Change of gene frequencies by natural selection under population number regulation

By incorporating a population number regulating mechanism into the formulation of genic selection involving a pair of alleles (A1 and A2) with respective frequencies x and I-x, it is shown that the change of x in one generation is given by deltax = sx(1-x)/W, in which W is the mean absolute selective value (in Wright's sense). It is also shown that, in the process in which advantageous allele (say A1) increases from a low frequency to a high frequency, quasi-equilibrium is rapidly attained where deltaW approximately 0. In this state we have W approximately 1 + (s2/c)x(1-x) in the case of logarithmic population number regulation, and W approximately 1 + s2x(1-x)/(cN) in the case of logistic regulation. In these expressions, s is the selective advantage of A1 over A2, and c is a coefficient relating to the total population number regulation. It is pointed out that the approximation formula deltax = sx(1-x) is valid under wider circumstances than usually suggested by the conventional treatment of genic selection.