conditions oscillates, so will the subpopulations adapted to them, resulting in population flushes and crashes. Breeding between individuals from a subpopulation with those of the central population will tend to diffuse the alleles that are distinctively adapted to the marginal conditions, except for alleles that may have been captured within a chromosome rearrangement. Interbreeding between the central and the marginal population in parapatric zones of contact will thus homogenize their genetic makeup, except for the alleles protected by the chromosome rearrangements, where new adaptive alleles will accumulate, including those that will promote reproductive isolation. Reproductive isolation will gradually evolve, yielding incipient speciation and eventually full species.
Coluzzi (1982) explored the variety of environmental and genetic conditions that may yield various possible outcomes, the most significant of which are (i) speciation, as just described, (ii) extinction of the distinctive genotypes adapted to the marginal conditions, and (iii) incorporation of the rearranged chromosomes into the main population as an adaptive polymorphism, particularly when the heterozygotes exhibit overdominance. Speciation would facilitate adaptation to the originally marginal conditions, leading eventually to full exploitation of new environments or ecological niches. The fixed X-chromosome inversions that differentiate A. arabiensis, A. quadriannulatus, and A. gambiae include reproductive isolation factors between these species and thus would have been instrumental in the speciation process, according to the suppressed-recombination model. Fixed and polymorphic inversions in other chromosomes, particularly on the right arm of chromosome 2, characterize genetically distinct subpopulations that have adapted to different regions and niches. Some are associated with incipient speciation processes that still prevail within this young species complex (see Fig. 4.2 and Coluzzi et al., 2002).
The suppressed-recombination model of chromosome speciation predicts that sympatric sister species will be more different with respect to fixed chromosome rearrangements than allopatric sister species. Such is the state of affairs prevailing among the sibling species of the A. gambiae complex. Fixed rearrangements occur between sympatric sister species, but not between the two allopatric species A. quadriannulatus A and A. quadriannulatus B, relics of a widely distributed species that genically diverged allopatrically after their geographic distribution became discontinuous.
The suppressed-recombination speciation model also predicts that while gene exchange persists between the diverging populations, genes protected by the rearrangements will accumulate allelic differences faster than genes in the colinear chromosomes, where gene flow occurs between populations. The recent origin of seven species of the A. gambiae complex and the continuing processes of incipient speciation throughout the com-