University of Arizona, Tucson
A biologist turns his attention to evolution’s neglected radiations.
Evolution’s spectacular adaptive radiations get a lot of press: Darwin’s finches and the Hawaiian silversword plants being textbook examples. These organisms, in adapting to environmental pressures, underwent both rapid speciation and radical morphological change.
Such episodes give rise to easily observable diversity and have stimulated extensive study. But how about those hyperdiverse clades in the tree of life in which many species have little morphological difference between them?
I first pondered this problem when musing about my thesis on the flowering-plant taxon Astragalus. I was cursed with perhaps 2,500 species, many remarkably similar. Their small differences were typically of uncertain adaptive significance.
Alas, I have counted barely ten papers since then that have addressed such radiations, which end up being labelled as ‘non-adaptive’. I hope the most recent will shake things up a bit.
It analyses the speciation rate of North American Plethodon, a clade of salamanders most diverse in the woodlands of the Appalachian mountains (K. H. Kozak et al. Proc. R. Soc. B. 273, 539–546; 2006). This group has an evolutionary history that runs back 28 million years and has spun off about 46 species, many of which are only diagnosable by molecular markers.
Remarkably, the rate of speciation in the group’s early days matched or exceeded rates seen in the textbook adaptive radiations. This suggests that we have a lot to learn about the evolutionary phenomena driving such radiations.
The authors make some interesting suggestions about the role of geography, ecology and adaptation in the salamanders’ evolution. For example, the lineages may have evolved by tracking the ebb and flow of favourable habitats.