  A more secular philosophical concern about essentialism is tied to longstanding debates in philosophy of biology about species (see Erefshefsky 1992). Gilbert (1992) foresaw from the HGP a DNA-based definition of Homo sapiens: “At the end of the genome project, we will want to be able to identify all the genes that make up a human being. For example, we will compare the sequences of the human and the mouse and be able to determine the genes that define a mammal by this comparison…. So by comparing a human to a primate, we will be able to identify the genes that encode the features of primates and distinguish them from other mammals. Then, by tweaking our computer programs, we will finally identify the regions of DNA that differ between the primate and the human—and understand those genes that make us uniquely human” (p. 94). While it is true that any stretch of DNA that belongs to all and only humans would be among those differences found by comparing a single human genome sequence to a single nonhuman primate or mouse genome sequence, any “uniquely human” differences could not be distinguished from the others without extensive infra- and inter-specific population studies which are not part of the HGP. Even if such population studies were carried out, Gilbert's assumptions about species essentialism—that species can be defined or represented by properties (in this case, certain stretches of DNA) universally shared among, and particular to, their members—have long been challenged by philosophers of biology (Gannett 2003a; Robert and Baylis 2003). Because evolution is a gradual process where species are constantly undergoing change, Aristotelian (essentialist) definitions of species need to be abandoned; from an evolutionary perspective, in David Hull's (1994) words: “The essence of a particular species is to have no essence” (p. 215). Species should instead be defined as cluster concepts (Hull 1965) or recognized to be individuals (i.e. spatio-temporally restricted, historically contingent particulars) to which organisms belong as parts, and not classes, sets, or natural kinds at all (Ghiselin 1974; Hull 1978). 