 The advocates of the neutral theory initially proposed it as an unrealistic simple model only to have subsequent data on the prevalence of neutral alleles convince them of its realism. That led to more explicit testing of the neutral theory and debates over the molecular clock (Dietrich 2006). To focus on the first of these, Dietrich (2006) suggests that although the neutral theory promised to generate numerous quantitative testable predictions—and did—testing it has in fact proved difficult. Is this because of the difficulty in identifying outcomes that are unique to drift? Perhaps so. For example, the neutral theory predicted a certain measure of heterozygosity, but when Francisco Ayala and colleagues failed to observe that predicted outcome in a study of natural populations of Drosophila, Jack King responded by saying that many of the assumptions made by the model could be the source of the discrepancy. In other words, as has been well-discussed in the philosophy of science, there are no “crucial experiments” between two theories because theories are always tested together with their assumptions, any one of which can be given up rather than rejecting the theory. Debates over the so-called “molecular clock” can also be understood in terms of the search for distinctive outcomes for drift. At first, it was thought that only drift could make sense of an apparent constant rate of evolution—that only drift would produce such an outcome—yet this time it is the selectionists who modified their assumptions such that their models would also predict a molecular clock outcome (Dietrich and Skipper 2007).