ON THE ROLES OF DIRECTED AND RANDOM CHANGES IN GENE FREQUENCY IN THE GENETICS OF POPULATIONS

Science has largely advanced by the analytic procedure of isolating the effects of single factors in carefully controlled experiments. The task of science is not complete, however, without synthesis: the attempt to interpret natural phenomena in which numerous factors are varying simultaneously. Studies of the genetics of populations, including their evolution, present problems of this sort of the greatest complexity. AMany writers on evolution have been inclined to ignore this and discuss the subject as if it were merely a matter of choosing between single factors. Mly own studies on population genetics have been guided primarily by the belief that a mathematical muodel must be sought which permits siinultaneous consideration of all possible factors. Such a model must be sufficiently simple to permit a rough grasp of the system of interactions as a whole and sufficiently flexible to permit elaboration of aspects of which a more complete account is desired. On attempting to make such a formnulation (\Vright, 1931) it was at once apparent that any one of the factors

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