Chapter two is dedicated to the complicated contemporary debate on the notion of biological species. After a short introduction and critical analysis of all major relational and intrinsic definitions of species, special attention is paid to the recent revival of the essentialist species concept, both in its contemporary and classical Aristotelian-Thomistic formulations, tested against the two major arguments denying their compatibility with evolutionary biology.

Evolution by natural selection is not a process by which objects called “species” change over time under selective pressures. There are no such objects. It is a process by which organisms that are related in salient ways, by which they are specific we might say (as opposed to being members of a species), are replaced by others. There is more than one way in which they can be specific. One is related compatibility, which some organisms, the xs, exhibit just when they have common ancestry, when they are reproductively compatible, and when every organism so related to one of the xs is among the xs. Given this way of understanding specificity, we current humans, the Homo sapiens presently inhabiting the planet, could give way to future humans that resemble us only insofar as they are reproductively compatible with us. What is more, our humanity is not essential to us. Neither is our origin, in that we might have originated in a different time and place. So in theory – given technology that is currently out of sight – we could change ourselves into creatures that are very unlike us.