simply a statement of fact. And to get from a fact to an ethical value, there needs to be some kind of compelling argument about the 'goodness' or 'evilness' of the fact. For this to happen we need an agreed upon 'good,' and an agreed upon 'evil'.
(Note, this is a very simplistic treatment of Social Darwinism - and arguments not based on 'is-ought' with respect to the 'goodness' of laissez-faire economics and eugenics have been made by many.)