Is specialization a dead end? The phylogeny of host use in Dendroctonus bark beetles (Scolytidae)

Ecological explanations for the prevalence of resource specialists are abun dant, whereas phylogenetic evidence on their origins is scarce. In this pap er, we provide a molecular phylogenetic study of the 19 specialist or gener alist species in the bark beetle genus Dendroctonus, which collectively att ack species in four different genera in the conifer family Pinaceae. Given substantial variation in diet breadth, we asked two general questions conce rning the evolution of resource use in this group. How conservative is the evolution of host use in these insects? Does specialization tend to be deri ved (i.e., a "dead end")? To answer these questions, we estimated the phylogeny of Dendroctonus using mitochondrial DNA sequences and mapped transitions in resource use on the resulting phylogeny estimate. The evolution of affiliations with Pinus and Picea hosts in Dendroctonus was conservative among beetle species (PTP test ; P < 0.012), but there was no significant correspondence between the phylo geny of these beetles and the phylogeny among their Pinaceae hosts (among g enera, P = 0.28; among Pinus species, P = 0.82). Degree of specialization, as measured in the proportion of hosts used, was bimodally distributed with "generalist" species utilizing greater than or e qual to 60% of the congeneric hosts within their range and six specialist s pecies utilizing less than or equal to 40% of the available hosts. Among th e generalists, we found a strong correlation between the number of hosts en countered and the number of hosts utilized (R = 0.97, P < 0.0001), whereas there was no significant correlation among the specialists (R = 0.27, P = 0 .59). The evolution of specialization in Dendroctonus proved highly labile- specialists arose from generalists at least six separate times (without rev ersal) all in derived positions, and closer examination of some specialists revealed instances where they appear to have lost particular host species from their diet. However, evidence from the ecological literature also sugg ests that several Dendroctonus generalists may have increased their range o f host genera within the Pinaceae.