The causes and consequences of ecological specialization, including specialization to a particular host species or group, are subjects of considerable interest, though mechanisms remain poorly understood. A prominent hypothesis is that greater host specialization effectively increases patchiness of host distributions, isolating the specialist populations into smaller units having greater barriers to dispersal. This fragmentation may predispose specialist populations to divergence, increasing the likelihood of speciation. This hypothesis predicts that specialists would tend toward smaller effective population sizes, due to reductions in dispersal and host patch sizes.
Composite-feeding species in the genus Crossidius (Coleoptera: Cerambycidae) are sympatric across broad geographical areas of western North America. Interspecific differences in diet breadth provide for a test of this prediction, using a coalescence-based approach. Analyses include estimation of the population parameter theta (Q=2NÁ) using Markov-chain Monte Carlo algorithms, for 1.4 Kbase sequences from the mitochondrial locus Cytochrome Oxidase 1. For each population locale included in these analyses, multiple individuals of the less specialized Crossidius ater and multiple individuals of one or both of the more specialized C. hirtipes and C. coralinus were collected and sequenced. Results of these analyses support the prediction that specialists have smaller effective population sizes.
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