Evolutionary genetics / edited by D.L. Jameson.

Date:
[1977], ©1977
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolutionary genetics / edited by D.L. Jameson. Source: Wellcome Collection.

    308/360 (page 288)
    of natural and artificial selection 135 If \ and fj, have opposite signs, the group of highest average viability will be favoured until X exceeds - X/fjb, i.e. until 1- e ~ dw J - Л/Ц If — \/fM is fairly large, say greater than 3, this becomes approxi¬ mately И' In practice however since z rarely exceeds 10^^, the direction of selection is not likely to be reversed if — /u, < Л,/7. In the figure q is plotted against z when At = 0, X, - lOX, - 2X, - IX - p., - The maximum intensit}' of selection is reached when z is greater or less than unity according as — fx¡\ is less or greater than unity. But лvherever it is not zero the results of slight and intense competition are in opposite directions, although the required com¬ petition may sometimes be too intense or the selection too slight to be of practical importance. This is in full accordance with the views of Bidder, who points out that, where cataclasms occa¬ sionally destroy the vast majority of a species, characters which are useless or worse under normal conditions may be selected. He specially mentions the case of a violent or erratic response of an animal by migration or otherwise to unfavourable environments, which would be likely to lower the average viability, but increase its dispersion. It is easy to extend the above arguments to a population con¬ sisting of many genotypes. To take one example, suppose that fi — 0, but Л. is normally distributed лvith a standard deviation cr. Then the new frequency of any value of X will be given by v27r (T The new mean value of X will therefore be = V27r <r (l + qX)e-^^'l''' d\ \^e ~ d\, approximately, 288