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drift, particularly if hybrid underdominance is only slight (in which case natural selection for reproductive isolation will be weak as well).

A speciation model of suppressed recombination was proposed by Coluzzi (1982) in his account of multiple speciation events within the species complex related to Anopheles gambiae, the main vectors in Africa for the transmission of malignant malaria, caused by the protozoan Plasmodium falciparum. The World Health Organization estimates that there are annually 300–500 million cases of malaria and >1 million deaths, mostly in sub-Saharan Africa, and the most of those who die are children. Seven species have been identifiedwithin the A. gambiae complex and have arisen, according to Coluzzi (1982; Coluzzi et al., 2002), within the last 5,000 years.

Suppressed-recombination models of speciation have recently been proposed by Rieseberg (2001) to account for speciation in wild sunflowers (Riesberg et al., 1995); by Noor et al. (2001a,b) and Machado et al. (2002), using evidence from the closely related Drosophila persimilis, Drosophila pseudoobscura, and D. p. bogotana; and by Navarro and Barton (2003a,b), who have mathematically modeled the process and supported the model’s predictions by comparing genomic DNA sequences between humans and chimpanzees.

We will successively examine the human–chimpanzee and Drosophila evidence advanced in support of the model and then return to speciation in the A. gambiae complex.

HUMAN SPECIATION

Genomic studies have confirmed that a substantial number of chromosomal rearrangements have occurred between humans and chimpanzees. In particular, nine chromosomes (nos. 1, 4, 5, 9, 12, 15, 16, 17, and 18) exhibit pericentric inversions between humans and chimpanzees, and human chromosome 2 represents a fusion of two acrocentric chromosomes present in chimpanzees (chromosomes 12 and 13) and other great apes (chromosomes 11 and 12 in gorillas and orangutans) (de Boer and Seuánez, 1982; Yunis and Prakash, 1982). If these chromosomal rearrangements occurred early in the divergence between ancestral populations of chimps and humans, they “would facilitate genic divergence during the time when the diverging populations are in parapatry, i.e., have limited gene flow” (Navarro and Barton, 2003b). The hypothesis proposes that alleles favored in one or the other population will be trapped at the chromosomal barrier and thus would cause the two populations to diverge genically as they adapt to their distinct prevailing environmental conditions. Accumulation of incompatibilities would gradually result in reproductive isolation and speciation. In regions not protected by chromosome



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