ISOLATION BY DISTANCE

Abstract

important S line of attack on the evolutionary problem.While such differences can only rarely represent first steps toward speciation in the sense of the splitting of the species, they are important for the evolution of the species as a whole.They provide a possible basis for intergroup selection of genetic systems, a process that provides a more effective mechanism for adaptive advance of the species as a whole than does the mass selection which is all that can occur under panmixia.TUDY of statistical differences among local populations is RANDOM DIPPERENTIATION UNDER THE ISLAND MODEL Mathematical consideration requires the use of simple models of population structure.The simplest model is that in which the total population is assumed to be divided into subgroups, each breeding a t random within itself, except for a certain proportion of migrants drawn a t random from the whole.Since this situation is likely to be approximated in a group of islands, we shall refer to it as the island model.The gene frequency (4) of a subgroup tends to vary about a certain equilibrium point (q) in a distribution curve (+(q)) determined by the net sys-* A portion of the cost of composing the mathematical formulae is borne by the Galton and Acknowledgment is made to the DR.WALLACE C. and CLARA A. ABBOTT MEMORIAL FUND Mendel Memorial Fund.of the UNIVERSITY OF CHICAGO for assistance in connection with the calculations.