Proposal (937) to South
American Classification Committee
Species limits within
the Megascops watsonii complex
Background: For decades there has
been considerable debate on not only species limits but even what taxa should
be included within the Megascops watsonii (Strigidae) complex (summary
in Introduction, paragraph 2, Dantas et al. 2021).
New
information:
Genetic data in Dantas et al. (2016) clarified relationships,
recognizing that M. atricapilla was part of the complex. In a subsequent
publication, Dantas et al. (2021) proposed that in the currently
recognized watsonii (sensu stricto) that four additional species be recognized.
The recognition of four additional species was based solely on genetic results
(3 mitochondrial, 3 nuclear genes) as morphology and vocalizations have not
provided concrete characters for species recognition within the watsonii
complex (Krabbe 2017, Dantas et al. 2021). In defining species based on
genetic data, Dantas et al. (2021) used a coalescent-based species
delimitation method that has been shown not be reliable in delimiting species
(Sukumaran and Knowles 2017; Chambers and Hillis 2019). Dantas et al.
(2021) failed to acknowledge this issue.
In addition, results
from Harvey et al. (2017) were “in sharp contrast” to the Dantas et
al. (2021) data set. As a result, I contacted Mike Harvey and asked his
opinion of the Dantas et al. data and interpretations.
Here is what Mike
stated in an email dated 28 January 2022:
“Based on reviewing both our results (see
attached figures), my conclusion is that the topologies from both our studies
are concordant, but the internal branches are much shorter in my data,
suggesting that the divergence times in Dantas's tree might be highly
overestimated, likely due to deep mitochondrial coalescence. In short, I
do not see strong evidence for treating watsonii as multiple species. I expand on these
points below.
“Trees are concordant:
My mitochondrial data (from off-target UCE
reads) is really fragmentary for this M.
watsonii, so we should ignore my
mitochondrial tree (in the supplement of the AmNat paper). The UCE tree in the
supplement was unrooted and, with the relatively short length of the internal
branches, difficult to interpret. However, I went back and estimated a
quick/rough rooted tree using BEAST analysis of the concatenated UCE SNPs
(attached, top panels). This reveals a clear topological congruence between my
results and those of Dantas et al. (attached, bottom panels). Both our trees
show the Guianan populations splitting earliest. I wasn't able to get Atlantic
Forest samples for my project, but I do have one sample from the SE Amazon. In
Dantas's tree both the SE Amazon and Atlantic Forest (his clades C, D, E, and
F) are included in a large clade that represents the next split (from the
western Amazonian populations). Based on my one sample from that clade, my tree
is concordant. So the three oldest clades are concordant between the two
studies. Without samples from Dantas's clades D, E, and F, I can't evaluate
those more recent divergences. Also, it looks like Dantas et al. lack sampling
from the Imeri/Rondonia regions, so we can't tell if their data support some of
the deeper divergences I found within the western Amazon clade.
“Internal branch lengths/divergence times:
All that said, the internal branches in my
rooted tree (those separating the major clades) are all very short,
consistent with the star-like shape of my original unrooted tree and my
population genetic structure analyses (which indicated a lack of substantial
structure) in the AmNat supplement. All of this suggests these populations are
very recently diverged or have experienced extensive gene flow across the
genome. I think the impressively deep divergences seen in Dantas et al.'s tree
must be driven by deep coalescence in the mitochondrial genes. More than half
the sites in their matrix (2454 of 3968) are from the mitochondrial genes, and
given that these likely contain most of the variation in the matrix, the
mitochondrial data likely drive the tree topology and branch lengths. Because
the mitochondrion is a single locus with a single gene history, deep
coalescence of mitochondrial haplotypes (due to the persistence of multiple
alleles for prolonged periods in ancestral populations) could drive the
apparent old divergence times in the Dantas et al. tree.
“My conclusions:
I think what's going on is that,
although the major clades recovered by Dantas are correct, the divergence times
between them are likely substantially younger than they found. I would take
those divergence times with a grain of salt. If the clades were supported by
clear phenotypic/vocal differences, then I would be okay with considering them
separate taxa, but it looks like this is not really the case in their analyses.
I therefore think chances are these populations are not very different
genome-wide and treating them as separate taxa and particularly separate
species is questionable. A study with geographic sampling similar to that of
Dantas et al. but wider sampling of the genome and including a time-calibrated
tree would probably be required to confirm polytypy in the species at any
level.”
