Insight into speciation from historical demography in the phytophagous beetle genus Ophraella

Speciation in phytophagous insects is commonly associated with shifts in ho st use. Using a phylogenetic framework to identify recently diverged taxa t hat have undergone a radical host shift, this study focuses on how reconstr uction of the historical demography of a species, in conjunction with branc hing patterns between species, provides insight into mode of speciation. An alyses of mitochondrial cytochrome oxidase I sequences indicate that the le af beetle Ophraella communa exhibits significant population structure, as s hown by patterns of genealogical relationships among mitochondrial haplotyp es and high F-ST-values. However, the absence of regional localization of o ld clades of haplotypes, negative Tajima's D, and unimodal rather than bimo dal frequency distribution of the number of pairwise differences between se quences suggests an absence of long-term barriers to gene flow. Furthermore , we found no evidence of isolation by distance. This pattern of genetic va riation is consistent with episodes of gene flow on a large geographic scal e, perhaps owing to Pleistocene changes in climate. Ophraella communa and its sister species O. bilineata diverged during the e arly Pleistocene. The evidence of dynamic population structure in O. commun a, potentially including episodic but massive gene flow, suggests that repr oductive isolation evolved quite rapidly on a localized geographic scale, b ecause speciation would probably have been reversed by gene flow if the evo lution of reproductive isolation had been prolonged. That is, gene flow occ asioned by range shifts during the Pleistocene would likely have interrupte d speciation unless it occurred very rapidly. Sequence diversity implies a large effective population size (> 10(6)) in b oth O. communa and O. bilineata. However, a model based on a drastic bottle neck did not have a lower likelihood than a model with no bottleneck, simpl y because the time since speciation has been great enough for coalescence t o a single ancestor that existed after the speciation event. Sequence diver sity in itself, without reference to the time since speciation, cannot prov ide evidence on the demography of speciation.