FIGURE 4.2 Geographic distribution of A. gambiae and six other closely related species. A. arabiensis, descended from a Pyretophorus species from the Arabian peninsula, is the likely ancestral species of the complex (see Fig. 4.3). Originally, A. arabiensis was zoophilic and exophilic, but it became anthropophilic and domestic by gradual adaptation to the human environment in Sudan and western Africa. A. quadriannulatus A and A. quadriannulatus B retain the original zoophily and exophily of their ancestral homonymous species, which also gave rise to A. bwambae and A. melas, and to A. gambiae, the most effective vector of malignant human malaria. A. gambiae and its strong anthropophily evolved <4000 B.P. with human invasion of the rain forest and introduction of slash-and-burn agriculture. In western Africa, A. gambiae is well represented in the Sahel region, extending up to 18° N, also the northern limit of A. arabiensis. In the Sudan, A. arabiensis, but not A. gambiae, is found along the river Nile upwards to the Egyptian border. Genetic data indicate that A. merus descends from A. gambiae and became adapted to breed in brackish, tide-dependent pools independently of A. melas.