Note
what Mike states is congruent with what Sukumaran and Knowles (2017) and
Chambers and Hillis (2019) concerning caution about using coalescent-based
species delimitation methods for species.
Recommendation:
After
digesting all of this, I recommend a NO vote for recognizing the four
additional species as I concur with Mike that more information is needed. I
would add that dense sampling is needed in potential areas of contact as
outlined by Dantas et al. (2021; figs. 2 & 9).
Finally,
if one does not recognize multiple species this brings into question the
validity of recognizing atricapilla as a species separate from watsonii
(Dantas et al. 2021, figs. 2 & 3). It appears that atricapilla
(Temminck 1822) would have priority over watsonii (Cassin 1848).
References:
Cassin, J. 1848. Descriptions of owls, presumed to be undescribed,
specimens of which are in the collection of the Academy of Natural Sciences of
Philadelphia. Proceedings of the Academy of Natural Sciences of Philadelphia
4:753‒774.
Chambers,
E.A. and D.M. Hillis. 2019. The Multispecies Coalescent Over-Splits species in
the case of geographically widespread taxa. Systematic Biology
69:184-193. https://doi.org/10.1093/sysbio/syz042
Dantas, S.M., J.D.
Weckstein, J. Bates, N.K. Krabbe, C.D. Cadena, M.B. Robbins, E. Valderrama, and
A. Aleixo. 2016. Molecular systematics of the new world screech-owls (Megascops:
Aves, Strigidae): biogeographic and taxonomic implications. Molecular
Phylogenetics and Evolution, 94:626‒634.
https://doi.org/10.1016/j.ympev.2015.09.025
Dantas,
S.M., J.D. Weckstein, J. Bates, J. N. Oliveira, T.A. Catanach, and A. Aleixo.
2021. Multi-character taxonomic review, systematics, and biogeography of the
Black-capped/Tawny-bellied Screech Owl (Megascops atricapilla-M. watsonii)
complex (Aves: Strigidae). Zootaxa 4949:401-444. https://doi.org/10.11646/zootaxa.4949.3.1
Harvey, M.G., A.
Aleixo, C.C. Ribas and R.T. Brumfield, 2017. Habitat association predicts
genetic diversity and population divergence in Amazonian birds. American
Naturalist, 190:631‒648. https://doi.org/10.1086/693856
Krabbe,
N. 2017. A new species of Megascops (Strigidae) from the Sierra Nevada
de Santa Marta, Colombia, with notes on voices of New World screech-owls. Ornitología
Colombiana 16:1-27.
Sukumaran,
J., and L.L. Knowles. 2017. Multispecies coalescent delimits structure, not
species. Proceedings of National Academy of Natural Sciences
114:1607-1612. https://doi.org/10.1073/pnas.1607921114
Temminck, C.J. 1822. Nouveau recueil des Planches Coloriées d’oiseaux. Vol. 2. Livraison
25. Pl. 145. F. G. Levrault, Libraire-editeur, Paris, 278 pp.
Mark
Robbins, February 2022
_______________________________________________________________________________________
Comments from Lane: “NO. In light of Mike Harvey's
comments and the lack of strong vocal evidence for species status of the clades
Dantas et al (2021) separated, I think Mark's recommendation is prudent. As for
lumping M. watsonii into M. atricapillus, I'd like to see
corroboration of the paraphyly using nuclear markers within the complex before
acting on this. For the time being, I vote to keep the status quo until the
relationships within the complex are better fleshed out.
Comments from Niels Krabbe (who has Remsen’s vote): “First, I am averse to
species that can only be safely identified genetically. My immediate response
to the finding that atricapilla is embedded within the other clades
would be to lump all under atricapilla. I acknowledge that one
population can speciate without the others doing so (resulting in a
paraphyletic tree), especially when it is isolated geographically, as seems to
be the case with atricapilla and "alagoensis", but
genetically, atricapilla is the least distinct in the group, and the
high pitch and rapid pace of its longsong and shortsong, its major vocal
characteristics, are matched by other populations in the group (I believe one
of the shortsongs listed under atricapillus: XC102229 is a misidentified
sanctaecatarinae). In addition, if the divergence time is overestimated
as Harvey's results suggest, then there is no reason to think that any two
members of the group would be genetically incompatible. Like Mark, I would call
for more nuclear gene sequences and more sampling from potential areas of
hybridization/intergradation before considering species rank for these forms.”
