Asked what attributes of the Creator were manifest in the natural world, the 20th-century biologist J.B.S. Haldane is said to have replied, “an inordinate fondness for beetles.” Beetles are, indeed, the most diverse group of animals on earth, accounting for something less than 40 percent out of five to ten million arthropod species, according to one estimate [PDF]. Naturally, evolutionary biologists would like very much to know how there came to be so many beetles* — and a new paper in this week’s PNAS proposes to answer this question for the largest beetle groups, the weevils.
It seems unlikely to be a coincidence that beetles are widely involved in interactions with the most diverse group of land plants, the angiosperms. In a now-classic 1998 paper, which took Haldane’s apocryphal quip as its title, Brian Farrell presented good circumstantial evidence that living and feeding on flowering plants is associated with beetle diversity. Farrell compared the number of species in groups of angiosperm-feeding beetles with the number of species in closely-related groups of non-angiosperm-feeders, and found that angiosperm-feeding groups were more diverse by orders of magnitude [$-a].
Interactions between beetles and their host plants could lead to hyper-diversity in two ways. The evolution of new plant defenses and herbivore counter-defenses could generate alternating cycles of diversification in each interacting group [PDF]. Under this process, diversification doesn’t really happen because of reciprocal natural selection between plant and herbivore — it occurs when plants “escape” their herbivores by virtue of a new defense mechanism, and when herbivores exploit a new food resource made available by innovative counter-defenses. Alternatively, plants and beetles might diversify more simultaneously, with natural selection from plants’ defenses actually driving the speciation of the insect populations that eat them, and vice-versa.
The new paper, on which Farrell is senior author, attempts to distinguish between these two possible scenarios [$-a] using a new phylogeny of the Curculionoidea, the superfamily of beetles more commonly known as weevils. Weevils are distinguished by the rostrum, a noselike appendage they use in feeding — and the estimated 220,000 weevil species feed on an enormous array of plant species. Using DNA sequence data, the paper’s authors reconstructed the evolutionary relationships between 135 weevil genera. They then calibrated the resulting evolutionary tree using the known dates of fossil weevils, so that they could compare the dates of origin of major weevil groups to the history of angiosperm diversification.
Based on this analysis, the oldest weevil groups had their origin millions of years before the first flowering plants. Many of the extant species in these groups still feed on gymnosperms, which predate flowering plants. The most diverse weevil families, which feed on angiosperms, did not emerge until well after the first flowering plants appear in the fossil record, and may not have diversified until angiosperms became the dominant land plants. This lag suggests that, at least on a very broad time scale, weevils diversified because of angiosperm diversity, but probably did not contribute much to creating that diversity:
Thus, the extraordinary taxonomic diversity of weevils appears to have been mediated predominantly by the presence of susceptible, abundant, and diverse host resources, and the ability of weevils to use those resources, rather than by the evolution of host taxa themselves.
In the strictest sense, then, it seems that coevolution isn’t responsible for weevil diversity — yet it is hard to conclude much from results at this broad scale. As weevils took advantage of the “ecological opportunity” created by angiosperm diversity, they would have created myriad opportunities for reciprocal natural selection. Patterns of strict-sense coevolution following the initial colonization of angiosperms may only be apparent over shorter time spans.
Ehrlich, P.R., & Raven, P.H. (1964). Butterflies and plants: A study in coevolution Evolution, 18, 586-608 DOI: http://www.jstor.org/stable/2406212
Farrell, B. (1998). “Inordinate Fondness” explained: Why are there so many beetles? Science, 281 (5376), 555-9 DOI: 10.1126/science.281.5376.555
McKenna, D., Sequeira, A., Marvaldi, A., & Farrell, B. (2009). Temporal lags and overlap in the diversification of weevils and flowering plants PNAS, 106 (17), 7083-8 DOI: 10.1073/pnas.0810618106
Ødegaard, F. (2000). How many species of arthropods? Erwin’s estimate revised Biol. J. of the Linn. Soc., 71 (4), 583-97 DOI: 10.1111/j.1095-8312.2000.tb01279.x
* Apart, that is, from the untestable and ultimately unknowable preferences of any putative Creator.