Author: Kevin de Queiroz

I’m posting these remarks at the request of Anole Annals founder Jonathan Losos in light of his suggestion that a proponent of the PhyloCode explain how this system works (with reference to anoles).  As one of the developers of the PhyloCode, as well as a systematic biologist who studies anoles, I guess I’m the logical person to do this.  These issues relate to the recent proposal to “split” Anolis into multiple “genera” following the rules of the Zoological Code (ICZN) in that the PhyloCode (ICPN) describes an alternative system for applying taxon names according to which the very idea of “splitting a genus” has no meaning (hence my use of quotation marks).  The reason is that unlike the Zoological Code, which is based on artificial ranks (e.g., genus, family), the PhyloCode is based on statements about phylogenetic relationships, which means that the PhyloCode ties names directly to clades (monophyletic groups), rather than tying them indirectly and loosely to clades through the intermediary of ranks, as in the case of the Zoological Code.  Clades are evolutionary groups about which scientists can make inferences (regarding properties such as composition, diagnostic characters, and age of origin); they are not things that scientists can “lump” or “split.”  In any case, some of the advantages of the PhyloCode are that names maintain more stable associations with clades, many unnecessary and disruptive name changes that occur under rank-based nomenclature can be avoided, clades can be named one at a time as the evidence permits (rather than requiring large-scale revisions to the taxonomy, many components of which may lack an adequate evidentiary basis), and much more information about phylogenetic relationships can be conveyed (because the system is not artificially constrained by ranks).  In the rest of this post, I’ll illustrate these points using examples involving anoles.

The Fundamental Difference

The fundamental difference between the Zoological Code and the PhyloCode concerns the way in which names are defined in the two systems.  Under the Zoological Code, the name Anolis is effectively defined as follows:  Anolis := [is defined as] the taxon ranked as a genus that contains the species carolinensis.  Now it turns out that no one has defined the name Anolis using the PhyloCode approach, which requires names to be defined explicitly.  The following examples are just two possible ways in which that name could have been defined prior to the proposal to “split” the “genus”:  Anolis := the least inclusive clade containing bimaculatus, lineatus, carolinensis, punctatus, and auratus (some of the species originally included by Daudin) or Anolis := the clade originating in the first ancestor of carolinensis that had adhesive toe pads synapomorphic with those in carolinensis (one of the diagnostic characters originally cited by Daudin).  Note that the PhyloCode style definitions tie the name directly to a clade, while that of the Zoological Code only ties the name to a taxon, which might or might not be a clade, and even if it is a clade, the tie is only indirect through the clade being ranked as a genus.  I also want to point out that PhyloCode methods for applying names are tree-based in that they require phylogenetic trees for determining the limits of the clades to which the names apply.  Although rank-based methods can be applied in the context of trees, they are not inherently tree-based in that first, their implementation doesn’t require trees (taxa can be “erected” however the taxonomist chooses), and second, the names are more strongly tied to artificial ranks (in this case the “genus”) than they are to any of the monophyletic groups (clades) implied by a tree.

Associations between Names and Clades

As a consequence of the indirect (and thus weaker) tie between names and clades under the Zoological Code, names governed by that code do not have stable associations with clades.  This should be obvious from the fact that the name Anolis is associated with a relatively large clade of ca. 385 (currently recognized extant) species according to the current widely accepted taxonomy, but that name is to be associated with a relatively small clade of ca. 44 species according to the proposed “split.”  By contrast, under the PhyloCode, names have more stable associations with clades.  Thus, if we were to adopt either of the phylogenetic definitions of the name Anolis described in the previous section, that name would apply to the same large clade of ca. 385 species under both the phylogeny of Poe (2004: Figs. 1–4), who treated the entire clade as a “genus,” and that of Nicholson et al. (2012: Fig. 4), who propose to “split” the “genus.”  The reason is that the name is defined as referring to a particular clade independent of arbitrary rank assignments (note that the phylogenetic definitions make no references to ranks).  In addition, any changes concerning hypothesized species composition under the PhyloCode can result only from revised phylogenetic inferences (i.e., new scientific results); they cannot result from artificial and non-scientific decisions to change ranks (whether a particular clade is a “genus” is not a scientific hypothesis).  Thus, if we were to adopt either of the phylogenetic definitions of the name Anolis described in the previous section, the phylogenies of both Poe and Nicholson et al. lead unambiguously to the conclusion that Anolis includes the species formerly referred to the “genera” Chamaeleolis, Chamaelinorops, and Phenacosaurus.  But this does not mean that those names must be “synonymized” with Anolis, as they would be under the rank-based Zoological Code.  Instead, the name Chamaeleolis can continue to be applied to the clade of giant twig anoles including Anolis chamaeleonides and it close relatives (rather than adopting the new and cumbersome name “Xiphosurus chamaeleonides species group” of Nicholson et al.).  Similarly, the name Chamaelinorops can continue to be applied to the clade of anoles with certain distinctive vertebral modifications that is currently considered to include only the single extant species Anolis barbouri (rather than applying that name to a larger clade including 8 other species that do not possess those vertebral modifications and were not previously included in Chamaelinorops, as Nicholson et al. were obligated to do by the rank-based Zoological Code when they chose to rank that clade as a “genus”).

Unnecessary and Disruptive Name Changes