The genetics and ecology of sympatric speciation: A case study

Mating occurs on the larval host plant in all Rhagoletis species (Diptera: Tephritidae). We show how this attribute, when coupled with certain differences in other biological traits, strongly influences the mode of speciation. In species of the suavis species group, host shifts have never occurred during speciation, and larvae feed in the husks of any walnut species (Juglans spp.), which are highly toxic. Taxa are allopatric or parapatric and exhibit deep phylogenetic nodes suggesting relatively ancient speciation events. Traits responsible for species and mate recognition, particularly in parapatric species, are morphologically distinct and strongly sexually dimorphic. All aspects of their biology, genetics and distribution are consistent with a slow rate of allopatric speciation followed by morphological divergence in secondary contact. In contrast, speciation in the pomonella species group has always involved a shift to a new, usually unrelated, non‐toxic host, and all taxa within these groups are sympatric, monophagous and morphologically indistinguishable from one another. Phylogenetic nodes are very shallow, indicating recent sympatric speciation. Sympatric divergence is promoted by genetic variation which allows a portion of the original species to shift to a new habitat or host. Evidence suggests that changes in a few key loci responsible for host selection and fitness on a new host may initiate host shifts. By exploiting different habitats, competition for resources between diverging populations is reduced or avoided. We provide evidence that in phytophagous and parasitic insects sufficient intrinsic barriers to gene flow can evolve between sister populations as they adapt to different habitats or hosts to allow each population to establish independent evolutionary lineages in sympatry.