Back to 3. Natural and Sexual Selection
The effects of truncation selection across generations acts via heritable vaiation and we can visualize selection in terms of the relationship between parents and offspring. Assume that only parents with very large phenotypic values are allowed to breed. The selection differential, s, is the difference in mean of the selected parents versus to unselected parents (red vs red+blue distributions on the horizontal axis). The response to selection, R, is the change in mean of the progeny distribution relative to parents. Response to selection will always be less than the selection differential because, h2, the slope of the line is less than one, or the proportion of phenotypic variation explained by additive genetic causes should be less than one. Another way to think about the relationship between R, h2, and s is to predict R (the rise in the progeny generation) as a function of the s (the run), where rise over run is the slope (h2). The rise is a function of the the run times the slope or R = h2s.