White blood cells are clearly altruistic. To the extent a species or even an ecosystem functions like an organism, there is an evolutionary advantage to evolving altruism. To quote Spock, "where the good of the many outweigh the good of the few, or the one."

There have been models that pretty convincingly show altruism arising without any special prerequisites. It's basically a balance between propagating directly versus ensuring propagation of enough of your kin. They may even be very distant kin, not sharing that many genes with you - but if your altruism helps sufficiently many by sufficiently much, you end up propagating those shared genes more successfully, in a sense of having more carriers of them in the follow-up generations. And if those shared genes include the ones that affected your altruistic behavior - again, in at least some of those you helped, not necessarily all - then it becomes a selectable trait.