Coleoptera (beetles) are one of the most successful groups of organisms on Earth. Their success in evolutionary terms is recognised by their extreme adaptive diversity (occupying almost every possible ecological niche) and their longevity (fossils from the Palaeozoic, 280 million years ago). But most of all, their success is indisputable in their sheer species numbers: with over 350,000 named species and many more to be described, they constitute about one fourth of all species on the Planet!
It is commonly accepted that phytophagous beetles and their host plants (mainly the likewise speciose angiosperms or flowering plants) have radiated in concert since the origin of both groups in the early Cretaceous. Indeed, this is a text-book example of coevolution and a straightforward interpretation of the forces driving evolution and the rise of new species.
However, a new molecular study by Dr. Jes๚s G๓mez-Zurita and collaborators in the Natural History Museum in London challenges this view. This study shows that at least in the leaf beetles (Chrysomelidae; 40,000 species) this association is apparently out of sync. A time-calibrated phylogenetic tree based on three genes of ribosomal RNA and the most extensive sampling of leaf beetle species to date shows that the phylogenies of both groups, beetles and plants, are neither congruent, nor are they contemporaneous.
Instead, the Chrysomelidae are likely to be younger by several tens of millions of years, placing their origin in the very latest Cretaceous. This finding is consistent with the fossil record of Chrysomelidae which is much younger than that of angiosperms. In addition, chrysomelids feeding on the ancient lineage of monocotyledons (grasses, palms, etc.) have arisen at least on two occasions, refuting earlier claims of their monophyly, their unique evolutionary origin, and illustrating the importance of host shifts.
Hence, the coevolution hypothesis can be rejected in this case, demonstrating a different, perhaps more sophisticated principle of speciation in such diverse groups: phytophagous chrysomelid beetles radiated on a pre-existing diverse resource.
Citation: G๓mez-Zurita J, Hunt T, Kopliku F, Vogler AP (2007) Recalibrated Tree of Leaf Beetles (Chrysomelidae) Indicates Independent Diversification of Angiosperms and Their Insect Herbivores. PLoS ONE 2(4): e360. doi:10.1371/journal.pone.0000360 (http://dx.doi.org/10.1371/journal.pone.0000360)
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