With more than 350,000 species found almost everywhere on the planet, beetles have got the world at their wingtips. They swim, they walk, they fly, they burrow. And they eat everything from dung to each other. Now an international team has published the most comprehensive look yet at the amazing diversity of the beetle family tree--a view that may overturn a popular theory about how the insects evolved.
If an insect has hard wing covers, it's classified as a beetle and slotted into the order Coleoptera. After that, things get messy: Coleoptera contains four suborders, 17 "superfamilies,” and 168 families. The distinctions among many of these groups are not clear-cut; and until now, working out how each is related to the other has been too daunting for biologists to tackle.
Undeterred, molecular systematist Alfried Vogler of Imperial College London and the Natural History Museum in London and his colleagues compiled the existing DNA data on beetles and began building an all-inclusive family tree. They also sampled 700 additional species to fill in missing branches. In all, the team compared nearly 1900 species from 80% of the beetle families, examining three genes--instead of one as many classification efforts do--to sort out how the beetles were related. They further combined the beetle fossil record with the degree of differences in the DNA sequence of living beetles to date when the various branches arose. Filling the gaps was often tough because it required the study of species that are rare or restricted to remote parts of the world, Vogler notes.
For the most part, the twigs and branches of the new beetle family tree matched the species arrangements previously determined through traditional morphological studies. But Vogler and his team break with the idea that beetle species became so numerous only because an explosion of flowering plant species provided many new eating opportunities and living arrangements. Instead, it seems beetle diversity stems in part from this group having been around--and evolving--longer than had been appreciated. Vogler's team reports in the 21 December issue of Science that the earliest branches in the Polyphaga, the biggest group of beetles, including all major plant-feeding groups, extend back to the Paleozoic era (270 million years ago). By the time the first modern flowering plants were blossoming about 140 million years ago, more than 100 modern groups of beetles were already thriving.
Experts praise the team's exhaustive approach. "For such a huge and diverse group as beetles, this [analysis] is extremely difficult," says systematist Michael Frohlich of the Royal Botanic Gardens Kew in Richmond, U.K. "[It] convinces me that angiosperm evolution didn't potentiate the great diversity of beetles." Brian Farrell, an evolutionary entomologist at Harvard University, is also impressed with the study, but he thinks flowering plants played more of a role in beetle speciation than the researchers are giving them credit for. The emergence of flowering plants must have influenced beetle evolution, he says, given that so many of the insect species depend on such plants.