Figure 4.

The power to estimate α using An. gambiae and An. arabiensis. The relative log-likelihood of α (the proportion of amino-acid substitutions that are adaptive) estimated using the modified McDonald-Kreitman approach [30]. The grey curve is calculated from all 102 genes for which both An. arabiensis and An. gambiae population samples were available in the dataset of Cohuet et al. [6]. The black curve shows an equivalent dataset of 102 genes from Drosophila melanogaster and D. simulans, with genes selected to be the same average length as those in the Cohuet dataset (D. J. Obbard, J. J. Welch and F. M. Jiggins, unpublished data). Despite both pairs of species having similar levels of diversity (π_s from 1.6% to 2.9%), for the Anopheles dataset the bounds (2 units of log Likelihood) stretch from -0.33 to 0.32 (and include zero) while for Drosophila the bounds only stretch from 0.26 to 0.44, and the maximum-likelihood estimate of α is 35%. The low precision in the second estimate reflects the very low power available due to the low divergence in An. gambiae-An. arabiensis comparisons