In contrast, we present a model in which assortative mating evolves in sympatry, and in which the resultant incipient species coexist.
It was Darwin (1871) who first stressed the importance of sexual selection in the divergence of incipient species, or the formation of races, and Fisher (1930) who stressed the potential of reproduction to promote genetic instabilities as a first step in the fission of species.
We have presented a model in which such shifts are not required, but this does not imply that the two mechanisms are mutually exclusive: the two mechanisms may often work together to accelerate the divergence between incipient species. Indeed, the success of our hypothesis is likely, in practice, to rest on its ability to account for those cases in which there is sympatric diversity in weakly heterogenous environments.
Not only do reproductive barriers frequently entail interactions between two or more genetically differentiated entities (e.g., incipient species
or sister species), but each of these entities may evolve in response to an array of selection pressures, including those that emanate from the other.
Evaluation of this host-shift model has concentrated on the testing of its genetic and behavioral assumptions in species thought to have recently undergone shifts (incipient species
or host races).
However, as Butlin (1987) noted, reinforcement can support speciation only if postzygotic isolation is not already complete between incipient species. The reinforcement hypothesis is also involved in other speciation mechanisms, for example, sympatric speciation (Barton and Hewitt 1985; Butlin 1987) or habitat divergence (sensu Templeton 1981).
Mating is usually one of the main behaviors capable of producing prezygotic reproductive isolation between incipient species (Spieth and Ringo 1983; Coyne 1992).
Alternatively, since the total land area of the separate islands is currently much less than that of Maui-nui, and if the periods of island separation were insufficient for complete speciation such that incipient species
coalesced when sea levels dropped, fewer species than expected may occur there.
A review of the kinds of evidence that might distinguish primary from secondary zones, and therefore clarify the origin of incipient species
, would have been a useful addition to this book.
Range expansion likely would have been characteristic of those peripherally isolated, incipient species
that survived, thereby obscuring evidence of their peripheral isolation.
An important difference between Mayr and Dobzhansky was their appraisal of reinforcement, the idea that natural selection can directly increase reproductive isolation between two incipient species
that become sympatric.