The “Sewall Wright Effect”

THE current state of understanding of the Theory of Natural Selection, and the degree of appreciation which it now enjoys as a presumptive agency of evolutionary change, constitute in effect a reversal of the opinions held by the majority of geneticists during the early years of the century. This reversal followed, we believe inevitably, from the better understanding afforded by the Mendelian system of the genetic structure of natural populations, and of selection within them. It is natural enough that progress in such understanding has not always been easy, and that workers with different preconceptions have not always given equal weight to the same circumstances. The widest disparity, however, which has so far developed in the field of Population Genetics is that which separates those who accept from those who reject the theory of" drift" or " non-adaptive radiation," as it has been called by its author, Professor Sewall Wright of Chicago. In a recent paper,' we criticised this theory of Sewall Wright. It claims that the subdivision of a population into small isolated or semi-isolated colonies has had important evolutionary effects; and this through the agency of random fluctuation of gene ratios, due to random reproduction in a small population. We have long felt that there are grave objections to this view, to several of which we referred, though briefly, as it was to one of them only that our new data were directly relevant. This one, however, is completely fatal to the theory in question, namely that it is not only small isolated populations, but also large populations, that experience fluctuations in gene ratio. If this is the case, whatever other results isolation into small communities may have, any effects which flow from fluctuating variability in the gene ratios will not be confined to such subdivided species, but will be experienced also by species having continuous populations. This fact, fatal to "The Sewall Wright Effect," appeared in our own researches from the discovery that the year-to-year changes in the gene ratio in a wild population were considerably greater than could be reasonably ascribed to random sampling, in a population of the size in question. We presumed that random sampling fluctuations must always be present, but that other causes must be acting