Altruism in this technical sense can evolve in two main ways. First, since relatives share genes, any gene that inclines an organism toward helping a relative will increase the chance of survival of a copy of itself that sits inside that relative, even if the helper sacrifices its own fitness in the generous act. Such genes will, on average, come to predominate, as long as the cost to the helper is less than the benefit to the recipient discounted by their degree of relatedness.
