What makes someone a human being? The idea that each human being shares with every other human being but with nothing else some essential, human-making feature goes back at least to Aristotle. He thought that each species was defined by an "essence" - a set of properties found in each individual of the species, but only there. That essence makes it the sort of creature that it is. [...]
In reality, however, there is no such thing as the "genetic essence" of a species. A central aspect of modern evolutionary theory is population thinking. Each population is a collection of individuals with many genetic differences, and these differences are handed on to future generations in new combinations. Populations change generation by generation. In many contemporary views of the nature of species, there is no upper limit to the amount of evolutionary change that can take place within one species. Over many generations a species may be transformed in appearance, behavior, or genetic constitution while still remaining the same species. Diversity is normal, and perhaps even functional, for lack of diversity makes a species vulnerable to parasitism and to extinction due to environmental change. So uniform populations in the natural world are unusual. [...] Contemporary views on species are close to a consensus in thinking that species are identified by their histories. According to these views, Charles Darwin was a human being not by virtue of having the field marks-rationality and an odd distribution of body hair-described (in Alpha Centaurese) in A Guide to the Primates of Sol, but in view of his membership in a population with a specific evolutionary history. [...]
The implications of this transformation of our view of species have been much discussed in philosophy of biology, although they have been surprisingly neglected in ethics. David Hull, in particular, has argued that nothing in biology corresponds to the traditional notion of "human nature". This idea is significant, for the concept of human nature has been historically important. It has underwritten the view that there is some way that human beings are supposed to be, and that other ways of being are deviant or abnormal. [...]
Biology is often supposed to provide some backing for this notion of normality: that there is a way that members of any species - including Homo sapiens - are meant to be, and that deviations from this are abnormal. But Darwinian species are continually evolving clusters of more or less similar organisms. There is nothing privileged about the current statistical norm. [...]
So no general biological principle suggests that human moral feelings, mental abilities, or fundamental desires should be any more uniform than human blood type or eye color.
- Sterelny, Kim and Griffiths, Paul E.: Sex and Death. An Introduction to Philosophy of Biology. Chicago: University Of Chicago Press 1996. p 7-8.