Proposal (348) to South American Classification Committee
Incluir Pterocnemia dentro de Rhea
Antecedentes: El "Lesser Rhea" fue descripto en 1834 por d´Orbigny como Rhea
pennata y así fue denominado por autores posteriores hasta 1871 cuando
G. R. Gray crea el género Pterocnemia, basado principalmente en que la
especie presenta la parte superior del tarso emplumado. Aparentemente, ese nombre
no fue usado posteriormente hasta 1919 cuando R. Dabbene lo utiliza
directamente para Pterocnemia tarapacensis sin aclaración alguna.
Posteriormente, otros autores (Peters 1923, Wetmore 1926, Hudson 1927, Zotta
1935, Olrog 1959, y la mayoría de los autores modernos) usan este nombre.
Nueva información: Sibley y Ahlquist (1990), basados en su estudio de evolución molecular
(DNA-DNA Hybridization), usa nuevamente el nombre Rhea pennata para esta
especie. También otros autores o instituciones utilizan este nombre, por
ejemplo: Animal Diversity Web del University of Michigan Museum of Zoology,
Wikipedia, Rossi Fraire, H.J & Martella, M.B en British Poultry Science y
ZipcodeZoo.com, etc.
Las especies en el campo son muy parecidas
morfológicamente y en comportamiento y según parece, las diferencias entre
ambos géneros, son sólo que Pterocnemia tiene la parte
superior del tarso emplumado y tiene menor número de escutelaciones
(restringidas a la parte inferior delantera del tarso).
Recomendaciones: teniendo en cuenta de que existe un análisis molecular que incluye Pterocnemia
en Rhea y que las diferencias morfológicas que separan a ambos
géneros son poco importantes (pensemos en otros géneros de aves como Tityra,
Myrmeciza, etc. que las especies presentan áreas con o sin plumas), yo
voto SI a esta propuesta.
Manuel
Nores, May 2008
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Comments from Cadena: "NO. I don't have a
copy of Sibley and Ahlquist here, but since there is only one species of Rhea
and one species of Pterocnemia, I do not understand how can their
molecular analysis reveal that Pterocnemia is included in Rhea as
described in the proposal. By definition, if you include two species of a
single clade in a phylogeny as did Sibley and Ahlquist you recover two
lineages, but since these two are sister to each other there is no reason to
change the taxonomy. Manuel's points on the similarity of the two are
well-taken, but as I said in the Pipromorpha-Mionectes proposal,
unless traditional genera are not monophyletic, I think we should not modify
classification at this level (and at higher ranks)."
Comments from Stiles: "A tentative YES. Although
more recent sequence data would be desirable, I agree that the evidence for a
generic distinction is not very convincing - more feathered tibia might be
simply an adaptation to a cooler climate and tarsal scutellation is notoriously
plastic in several other groups."
Comments from Remsen: "NO, but reluctantly. I
agree with Manuel that the characters used to place these two species in
separate genera are not very impressive, and I am struck by the overall
superficial similarity between these two. The decision to use 1 or 2 genera in
this case is largely arbitrary. Genetic distance might provide some guidance,
but the problem is that as far as I can tell Sibley and Ahlquist (1990) did not
sample Pterocnemia, and they started from the standpoint that Pterocnemia
is included in Rhea without further discussion (that I can find). Sibley
& Monroe (1990) implemented the merger without comment. Given that most
other classifications, including Davies 2002 book on ratites and tinamous
(Oxford's "Bird Families of the World" series), retain Pterocnemia,
I would like to see additional analyses before adopting a merger. So many
papers have been written on ratite classification that I suspect comparative
data in terms of genetic and morphological distance are already in the
literature, but I would like to see this synthesized before changing status
quo. That said, I suspect that such a synthesis will show that a merger is
warranted."
Comments from Pacheco: "YES. Creio que a proposição é perfeitamente pertinente. Há
pelo menos uma evidência em favor publicada em:
Delsuc et
al. (2007) Molecular evidence for hybridisation between the two living
species of South American ratites: potential conservation implications. Conservation
Genetics 8(2): 503-507.
Abstract:
n a private wildlife research facility and rhea farm of Argentina, artificially
incubated eggs of putative hybrid origin between the Greater Rhea (Rhea
americana) and the Lesser Rhea (R. pennata) hatched and gave birth
to healthy chicks. Molecular genotyping by the analysis of mitochondrial
Cytochrome b (Cyt b) and nuclear Chromo-Helicase DNA binding
(CHD1) gene sequences confirmed the hybrid origin of these chicks which were
molecularly sexed as females. The possibility of hybridisation argues for
careful management of captive populations of these species, especially if
individuals are to be released in the wild for conservation purposes.
"I assume that the existence of hybrid among Greater Rhea and
Darwin Rhea is a good indication of a closer relationship than admitted
before."
Additional comments from Remsen: "On the basis of the
paper and abstract that Fernando found, I change my vote to YES. Intergeneric
hybrids are know, of course, in Trochilidae, Anatidae, and other families, and
the instance above came from captive birds. But combine this with the absence
of discrete morphological characters to distinguish them and I think
burden-of-proof falls on maintaining separate genera. My subjective definition
of 'genus', at least for non-passerines includes diagnosability at the
skeletal/fossil level."
Comments from Stotz: "YES. While I tend to vote
for the status quo in the absence of clear evidence to the contrary, I have to
say that two monotypic Rhea genera seems like a taxonomic treatment that
doesn't provide useful information. These are not very distinctive taxa, and
there can't be any question that they are sister taxa. I can't see that there
is any value to continuing to maintain two genera of Rheas."
Comments from Zimmer: "YES, for reasons perhaps
best summarized by Doug."
Comments from Jaramillo: "YES - I think that
separating these species into different genera is not all that informative. It
feels a bit uncomfortable to merge these in the same genus because I have
become so accustomed to "Pterocnemia" but concede that there
is little reason for doing so. There may be stronger arguments for separating
the two taxa of Pterocnemia as different species though, but with
this question it would also be useful to see how separate the various lineages
in Pterocnemia/Rhea are."
Comments from Schulenberg: "NO. That's a reluctant
vote, since the differences between the two are slight. According to Blake
1977, the diagnosable differences simply are size and whether or not the tarsus
is feathered are the tarsal scutellation. I'm not impressed by those
differences in morphology, but who knows what a genetic perspective would
reveal. Like Van, I have to wonder, are there no genetic data already available
bearing on this question? Has anyone looked? Or have all the reams and reams of
ratite relationship projects managed to overlook this central question?"