Dan Rabosky and co-authors have just published an important report on patterns of organismal diversity in PLOS Biology, with one of their main conclusions being that clade age does little to explain species richness. Luke Harmon has a commentary on this article in the same issue of PLOS Biology, and I’ll refer readers there for a general summary of the work’s implications. I wanted to give this article a shout-out here at Anole Annals because they used an anole as their icon for squamates in Fig. 3 (see above).
Inspection of their supplemental Table 2 and consultation with the authors, however, reveals that anoles were inadvertently left out of the final analyses due to a book-keeping error involving use of the timetree age for Iguanidae sensu Schulte et al. 2003 but the species richness for Iguanidae sensu Frost & Etheridge 1989. (A quick taxonomic review for the uninitiated: The family diagnosed as Iguanidae by Frost and Etheridge included only a subset of the species previously regarded as members of the much larger family Iguanidae. Frost and Etheridge assigned Anolis and many of the other genera previously included in Iguanidae to other newly defined families. They considered this taxonomic revision necessary because they did not recover a monophyletic Iguanidae sensu lato. Because molecular phylogenetic analyses do tend to recover a monophyletic Iguanidae sensu lato, some subsequent authors, including Schulte et al. 2003, have advocated retention of Iguanidae sensu lato and treatment of Frost & Etheridge’s families as subfamilies [see Daza et al. 2012 for another perspective on this taxonomic debate].)
If we imagine crudely adding a circle to represent Anolis in Rabosky et al.’s figure 3 (assuming an age of ~50 mybp and species richness of ~400 for the genus), its clear that anoles would be among the youngest, yet also most species rich, of all squamate clades, providing further support for Rabosky et al.’s main conclusion that clade age has little role in explaining clade richness.
When alerted of this issue, Rabosky and his co-authors re-ran their analyses including anoles and their relatives (i.e., Polychridae/Polychrotidae of Frost and Etheridge) as well as all of the other Frost and Etheridge families that were overlooked for the same reason (e.g., Tropiduridae, Phyrnosomatidae). Rabosky sent me a figure that illustrates the position of all these missing clades (in blue), including the clade that includes Anolis (in red) as well as the other squamate clades in the original analysis (in grey). Because many of these clades stem from series of basal branching events within Iguanidae sensu lato and are relatively similar in age, they rather nicely illustrate the reported absence of a correlation between clade age and species richness. Not surprisingly, Rabosky et al.’s overall conclusions about clade age and species richness are unchanged by inclusion of these additional datapoints.
At the end of the day, this discussion nicely illustrates how monkeying around with the names of formal Linnean ranks can cause chaos for anyone who is not intimately familiar with a particular name’s complete history.
Rabosky, D. L., G. J. Slater, and M. E. Alfaro (2012). Clade Age and Species Richness Are Decoupled Across the Eukaryotic Tree of Life PLOS Biology DOI: 10.1371/journal.pbio.1001381
- JMIH 2014: Early Records of Fossil Anolis from the Oligocene and Miocene of Florida, USA - August 13, 2014
- JMIH 2014: Relative Contribution of Genetic and Ecological Factors to Morphological Differentiation in Island Populations of Anolis sagrei - August 7, 2014
- JMIH 2014: The Ultrastructure of Spermatid Development within the Anole, Anolis sagrei - August 5, 2014
Skip Lazell
Iguanidae sensu lato: keep it!
Jonathan Losos
This mistake clearly illustrates the downside of the common practice of splitting long-recognized taxonomic groups into many taxa. Like it or not, the fruits of systematics are used by the rest of biology, and changing names does come at a cost of sowing confusion. In this case, the benefits of a taxonomy recognizing subclades within the old Iguanidae is countered by the loss of information in such a phylogeny that these subclades form a larger clade, the old Iguanidae, and one for which there is a large literature.
Harry Greene
Isn’t it the content of named clades rather than “formal Linnean ranks” that’s at issue?
Rich Glor
I don’t think this problem would have occurred in a rank free system because taxonomists would have just come up with new names for the clades that Frost and Etheridge diagnosed rather than transferring the family name Iguanidae from one clade to another.
Jonathan Losos
I agree. If we want our classification system to reflect phylogeny, then we have to give up on the idea that a binomial system can do the job. This is especially true when, because of the rules of priority and custom, old names get re-used, but with different contents.
Shai Meiri
For the Rabosky et al. analysis it would not have mattered because they use clades, which can work regardless of rank (unless you make the above-mentioned mistake of using one meaning of the name for richness, another for age), but the assumption – a naive version of the cone of ever increasing diversity, ignores the fact that while clades are born with a single species, they also die with one – for each waning clade there will be a waxing one, so what they got is the expectation, not simply the null