“I
note that the sequences in the FGB intron 5 (fib5) includes two indels. One is
shared by two birds from N Peru and one (MZUSP JF297) of two birds from Jufari
300 km NW Manaus, Amazonas (all Clade B, usta), the other by an usta
from Rondonia (MPEG 70663) and one (MPEG 70433) of three birds from Benevides,
Para (Clade D, "ater"). In both cases the lack of the indel in other
specimens from the same localities indicate that two different alleles were
used for the comparisons. Additionally, in the latter case, the great
geographic distance between Rondonia and Benevides, as well as the presence of
a geographically intervening "stangiae" suggest that the
allele sequenced was present in the ancestral population, similar to what
Harvey suggested was the case for some mitochondrial haplotypes.
“In
the mitochondrial sequences a sample from La Paz, Bolivia (LSUMZ B947) stands
out in both the COI gene and (especially) the cyt-B sequence, including ties to
both Clade A and to Clades C-F as well as having several unique sequences, and
could be such an ancient haplotype or even a pseudogene.”
“The
same might be the case for sequences with an insertion of a single nucleotide
in the tRNA region of the ND2 sequence in four birds, three from Guyana (NMNH
11476, ANSP 21937 and 188291) and one from north of Manaus (LSUMZ B20185), but
not found in a bird from Óbidos, N Para (MPEG 6663), all in Clade A. A unique
point mutation in the tRNA link the same four specimens.
Comments
from Areta:
“NO. I have been waiting for Aleixo´s input on
comments by Mark and Mike Harvey, but I decided to vote based on what we have
at hand. I expect species in Megascops
to be clearly diagnosable by vocalizations, but this is not apparently the
case. Further bioacoustic exploration is necessary. Although Megascops taxa are notoriously difficult
to diagnose based on plumage, I am also hesitant to accept new species that can
only in part be separated by others in song, which do not exhibit deep genetic
divergences and for which no single morphological or plumage features are
diagnostic. In sum, I think that Dantas et al. (2021) have provided an
interesting perspective of the situation in the Megascops watsonii complex but that further testing of their
taxonomic proposal is needed, based on an improved genomic dataset and more
thorough vocal analyses with more sampling. As a side note, I´ve found the
paper difficult to read, especially because I had to spend a lot of time trying
to link the arbitrary clade letters to the available (and new) taxon names, and
because of the poor labelling on the figure themselves.”
Comments from Bonaccorso: “NO. Not being an expert on the
group, it seems clear that more genetic information on a broader geographic
coverage, including hybrid zones, as well as vocal data for these species are
crucial to making a change.”
Comments from Stiles: “NO. The comments by Krabbe,
Areta, and Harvey show that the evidence presented is insufficient to justify
it.”
Comments from Remsen: “NO. Species limits in owls are all about
vocalizations as an index of likelihood of free gene flow. They are not about differences in DNA
sequence data for a few neutral loci.
The latter are useful for delineating population structure, but by
themselves, useless for determining species limits unless they illuminate
paraphyly, and even then, such a result could be ILS. I also continue to decry the use of any
comparative genetic distance measures, which are on a continuous scale and
depend on N of individuals and geography of sampling, as a metric for species
limits. In my view, taxa become species when they have diverged to that free
gene flow is no longer possible between those lineages, either determined
directly by a contact zone or inferred from comparative data on vocal
differences etc. among other members of the group treated as species.”
Comments
from Pacheco:
“NO. Harvey's arguments presented here put the
results of Dantas and colleagues under scrutiny. Perhaps, Aleixo can add an
alternative interpretation to the set of data present in the article.”
Comments
from Jaramillo:
“NO – Broad study of vocalizations would be necessary before making any change
here.”
Comments from Claramunt: “NO. A very complicated situation. A strong
genetic structure suggests that more than one species is involved, but I concur
with others here in that the situation is not completely clear. Results from
coalescent-based species delimitation are to be taken with great skepticism.
These methods detect departures from panmictic populations. They don’t take into account the spatial dimension. Any geographical
structuring, even clinal, would be taken as evidence for the presence of more
than one lineage. So, they are expected to be prone to detecting spurious
species. Plumage and song variation is complex and doesn’t seem to match
genetic variation. The paraphyly generated by the inclusion of atricapillus
in the complex is problematic, but I would like to see how strong the evidence
of paraphyly is or if we can split the complex in some logical way that strikes
a balance between information from genes and the phenotypes.”