When we analyze an ethical theory, we ought to bear in mind the following:
1/ the ethical theory’s consistency or internal coherence;
2/ the consistency or internal coherence of that specific ethical theory when seen in relation to other conceptions of the theory’s author (in this case, Peter Singer);
3/ the reaction of our own ethical feelings regarding that ethical theory, or the relation between the ethical theory and the common-sense sentiments.
If, in point 1 and 2, the conclusion is negative, the ethical theory is not valid, as surely there is an intellectual error. But if the conclusion it is negative only in point 3, we cannot say that the author committed an error, but only say that we simply do not agree with him.
Now, let’s see.
Peter Singer bases his ethical theory in the opposition to the concept of “Speciesism” (see http://goo.gl/ggYDU ).
In this context, Speciesism is allegedly an ethical doctrine that determines that the human being is ontological, biological and morally superior to other animals.
The argument of Peter Singer against Speciesism is that “the belonging to one determined biological species, per se, does not have any moral, biological or ontological relevance”.
However, the ethical theory of Peter Singer, understood as an antithesis of Speciesism, is incompatible with Darwinism that considers the human being as the ultimate “product” of biologic evolution (point 2) — notwithstanding the fact that Peter Singer also adopts Darwinism (point 1) in his ethical conception.
This means that there’s an incoherence or an inconsistency between the ethical theory of Peter Singer (against Speciesism), on one hand, and other philosophical conceptions of the same author (in favour of Darwinist evolution), on other hand.
Therefore, Peter Singer incurs in an intellectual error and his ethical theory is not valid.