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of speciation, genomic regions associated with reproductive isolation between D. pseudoobscura and D. persimilis show less evidence of gene flow than regions not so associated.


Fig. 4.2 shows the geographic distribution of seven species of the A. gambiae complex, and Fig. 4.3 shows their phylogeny. Epidemiologically most important are A. gambiae and Anopheles arabiensis. Where the geographic distributions of these two species overlap, there is competitive exclusion between them, with A. gambiae prevailing in the rain forests and other messic habitats and A. arabiensis prevailing in xeric habitats. The seven species are siblings that are morphologically nearly indistinguishable, but they differ in genetic and ecological attributes, including breeding sites, as well as egg configuration and some subtle morphological traits. Chromosome rearrangements are common, so that the species are identified by their chromosome configurations, primarily inversion polymorphisms of the second and third chromosomes, which are particularly extensive in A. gambiae and A. arabiensis and are distinctive for each species (Coluzzi and Bradley, 1999; Coluzzi et al., 2002).

A. arabiensis is considered the most likely ancestral species, which may have originated in the Middle East and reached Africa through the Arabian peninsula. Two sources of evidence support A. arabiensis as the ancestral species: It is the only member of the complex present in the Horn of Africa and in the Arabian peninsula, and it exhibits a fixed second-chromosome arrangement (labeled 2La) (Coluzzi et al., 2002), which is thought to be ancestral because it is also present in other species groups such as the Anopheles subpictus complex, where it is fixed in at least one of the siblings. Various sources of evidence indicate that A. arabiensis was originally zoophilic and exophilic but acquired anthropophily and domesticity secondarily in Sudan and West Africa, the regions where this species exhibits its most extensive chromosome polymorphisms. A. arabiensis may have first dispersed in East Africa starting >6,000 years ago and soon reached Madagascar, where it remains zoophilic and exophilic, having failed there to adapt to human environments perhaps because low human density has not provided suitable selective pressure for this adaptation. In East Africa, where human density is higher, A. arabiensis gradually became anthropophilic, although never to the extent of A. gambiae.

Chromosome inversion patterns indicate that A. arabiensis gave rise to Anopheles quadriannulatus, from which it notably differs by three X-chromosome inversions, where factors are located that account for the reproductive isolation between the two species and which has retained the ancestral condition of being zoophilic and exophilic. A. quadriannulatus

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