I am anxiously awaiting the publication of David Buller's Adapting Minds: Evolutionary Psychlogy and the Persistent Quest for Human Nature(link to PDF table of contents). I took Buller's evolutionary psychology course in 1997, and I think it was the best course I've ever had. David's amazing crisp clarity enabled him to convey huge amounts of empirical information while simultaneously framing the philosophical debates surrounding philosophy of biology and evolutionary psychology in vivid and compelling terms. David's been working on this book since then, at least, and I expect it to be outstanding.
It's because of this course that I gave up on my facile Randian views about “human nature.” If I'm not misremembering, I think an earlier iteration of the book's tite was . . . the Persistent Myth of Human Nature. I'm not sure if this is David's own view, but I was eventually persuaded, despite very strong initial resistence, by the Hull/Ghiselen argument that species are not really natural kinds at all, but are rather a special kind of individual, like a very old club.
The members of a species are not members of a kind bound together by a shared essence. Members of a species are more like members of the Daughters of the American Revolution, bound together by a geneological fact. You and I are both part of the club of humanity because we have a shared ancestor: the first human. This, however, implies nothing about our having a metaphysically deep shared natured. Evolution works on selection over natural variation. That is, evolution works because members of a species are not homogenous. So at any time, there is simply a distribution of traits throughout a population. Maybe the distribution is a normal curve. Maybe it isn't. In any case, the distribution changes over time, and thus so do the traits of the “typical” member (if there is one). There simply is no non-contingent common core of traits that ties us together other than our shared lineage and consequent genetic similarity.
This is why I find the idea that there is a right way to live according to nature extremely dubious. (This is all me, from here on out, and not Buller, or anyone else.) We have no “deep” nature. Right now, in this neighborhood of our evolutionary history, there is a distribution of traits that one might call “typical” in a statistical sense. But this has no more deeply normative significance than would the fact that 90% of us prefer almonds over pistacchios. It makes no sense to argue that we thus ought to prefer pistacchios. People with statistically “deviant” behavioral dispositions are by definition not “normal,” but their behavior is not a scintilla less “natural” than that of the normals.
This is not to say that our contingent, temporary statistical “nature” is normatively irrelevant. Far from it. Our intuitions about morality, justice, and so forth, and our behavioral dispositions arise from within this “nature.” Our understanding of what we have reason to do isn't seperable from what we happen to be like. The ends we take ourselves to have reason to pursue depends on what we happen to be like, and what we happen to be like tells us a great deal about the necessary means to those ends. Given the ends that most of have, and take ourselves to have reason to realize, together with what most of us are like, it is possible to get fairly stable general principles about what we ought to do.
But we mustn't kid ourselves. These principles simply aren't universal, or universally binding, because there is no unviversal human nature. Some “deviants” will find a society hospitable to the lives of “normals” incompatible with their needs. And this is simply tragic, no more, no less. The deviant will either be unhappy or will act contrary to the principles of normals. If the latter is threatening to the order required by the normals, then they will lock up, institutionalize, or otherwise rid themselves of the deviant menace. But it is important to see that although the deviant is acting wrongly from the perspective of “morality,” construed as the system of rules that facilitates decent life among the normals, from a broader perspective they are just very unlucky. Foreign cells rejected from a host body have done nothing wrong; they are just incompatible with the principles governing the local order.
There's a lot more to say about this, but that's all for now.