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*)Σ_{i}(*W*_{i} – )^{2}. Often, the variance is estimated as [1/(*n* – 1)]Σ_{i}(*W*_{i} – )^{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.