Proposal (468) to South American Classification Committee
Species
limits in Arremon torquatus
Background: Species limits in Arremon
(formerly Buarremon and Atlapetes) torquatus have been controversial over the years, as reviewed by
Remsen and Graves (1995) and more recently by Cadena and Cuervo (2010). The
complex consists of 14 forms, currently treated as subspecies, ranging from
Costa Rica to Argentina; 12 of these occur in the area of SACC coverage (13 if
one counts the form tacarcunae from
eastern Panama). The difficulty in establishing species limits in the group
despite the existence of marked geographic variation has been, as usual for
many groups, the allopatric distribution of nearly all of these forms.
New analysis: Cadena and Cuervo (2010[CC1])
examined species limits in the group by using a molecular phylogenetic analysis
(Cadena et al. 2007) as a basis to examine patterns of phenotypic, vocal, and
ecological variation in the group. In short, they found that forms assimilis from high elevations of the
northern Andes and atricapillus from
lower elevations of Panama and Colombia can be considered geographically
sympatric because their favored environments are intermingled over a large geographic
area where they are not separated by any physical barriers. Despite their
sympatry, the two forms are strongly differentiated genetically (they are not
even sister taxa and differ by more than 6% in mtDNA distances),
morphologically (in plumage color and pattern and bill size and shape), and
vocally. Therefore, these two taxa are clearly two different species. Cadena
and Cuervo also documented that two other forms, poliophrys and torquatus,
seem to come together at a presumably narrow contact zone over relatively
continuous environments near the Peru-Bolivia border, where hybridization is
likely limited. These two taxa represent strongly differentiated non-sister
mtDNA clades and differ vocally and phenotypically (plumage and morphometrics),
so they are good candidates for species status as well, but information on
patterns of variation in all the relevant traits near the contact zone was
sparse.
After determining that assimilis and atricapillus (and possibly poliophrys
and torquatus) are different species,
the difficult part of the analysis was deciding what to do with the rest of the
complex based on incomplete information and considering the allopatric
distributions of all other forms. I note that, although incomplete, the
available information was nonetheless sufficient to show that different sets of
traits do not vary in parallel, so using divergence between the two sympatric
taxa as a yardstick to gauge the degree of reproductive isolation between other
taxa would be extremely difficult and problematic (see the paper for a -long-
discussion of this issue, including other more general problems that may apply
to other situations in which we have applied this comparative yardstick
approach). Being more of a conceptual and methodological paper focused on the
challenges of species delimitation in difficult groups than an empirical
analysis, our discussion focused on contemplating several taxonomic
possibilities and then concluded that none was truly satisfactory.
But we needed to decide something. We
thus proposed splitting the complex into eight species, which are admittedly
supported largely (and, in all fairness, sometimes only) by mtDNA variation.
The other alternative would have been to treat atricapillus as a different species from assimilis and leave everything else unchanged, but this created two
difficulties. First, to which of the two species should one assign all other
allopatric populations? We were unable to answer this in several cases and when
we could make some educated guesses, invariably we were faced by the second
difficulty: the recognition of non-monophyletic species. Non-monophyletic
species are not at odds with the BSC, so this is not a problem per se, but it
was simply too hard to find arguments for supporting the existence of paraphyletic
units one would call species in this group given all the variation (i.e. it
would not be entirely clear which form goes where).
The eight proposed species, seven of
which occur in South America would be:
(1) A. costaricensis from Costa Rica and
western Panama
(2) A. atricapillus from central and eastern
Panama and the Colombian Andes (includes atricapillus
and tacarcunae)
(3) A. basilicus from the Sierra Nevada de
Santa Marta, northern Colombia
(4) A. perijanus from the Serranía del
Perijá, northeast Colombia and northwest Venezuela
(5) A. assimilis from the Andes of
Venezuela, Colombia, Ecuador, and most of Peru (includes larensis, assimilis, nigrifrons, and poliophrys)
(6) A. torquatus from the Andes of extreme
southern Peru, Bolivia, and Argentina (includes torquatus, fimbriatus, and borelli)
(7) A. phaeopleurus from the Cordillera de
la Costa, northern Venezuela
(8) A. phygas from the Cordillera de la
Costa Oriental, northeast Venezuela.
