FIGURE 4.3 Phylogeny of seven species and three incipient species related to A. gambiae. The likely ancestral species is A. arabiensis, which differs from A. quadriannulatus by three X-chromosome inversions and differs from A. gambiae by two additional X-chromosome inversions. Reproductive factors among these species are primarily located on the X chromosome. Several incipient species can be recognized that are related to A. arabiensis, A. melas, or A. gambiae. Three incipient species related to A. gambiae, labeled Mopti, Savanna, and Bamako, are shown.

has split into two species, A. quadriannulatus A in southern Africa and A. quadriannulatus B in Ethiopia, which have homosequential chromosomes (although two polymorphic inversions are present in species A). These two allopatric species represent relics of the ancestral species, which genically diverged from each other after their geographic distribution became discontinuous. Two lineages originated from A. quadriannulatus: one giving rise to Anopheles bwambae, with a restricted geographic presence in northeast Uganda, and Anopheles melas, with a narrow distribution along the western coast of Africa, and a second, far more important lineage giving rise to A. gambiae.

The origins of A. gambiae can be traced to the late Neolithic, <4000 B.P. A. gambiae exhibits the primitive chromosome arrangement 2R (used within the complex as the standard of reference), which is adapted to the African rain forest, where, nevertheless, A. gambiae can only breed in environments modified by human agriculture, given that the larvae are “eliophilic,” requiring sunlight for breeding (Coluzzi et al., 2002). Agriculture was introduced in Africa ≈8000 B.P., imported from Mesopotamia into the lower Nile valley, but the forest remained for a long time impenetrable, without any traces of agricultural activity up to 4000 years B.P. Extensive penetration of the forest began ≈3000 B.P., made possible by climate change and the temporary “savannization” of much of the central African rain forest, a process which began ≈2800 B.P. and lasted ≈5 centuries (Willis et al., 2004). When the forest belt regained its original range of distribution, ≈2300 B.P., it was invaded by Bantu agriculturalists who adopted “slash-and-burn” agricultural techniques. Increase in rainfall and