I will try to put forward the beginning of an answer to this worry, but a full-blown defense of the view that I find most attractive will need, obviously, much more detailed argument than I can give in a forum like this. The full text of my student's objection can be found here.
The heart of my student's objection is this:
The basic sort of objection I have in mind runs something like this: Darwinian theory shows that species develop through the mechanisms of random mutation and natural selection, and in the case of human beings perhaps 'cultural selection' has played some role. Because the functions of the parts of animals (granting for the sake of argument that they have real functions, which not all biologists and metaphysicians of biology will concede) are the products of random genetic mutation and natural selection, with the individuals of a species inheriting genetic traits from their ancestors, what we have in a species is not the sort of determinate thing that the natural-law appeal to nature assumes they are. A species is, rather, a group of individuals with a common ancestry and a capacity, derived from their shared genetics, to produce offspring. What human beings share, then, is a range of genetically-determined dispositions to behave in certain ways, and not some essential nature which they can complete to a greater or lesser degree. Thus, what individual members of the human species seek by nature may differ from individual to individual and from population to population, and in the strictest sense the only criterion of membership in the human species is a sufficiently similar collection of traits. If, then, any moral theory can be grounded in what all human beings share, it will only be grounded in what we share accidentally, because there is nothing essential to the human species.To paraphrase very briefly: different human beings have different preferences, that is, they seek out different things depending on their own sets of values. Biological science does not provide a framework to support the view that any one set of values is superior to any other. In particular, biology does not support the idea that there is any such thing as a human "essence". So biology cannot support the view that there is anything common to all humans. So both in terms of human preferences and in terms of human biology there is nothing that can be held to be normatively the same for all and hence determinative of the end for man.
The objection here, if I understand it correctly, is grounded in a certain sort of assumption, namely a reductionist sort of assumption that appears to hold that if we can render an explanation of some aspect of human existence in materialistic terms, as the adaptationist explanation of human phenotypes aims to explain why we exhibit the traits that we do, then this fact in itself renders otiose--and thereby disproves--competing explanations of the sort that I'm offering.
Let me start by addressing the notion of function. It is true that not all biologists or philosophers of biology mean the same thing when they speak of "function" in organisms, but there is certainly a theoretically neutral notion of function that seems hard to deny, one which says only that certain structures appear to have certain capacities (this has been called the notion "service functions"). The eye, for example, responds to photons striking the retina and in that sense is capable, in conjunction with the visual cortex of the brain, of detecting light and color. We may say, in a limited way, that this is one of the "functions" of the eye, and as long as we make this claim very limited I don't think that there are any biologists or even philosophers of biology that are going to worry too much about the use of the word "function". The worry arises more often because of Aristotelians who want "function" to mean something like a "final cause" in the sense I described in my post of the other day. It would be possible to claim that that kind of worry borders on begging the question against the Aristotelian.
But a better answer goes this way. What is at stake, clearly, is not the question of whether certain structures have functions in some sense of the word "function." Rather, the worry is over whether organisms as a whole have functions in the Aristotelian sense, the sense grounded in the notion of nature. Only if humans as such have a single nature can there be such a thing as a "final cause" of mankind.
Biologically speaking, I'm not so sure I would be as quick as some to dismiss the notion that there is such a thing as a single human nature. It is an objective fact of biology that humans cannot interbreed with other species, for example. In fact, on one view, this breeding barrier is the sufficient condition for counting as a species. As with everything else, however, there is a fair amount of controversy--at least among philosophers of biology--as to what the necessary and sufficient conditions for biological species might be. But it does not strike me as plausible to say that it is literally impossible to tell the difference between a human being and, say, a snake. When we look at a human and recognize it as such, is this nothing more than a social construct? If so, it would be grounded in some sort of similarity conditions: that thing over there seems to me to be sufficiently similar to me to count as the same kind of thing as I. I suppose that kind of view is logically possible, but I think that, given the wide latitude of organisms that are able to "recognize their own kind" compared to the relativelly narrow latitude of organisms that are capable of social construction, it seems to me relatively implausible as an account of what it is we're doing when we recognize a member of our own species.
