i have recommended sober/wilson's book on this list before and i agree its an excellent book. they document not only instances of "altruistic" behaviour (in quotes since i do not want to get involved in a debate of what qualifies as altruism), but also show models/scenarios where group selection might apply (w.r.t the original post regarding the UCLA study, i am surprised by claims that this is the first of its kind or that the results are new. previous research from evolutionary and game theoretical perspectives have yielded similar findings).
however, singer, pinker, dawkins, et al may respond thus, to the last section above: the argument against the human nature excuse is not that human's do not have a nature (i.e., they are a blank slate) but that their nature does not exclusively determine their behaviour and that of societies.
a lot of very important work was done in the mid-20th-century against the notion of a complete, predetermined and unchanging inner nature that determines each individual's personality, position, etc. as i understand them, none of these authors oppose that work and to a large extent would like to extend the humanist/leftist tradition that motivates and benefits from such work. i read their suggestions (to varying degrees), rather, as a warning against a) falling prey to the effort to ground human outcomes in innate natures and b) being forced as a result to deny all innate natures. in particular, the warning is that human beings might indeed have instincts, including selfish ones, and it would be scientifically disastrous to claim otherwise (as i mentioned earlier, i believe singer misunderstands lewontin, in particular, whose position is a lot more nuanced).
perhaps one example may be the notion of innate homosexuality: is there a homosexuality gene? are gay people gay at birth? one may be tempted to argue from a biological position, and if research demonstrates a varying range of sexuality in the species, that information is quite valuable in the debate. however, the defense may be better grounded in the field of ethics/morality/rights, than in biology: it should not matter if a person is gay "biologically" or not. or to approach it from the opposite direction, this is the oft-repeated warning that "is" does not mean "ought".
i believe that singer's warning is timely for certain segments of what i see as the left (not marxists, whose positions and philosophy i have very poor grasp of): perhaps the "new age" left? i am not quite sure however that singer, dawkins, others thoroughly engage (or even fully understand) the deeper critique of certain "nurture" theorists, behaviourists and philosophers of science (critics of reductionism and biological determinism).
in addition to wilson/sober's excellent book, i would like to recommend: exploding the gene myth -- hubbard/wald, along with the usual suspects ;-) from lewontin, steven rose, kamin, kitcher...
--ravi