Isolating a role for natural selection in speciation: Host adaptation and sexual isolation in Neochlamisus bebbianae leaf beetles

Muller (1942) and Mayr (1963) hypothesized that natural selection indirectl y causes the evolution of reproductive barriers between allopatric populati ons by causing adaptive genetic divergence that pleiotropically promotes pr ezygotic or postzygotic incompatibility. Under this mechanism, herbivorous insect populations should be more prone to speciate if they are adapting to different host plants, because the evolution of reproductive isolation wil l be accelerated above the rate promoted by genetic drift and host-independ ent sources of selection alone. Although the Muller-Mayr hypothesis is wide ly accepted, little direct evidence has been collected in support of select ion's role in allopatric speciation. This paper offers a method for isolati ng and evaluating the contribution of host plant-related natural selection pressures to the reproductive isolation between allopatric herbivore popula tions. The host-related selection hypothesis (HRSH) predicts that herbivore popula tions using different host plants should be more reproductively isolated th an those using the same host, other things being equal. Here, I test this h ypothesis using Neochlamisus bebbianae, an oligophagous leaf beetle with a geographically variable host range. In each of two sets of experiments (con trast I, contrast II), I compared two beetle populations (Georgia and New Y ork) that use the same host (Acer) in nature and a third population that na tively uses a different host (Betula in Oklahoma [CI], Salix in Ontario [CI I]). Experiments showed that "different-host" populations were more strongl y differentiated in host-use traits (oviposition, host fidelity, feeding re sponse, larval performance) than were "same-host" populations and that each population most readily uses foliage from its native host. As predicted by the HRSH, sexual isolation was also greater between the adaptively diverge nt different-host populations (from Betula vs. Acer, from Salix vs. Acer) t han between the same-host populations (from Acer), which were undifferentia ted in host-use traits. Interpreting these results in a historical context provided by mtDNA sequen ces from test populations indicated: (1) that Acer- and Betula-associated N . bebbianae represent separate sibling species whose causal origins have be en lost to history, and whose incomplete sexual isolation is fortified by h ost-associated ecological and "physiological" isolation; and (2) that incip iently speciating Acer- and Salix-associated populations are more closely r elated to each other than are the two Acer-associated populations, which is consistent with the HRSH. This study thus illustrates the consequences of host-related selection for both the origin and maintenance of reproductive isolation. More important, it provides evidence that the pleiotropic effect s of natural selection promote allopatric speciation.