This last point is important for students of systematics and speciation because it means that some phenotypic divergence assumed to mark species may in fact represent intraspecific alternative phenotypes or, in paleontology, morphotypes assumed to be species when they are in fact complex alternative forms that represent gene-expression, not genetic, differences between individuals. More importantly for the process of speciation, divergent developmental pathways within species enable the exploitation of different conditions and resources by members of the same species as adaptive options, and assortative mating by developmentally similar individuals can then contribute to speciation, whether in sympatry or in geographically isolated populations. In either setting, selection for a single alternative would speed the specialization of an increasingly monomorphic subpopulation, because an approach to phenotypic fixation is expected to accelerate the (genetic) evolution of the fixed form (Clarke, 1966; West-Eberhard, 2003). This acceleration would contribute to the evolution of reproductive isolation between populations with different alternative phenotypes, insofar as genetic divergence contributes to the likelihood of pre- or postzygotic reproductive incompatibility between them (for tests of this largely unexamined condition, see Funk et al., 2002).
Consistent with the hypothesis that ancestral developmental alternatives can precede and contribute to speciation, there are many species differences that parallel differences between alternative phenotypes within closely related species (Schlichting, 2004; West-Eberhard, 1989, 2003). Multiple kinds of evidence support the hypothesis that species differences originated before reproductive isolation in a variety of organisms, including buttercups, butterflies, aphids, migrant fishes and birds, and socially parasitic ants (reviewed in West-Eberhard, 2003). Intraspecific divergence in host-specific behaviors and lifehistory characteristics has repeatedly been suggested as possibly contributing to speciation in apple maggot flies (Rhagoletis) (Carson, 1989; Walsh, 1864; West-Eberhard, 2003), but the possible contribution of preisolation developmental divergence in the form of a host-associated polyphenism or polymorphism has never been systematically investigated in that genus. Despite the experimental evidence for speciationrelated preisolation phenotypic divergence in host-switching phytophagous insects, given long ago by Walsh (1864) and mentioned by Bush (1994), speciation research has focused primarily on genetic divergence accompanying or following breeding isolation in Rhagoletis and other organisms.
Research like that of Schluter and associates (2000, 1995) shows how traits such as body size, which have diverged under natural selection, can contribute to the origin of reproductive isolation. Variation in body size is often associated with the evolution of condion-sensitive, faculatatively expressed alternative phenotypes within species, including in fishes and other