the return of the forest were determinant factors for the spreading of the tze-tze fly (Glossina), vector of the lethal animal trypanosomiasis (Trypanosoma brucei), which decimated cattle, thus promoting the adaptation of A. quadriannulatus to humans, who became an easy host for the blood meal and, more importantly, caused the opening of the forest canopy. These conditions promoted strong selection for anthropophily and domesticity, which facilitated the evolution of A. gambiae (Coluzzi, 1982; Willis et al., 2004). There was a consequent increase in the rate of human infection by P. falciparum, which in turn led to strong selection for highly virulent strains, such as are the current forms of this parasite. Genetic evidence indicates that the expansion of malignant P. falciparum in Africa and throughout the world tropics occurred only within the last few thousand years (Hartl, 2004; Rich et al., 1998; Tishkoff et al., 2001).
A. gambiae differs from its ancestral A. quadriannulatus by two X-chromosome inversions, where factors responsible for their reproductive isolation are located (as is the case for the three X-chromosome inversions fixed between A. arabiensis and A. quadriannulatus; see above). A. gambiae has a nearly continentwide distribution in sub-Saharan African and is highly anthropophilic, like A. arabiensis, with which it accounts for most of the transmission in Africa of P. falciparum (but surpasses it). Further speciation is occurring within A. gambiae (as well as within A. arabiensis and A. melas). In southern Mali and northern Guinea, there are three chromosomally distinct populations of A. gambiae that are partially sympatric or parapatric and manifest enough assortative mating to qualify as incipient species. These populations are named “Savanna,” “Mopti,” and “Bamako” (Fig. 4.3). Chromosome-rearrangement evidence indicates that A. gambiae is the ancestor of A. merus, a species adapted to breed in tide-dependent pools.
A suppressed-recombination model of speciation was proposed by Coluzzi (1982) to account for the speciation patterns elucidated in the A. gambiae complex of species, particularly the successive adaptation of new species to environments quite different from the xeric conditions to which A. arabiensis was originally suited, such new environments as the rain forest or brackish waters. In outline, the essential components of the model are as follows (Coluzzi, 1982). A chromosomally monomorphic population colonizes a suitable environment and expands in numbers. The population will encounter adaptively marginal conditions, whether at the periphery of its distribution or at different times as a consequence of the seasons or other climatic and ecological oscillations. Mutations that increase adaptation to these marginal conditions will be favored wherever such conditions exist, yielding new “ecotypes” [i.e., “intraspecific groups having distinctive characters that result from the selective pressure of the local environment” (Lincoln et al., 1998)]. As the prevalence of the marginal