At this point I don't see what difference it makes where we locate "human nature", that is, the "what-it-is-to-be-human", whether in some set of phenotypic traits, some section of the genome, some set of dispositional properties, or what have you. We don't need to be Aristotelian essentialists in order to be essentialists. Just so long as there is something that we can say is present in instantiations of human beings most of the time, then it seems to me that we are in a position to say that there is something that we can call a "human nature".
Apart from the details of how one would go about establishing such a thing, however, I don't think there's much controversy in such a notion. To borrow an argument that Aristotle himself often used (see, for example, Metaphysics Gamma), we may note that even the most stalwart defender of materialistic reduction still acts as though there is a real difference between a human being and a dog. To put it another way: you don't see many of these folks trying to have children with their dogs, even though they claim to "love" them. In short, the people who deny that there is such a thing as a human essence usually only make the denial because they have an impoverished notion of what sorts of things might be countable as essences.
Philosophically--historically, anyway--the prime candidate for what it is that separates human beings from other organisms has been our capacity for a certain kind of rational activity. I'm not all that sure what is supposed to be wrong with this candidate, but it is true to say that in recent times the idea that human beings are "rational animals" has come under relatively heavy skeptical fire. Sometimes the objection to it is rather banal: chimps, crows, dolphins, etc. are all very "intelligent" animals, so "intelligence" per se cannot count as the differentia of the human species. But there are more sophisticated objections: what, exactly, is rationality itself? Is it not a social construct in its own right? What reasons do we have for thinking of it as an objective feature of reality?
The nature of rationality is a very difficult topic. My own view, to put it bluntly, is that human rationality is constrained by things that are external to us, viz., the laws of logic and mathematics. These laws determine what is and what is not a valid inference. To the extent that our reasons are compatible with these laws, our reasons are good; otherwise they are bad. One builds from there. The building is, of course, difficult, but it can be done. For what I take to be a particularly good example of how the building should go, one may consult Hadley Arkes' fine book, First Things: An Inquiry Into the First Principles of Morals and Justice (Princeton, 1986).
Apollodorus (my student's nom-de-login) closes his post with:
What I want, I guess, is to understand why it is that Darwinian theory does not compel us in this direction [sc. towards nominalism and anti-essentialism]. Do we have to reject the theory, or accept it in some anti-realist way? Or is the theory perfectly compatible with a more essentialist view of species?My own view is that all of science can only be acceptable in an anti-realist way, but that, too, is a very large topic for discussion. But since I am a global anti-realist about science, I am a fortiori an anti-realist about evolutionary theory. In my opinion, all Christians must be global anti-realists about science, as I've argued in most of my posts about science. But you don't have to be a Christian to be an anti-realist about science. One of my colleagues, who specializes in philosophy of science and who is a militant atheist, has said that you'd have to be a moron to be a realist about science these days (I wish he would say what he really thinks one of these days). In short, I don't think that evolutionary theory, either in its Darwinian or neo-synthesis form, compels us to either nominalism or anti-essentialism unless we presuppose materialist reductionism. If you make that a priori assumption then, sure, lots of things will follow that are not very congenial to the view I'm defending. Like Aristotle, however, I am an anti-reductionist.
There will be time in the coming weeks to debate this matter more deeply. It is a matter of some interest to me and, I suspect, to a few of my readers (well, to one of them at least). For the Christian, much hangs on this, but to judge from my student's post I suspect that much hangs on it for the non-believer, as well, since it is possible to build a moral theory out of this kind of stuff. If one is serious about morality, then one wants to know that one's moral theory is well-grounded. This appears to be of interest even to the Humeans out there like Simon Blackburn. So the arguments need to be well-crafted, and the objections well met.
On at least one account, crafting the arguments and meeting the objections constitute a major part of our natural end, so it would be wrong to take this too lightly.