 In his response to their analyses, Wright (1948) pointed out, first, that his theory of evolution explicitly involved the simultaneous action of several forces (selection, drift, mutation, and migration) and he emphatically rejected the paradigm of Fisher and Ford that either selection or drift alone had to be responsible for all of the observed fluctuation in gene frequencies (see the entry on  population genetics).  Wright noted that, in order to reach their statistical conclusion, Ford and Fisher had to include gene frequency data from a decade before the more careful study, notably a period without any estimates of population size. Without this earlier data point, the average fluctuations were much smaller and not significant. He pointed out that, like the mark-recapture estimates of population numbers, the gene frequencies themselves were estimates whose variation, based on the reported sample sizes, accounted for more than half (55.2%) of the observed variance that Fisher and Ford were trying to explain. He then showed that, if one assumed only the unitary explanation of natural selection, then the observed gene frequency fluctuations were so large even without the sampling variance that the temporal variations in the allelic selection coefficients must range from near lethality (or sterility) to tremendous advantage (i.e., from −0.50 to +0.50). However, Fisher and Ford (1947) provided no indication of comparable levels of temporal variation in any environmental factor acting as a selective agent. Wright argued that the effective population sizes used in the analysis were almost certainly too large, possibly by an order of magnitude, and that Fisher and Ford had made no attempt to estimate the factors expected to reduce effective size, like temporal variation in breeding numbers, non-random mortality among larvae (mortality clustered within families as might affect a species which experiences &gt; 85% pupal mortality owing to viral infection), or other causes of the variance in offspring numbers (such as variation among females in egg numbers or variation among males in mate numbers). In an unyielding reply, Fisher and Ford (1950) labeled chance or random fluctuations in gene frequency, the Sewall Wright Effect, a term which has endured to the present day as a synonym for random genetic drift.