  The deeper question, of course, is how we might understand a single human genome sequence, a composite that belongs to no actual individual in its entirety and only a handful of individuals in its parts, to be representative of the entire species. This seems to ignore the extensive genetic variability which exists. The functional equivalence of many DNA polymorphisms led two early critics of the HGP to argue that “there simply is no such entity as a ‘representative sequence’ or the human (or any) genome” making it “fallacious and even dangerous to call any one ‘normal’” (Sarkar and Tauber 1991, p. 691). Another critic pointed out that problems with the idea of a representative sequence persist even when consideration is limited to DNA differences that are not functionally equivalent but related to health and disease: the sequence will contain unknown defective genes (since no one, including donors, is free of these), there is a heterogeneity of mutations even in so-called single gene diseases, and it is impossible to identify the genetic basis of a disorder simply by comparing the sequences of sick and well people since there will be many differences between them (Lewontin 2000 [1992]). For Gilbert (1992), these criticisms of representativeness arise from a failure to appreciate the difference between the approaches of molecular biologists who attend to similarities and evolutionary biologists who attend to differences within the species: “The human genome project … is directed toward a molecular biologist's view of a species rather than a population biologist's view. The latter views a species as the envelope of all possible variants that can breed together; the importance of that envelope is that different aspects of a species population will be drawn forth if you change the environment. Molecular biologists generally view the species as a single entity, sharply defined by a set of genes and a set of functions that makes up that entity” (p. 84). Gilbert held that the two approaches are consistent with each other, but many evolutionist critics of the HGP—both scientists and philosophers—did not, deriding the aims of mapping and sequencing the human genome as a throwback to anti-evolutionary, preDarwinian, typological, and essentialist  thinking.[16] The functional approach of molecular biologists alluded to by Gilbert is said to represent genetic variation in improperly normative ways, whereas “in evolutionary biology, variation is not the same as deviation” (Hull 1994, p. 208). When molecular geneticists view mutations as abnormal, not in the sense that they are rare or a change in form, but as “errors” in the genetic code or “damage” to the genome's proper structure, they impose an arbitrary a priori categorization: “it is genetic ‘errors’ that made us as a biological species: we humans are integrated aggregates of such ‘errors.’ Genetic variation is the source of evolution; it is the reason why there could be primates and not just protists or their precursors” (Limoges 1994, p. 124).