Figure WN19.3 - Deviations from the standard model of neutral evolution can be detected by comparing different measures of genetic diversity.

Figure WN19.3. Deviations from the standard model of neutral evolution can be detected by comparing different measures of genetic diversity. In each of these diagrams, there is a sample of 6 genes, which includes 11 segregating variants (crosses). This gives an estimate of θW = 11 / = 4.82 (A) The average number of pairwise differences is θπ = 5.4. These estimates of diversity are similar; this example is typical of the standard neutral coalescent. (B) A severe bottleneck or selective sweep leads to a “star” genealogy, with an excess of rare variants. Here, θπ = 2.67, and so Tajima’s D = (θπ – θW)/s.d. is negative. (C) In this example, a partial selective sweep has raised five variants to high frequency (green). θπ = 3.6, so that Tajima’s D is weakly positive. Fay and Wu’s (2000) measure of diversity, which gives greatest weight to derived variants at high frequency, is θH = 8.93. Hence, their H statistic is strongly positive, indicating a partial selective sweep.