Using trees to learn about the evolution of diversity: The beetles' diet
If you were to randomly pick an extant animal species, odds are that it would be a beetle. While there are 250,000 described species of plants, 12,000 described species of roundworms, and only 4,000 described species of mammals, there are over 350,000 beetle species described, with many more beetles yet to be discovered!
What accounts for all these beetles? Brian Farrell hypothesized that their diets might have something to do with it, and he performed a phylogenetic study to test that idea. He reconstructed the phylogeny of all the major groups of beetles and noted their feeding characteristics. This research allowed him to infer what the ancestral beetles were likely to have been eating and when each lineage switched to a new type of food. His evidence suggests that different beetle lineages switched to feeding on flowering plants (angiosperms) several times during their evolutionary history.
To understand what happened when these switches occurred, Farrell compared sister groups. He saw the same pattern again and again (as shown below): the lineage that switched to angiosperms speciated frequently and became very diverse, while the lineage that did not switch to angiosperms had a lower rate of speciation and did not become very diverse. Feeding on angiosperms is associated with higher rates of speciation (or lower rates of extinction it's hard to tell). This link between food and diversity is particularly compelling because it has played out several times in beetle evolution nature replicated the same experiment over and over again.
What remains to be discovered is why switching to angiosperm feeding is associated with beetle radiations. One possibility is that switching to angiosperms provided beetles an entrée to new niches. Consistent with this explanation is the fact that once beetle lineages switched to angiosperms, some of them diversified into lineages that specialize, feeding on different parts of the plant (root, seed, leaf, etc.). This diversification would then constitute an adaptive radiation. However, this explanation still needs to be tested with more data.