INSIGHT INTO SPECIATION FROM HISTORICAL DEMOGRAPHY IN THE PHYTOPHAGOUS BEETLE GENUS OPHRAELLA

Speciation in phytophagous insects is commonly associated with shifts in host use. Using a phylogenetic framework to identify recently diverged taxa that have undergone a radical host shift, this study focuses on how reconstruction of the historical demography of a species, in conjunction with branching patterns between species, provides insight into mode of speciation. Analyses of mitochondrial cytochrome oxidase I sequences indicate that the leaf beetle Ophraella communa exhibits significant population structure, as shown by patterns of genealogical relationships among mitochondrial haplotypes and high F_ST-values. However, the absence of regional localization of old clades of haplotypes, negative Tajima's D, and unimodal rather than bimodal frequency distribution of the number of pairwise differences between sequences suggests an absence of long-term barriers to gene flow. Furthermore, we found no evidence of isolation by distance. This pattern of genetic variation is consistent with episodes of gene flow on a large geographic scale, perhaps owing to Pleistocene changes in climate. Ophraella communa and its sister species O. bilineata diverged during the early Pleistocene. The evidence of dynamic population structure in O. communa, potentially including episodic but massive gene flow, suggests that reproductive isolation evolved quite rapidly on a localized geographic scale, because speciation would probably have been reversed by gene flow if the evolution of reproductive isolation had been prolonged. That is, gene flow occasioned by range shifts during the Pleistocene would likely have interrupted speciation unless it occurred very rapidly. Sequence diversity implies a large effective population size (> 10⁶) in both O. communa and O. bilineata. However, a model based on a drastic bottleneck did not have a lower likelihood than a model with no bottleneck, simply because the time since speciation has been great enough for coalescence to a single ancestor that existed after the speciation event. Sequence diversity in itself, without reference to the time since speciation, cannot provide evidence on the demography of speciation.