The "argument from species overlap" (abbreviated ASO) claims that some human and nonhuman animals possess similar sets of morally relevant characteristics, and are therefore similarly morally significant. The argument stands as a general challenge to moral theories, because many theories hold that all humans possess greater moral significance than all nonhuman animals. In this thesis I discuss responses to the ASO, primarily those of Peter Carruthers, Tom Regan, Evelyn Pluhar, and Peter Singer. Carruthers denies the conclusion of the ASO, while the other three do not. I argue that the ASO is a sound argument, and that Carruthers's attempts to counter it via his contractualist theory are unsuccessful. I next discuss the rights-based theories of Regan and Pluhar, which agree with the conclusion of the ASO but which, I believe, encounter significant theoretical difficulties. Finally, I address the ASO from a utilitarian perspective, first from Singer's utilitarian formulation and then from a "welfare-utilitarian" formulation. I answer a number of critical objections to welfare utilitarianism, and argue that the theory is most successful in facing the challenge of the ASO.