Recommendation: With all the above being said, and trying to be consistent
with what we concluded in the paper, I recommend voting YES to accept our
proposed classification. I note, however, that I am the first to admit that
this classification is quite far from being perfect (again, see detailed
discussion in the paper). However, I think that this proposed change is far
better than doing nothing, considering we already have very strong evidence for
species status for two (and maybe four) of the forms. I suggest we take this as
a provisional classification we may end up modifying soon, accepting that while
being imperfect it is better than the one we have followed thus far.
Literature Cited
Cadena, C. D., and A. M. Cuervo. 2010.
Molecules, ecology, morphology, and songs in concert: How many species is “Arremon
torquatus” (Aves, Emberizidae)? . Biological Journal of the Linnean Society
99:152-176.
Cadena, C. D., J. Klicka, and R. E. Ricklefs.
2007. Evolutionary differentiation in the Neotropical montane region: molecular
phylogenetics and phylogeography of Buarremon brush-finches (Aves,
Emberizidae). Molecular Phylogenetics and Evolution 44:993-1016.
Remsen, J. V., Jr., and W. S. Graves. 1995.
Distribution patterns of Buarremon Brush-finches (Emberizinae) and
interspecific competition in Andean birds. Auk 112:225-236.
C. D. Cadena, September 2010
=========================================================
Comments from Remsen: “YES. I’ve reviewed Daniel and Andrés’ paper, and
this is a near-and-dear-to-me group. I’m
convinced that this is the best way to carve up the taxa into species units
given the limitations we have on available data, as outlined clearly and
objectively by the authors.”
Comments from Stiles: “YES
– after rereading Cadena & Cuero’s paper, I can see no better way to
recognize the complex patterns of variation in this decidedly complex group: it
certainly represents a major step forward.”
Comments from Nores: “NO. Although very probably several of the eight forms were really different species, for example atricapillus, I do not think that they can be elevated to whole species. Neither do I see logic in eight new species being created because the authors have the need to decide something based on a classification that is far from perfect. I think that the proposal should be separated into lesser proposals or have several parts for voting. For example, in the molecular analysis of Cadena et al. (2007) the subspecies perijanus and phaeopleurus are not included, so they should not be included in the eight species proposed. Moreover, polyophrys and larensis could also have been proposed as species because they are in a similar situation to that of assimilis.”
Comments from Robbins: “YES.
Cadena and Cuervo have presented a thorough and very objective approach
to dealing with this confusing complex. Given the current understanding, their
recognition of eight species appears to be the best recourse.”
Additional comments from Cadena:
“Manuel mentions one could split our treatment
into several subproposals, but I do not get how one could do this. Let's take
the fact that atricapillus and assimilis are different species. If that
were the only proposal, then what should one do with all of the remaining 12
forms? In which of these two species should one put them? I simply do not know,
which is one of the reasons why we went for a whole set of splits at once. If
someone has any suggestions for alternatives, I would be more than happy to
consider them (I am the first to admit that our proposal is not free of
problems). Note also that "subspecies" perijanus *was* indeed included in the analyses in the 2007 paper.
Subspecies phaeopleurus was not
included back then, but as we described in the 2010 paper that forms the basis
for the proposal, we already have sequence data for this taxon. Indeed, the
paper documenting this has just been published online:
Cadena, C.D., Z.
A. Cheviron & W. C. Funk. 2010. Testing the molecular and evolutionary
causes of a “leapfrog” pattern of geographical variation in coloration. Journal
of Evolutionary Biology: doi: 10.1111/j.1420-9101.2010.02175.x"
Comments from Zimmer: “YES. This proposed classification, although imperfect, is a clear improvement on the current classification, as supported by the data presented by Cadena and Cuervo 2010. As far as I can tell, it also makes sense on biogeographic grounds, and advances our understanding of the complex, even if further work reveals the need to make some changes. I am very sympathetic to the problems of where to place the bulk of the taxa in the event of a simple two-way split (assimilis and atricapillus) – it would seem that any such attempt would be entirely arbitrary, and would only further confuse the issue.”
Comments from Pacheco: “YES. Da minha parte, também considero apropriada a solução sugerida por Cadena. Em vista das informações disponíveis acerca do complexo, sobretudo a ausência de monofilia nos táxons agora subordinados a torquatus, seria contraproducente dispor os oito táxons assimilis e atricapillus, sem implicar em arbitrariedade extra.”
[CC1]http://evolvert.uniandes.edu.co/Biologia_Evolutiva_de_Vertebrados/Publications_files/bjls2010.pdf