Distinguishing Coevolution from Covicariance in an Obligate Pollination Mutualism: Asynchronous Divergence in Joshua Tree and its Pollinators

Obligate pollination mutualisms-–in which both plants and their pollinators are reliant upon one another for reproduction-–represent some of the most remarkable coevolutionary interactions in the natural world. The intimacy and specificity of these interactions have led to the prediction that the plants and their pollinators may be prone to cospeciation driven by coevolution. Several studies have identified patterns of phylogenetic congruence that are consistent with this prediction, but it is difficult to determine the evolutionary process that underlies these patterns. Phylogenetic congruence might also be produced by extrinsic factors, such as a shared biogeographic history. We examine the biogeographic history of a putative case of codivergence in the obligate pollination mutualism between Joshua trees (Yucca brevifolia) and two sister species of pollinating yucca moths (Tegeticula spp.) We employ molecular phylogenetic methods and coalescent-based approaches, in combination with relaxed-clock estimates of absolute rates of molecular evolution, to analyze multi-locus sequence data from more than 30 populations of Y. brevifolia and its pollinators. The results indicate that the moth species diverged significantly (p < 0.01) more recently than their corresponding host populations, suggesting that the apparent codivergence is not an artifact of a shared biogeographic history.