Best for whom? The enviroment may change so that a particular species of tree is no longer very successful at propagating itself. If we do nothing, it will go extinct. But it might be much better for us if we did not let that happen, because the bark of the tree contains a chemical that is highly beneficial to us.

I agree with Nicholas that where we can take natural to mean 'conducive to human flourishing', in Aristotle's sense, there will be a connection between being natural and being good. But there are natural functions that do not carry this meaning. In biological cases, functions often correspond to 'selected effects'. Thus the function of the white fur of a polar bear is camoflage, and that coloration is the result of natural selection. Bears in that environment with white fur did better at reproducing than their more colourful cousins. Selected effects are in that sense natural: they are what the trait is for. From a moral point of view, however, selected effects may be bad and unselected effects may be good. Thus we may have evolved a tendency to deceive other people in certain circumstances, even if this is not morally decent behaviour, and someone who decently resists this temptation may be bucking that evolved inclination. Selected effects may be conducive to what we might call ...

This is a good question: it's hard to know what people mean when they say that an act is unnatural. One possibility is to appeal to a distinction between the functions some things have and alternative uses to which they may be put. A desk clock has the function of telling the time; if I use it as a door-stop, that might count as an unnatural act. That's not what clocks are for. Biological traits also have functions. For example, the function of the veins and arteries in your legs are to help to circulate blood down there. But suppose that surgeons translplant pieces of some of those veins or arteries to your uppper body when they are performing heart surgery: that too might count as an unnatural act. So we may be able to make sense of an unnatural act as the appropriation of something that has one function in order to perform a different function. But people who talk about unnatural acts often claim that those acts are bad, and bad because they are unnatural. The fact that an act is...