Adaptive Radiation and the Topology of Large Phylogenies

The idea that some organisms possess adaptive features that make them more likely to speciate and/or less likely to go extinct than closely related groups, suggests that large phylogenetic trees should be unbalanced (more species should occur in the group possessing the adaptive features than in the sister group lacking such features). Several methods have been used to document this type of adaptive radiation. One problem with these attempts is that evolutionary biologists may overlook balanced phylogenies while focusing on a few impressively unbalanced ones. To overcome this potential bias, we sampled published large phylogenies without regard to tree shape. These were used to test whether or not such trees are consistently unbalanced. We used recently developed null models to demonstrate that the shapes of large phylogenetic trees: 1) are similar among angiosperms, insects, and tetrapods; 2) differ from those expected due to random selection of a phylogeny from the pool of all trees of similar size; and 3) are significantly more unbalanced than expected if species diverge at random, therefore, conforming to one prediction of adaptive radiation. This represents an important first step in documenting whether adaptive radiation has been a general feature of evolution.