NOTE 17A: Most cases of natural pesticide resistance do involve a single gene of large effect, as expected from this argument.
NOTE 17B: The variances should be calculated as the average squared deviation from the mean (1/n)Σi(Wi – )2. Often, the variance is estimated as [1/(n – 1)]Σi(Wi – )2, which is almost the same for n = 40 (see Chapter 28).
NOTE 17C: Recall that the selection differential is approximately the selection gradient times the phenotypic variance; see Eq. 17.3 (p. 478).
NOTE 17D: If fitness increases exponentially with the trait value and the trait is normally distributed, then selection does not alter the variance of the trait. Other forms of selection (e.g., truncation selection) do alter the variance, however.