I'm having trouble seeing what evolution has to do with it. Many animals, Singer supposes, feel pain. Pain (roughly; the refinements won't matter here) is intrinsically bad, no matter what sort of creature experiences it. Whether animals (or humans) feel pain because of evolution, because a God made them that way, or because we're all sentient animaldroids, designed by mad scientists from Mars is beside the point. Singer's thought is that pain is bad for us, and animals are no different from us in that respect. He isn't making a point about the phylogenetic tree. Cabbages don't feel pain; cats do. So when we're calculating the balance of pleasure to pain, the cat's pain (or pleasure) should be included in the calculation. But since cabbages aren't sentient (so far as we know), there's nothing about the cabbage to add or subtract.
Read another response by Allen Stairs
I find Peter Singer's argument that animals' (specifically mammals') capacity to feel pain which according to him makes them intrinsically worthy of special status rather faceious as evolution is scientifically proven to not be teleological. If I uproot a cabbage (in the process killing microbes and insects) and eat it, how am I any more immoral than if I kill a cow or a dog and eat it? Why is an organism's place on the phylogenetic tree so special?