Figure 6
Hypothetical evolutionary path leading to adaptive ecological divergence of some chromosome-2 variants of Anopheles gambiae molecular forms. The figure shows second-order polynomial 'species' response curves [58, 69] fitting the data set of karyotype frequencies (P < 0.01 in all cases). The response on the ordinate is a measure of relative abundance that is taken as a proxy of fitness. Axis 1 in (A) and Axis 2 in (B) on the abscissa are the same ordination axes as in Figure 5. They are interpreted to represent environmental gradients related to a major eco-geographical cline (Axis 1 – xeric conditions at higher latitudes on the left, mesic conditions at lower latitudes on the right), and to general habitat quality (Axis 2 – increasing habitat quality from left to right). The curves visualize the optimum response (the point on the abscissa falling at the maximum of the curve), and the degree of tolerance (the width of the curve around the optimum) of each karyotype along the environmental gradients. In (A), arrows point to a postulated sequence of chromosomal mutation and allele assortment events leading to a habitat shift from a monomorphic standard karyotype (00000S) to a typical Cluster 2/M karyotype (02200M) via typical Cluster 1/S (02000S) and then 'atypical' (02000M, 02100M) karyotypes. The letter "M" marks the appearance of ecologically adaptive genes in the independently segregating pericentromeric region of the X chromosome. The figure also shows that 02100S karyotypes share similar habitat optima and tolerance but lower fitness than 02000S karyotypes. In the face of competition with 02000S, therefore, 02100M and the 'atypical' 02000M karyotypes compete less against 02000S by occupying more marginal habitats, particularly on the habitat quality gradient (Axis 2 in B), compared to 02100S. In (B) it is apparent the greater degree of tolerance of M karyotypes, with optima shifted to habitat of overall lower quality relative to S. This evolutionary path is not exclusive and it is taken as an example for illustrative purposes: other paths involving different sets of karyotypes are also possible (not shown).