Change South American siskins from the genus Sporagra to the genus Spinus

Proposal (675) to South American Classification Committee

Change South American siskins from the genus Sporagra to the genus Spinus

New World siskins are currently classified in separate genera by the North American and South American checklist committees, respectively. Spinus includes all siskins found exclusively in North America or Central America as well as one Eurasian species (Chesser et al., 2009), whereas all of the siskin species that occur in South America are included in the recently resurrected genus, Sporagra (see successful SACC Proposal 488). In light of a new, multi-locus phylogeny of siskins and goldfinches, with comprehensive taxon sampling of the focal clade (Beckman and Witt, 2015), we propose that the SACC should treat all of the New World siskin species under one genus name, Spinus.

Background:

The New World siskins plus the Eurasian Siskin (hereafter referred to as the New World siskins) constitute a monophyletic group that is sister to a clade comprised of the New World goldfinches, according to two recent multi-locus studies (Zuccon et al., 2012; Beckman and Witt, 2015). Support for each clade is high (≥95 bootstrap support and ≥0.99 posterior probability) regardless of tree-building method and with mitochondrial DNA and nuclear DNA analyzed separately or jointly (Zuccon et al., 2012; Beckman and Witt, 2015). The broad sampling of Zuccon et al. (2012), which included 93 of ~130 species and all but one of the genera in Fringillidae, demonstrated that the reciprocal monophyly of these two clades is unlikely to be the result of the accidental omission of an important taxon (as could occur in a study with limited geographic or phylogenetic scope).

Previously published phylogenies indicated non-monophyly of the siskins but had poor support, few phylogenetically informative sites, and sparse taxon-sampling (Arnaiz-Villena et al., 1998; Zamora et al., 2006; Nguembock et al., 2009). Arnaiz-Villena et al. (1998) and Zamora et al. (2006) each analyzed a single locus (cytb) and tree topologies were sensitive to the tree-building method. Nguembock et al. (2009) used two loci; however, the study included only a single Pine Siskin mitochondrial sequence to represent all North American and Central American siskin diversity. Perhaps due to incomplete taxonomic sampling or specific locus choice, the placement of the Eurasian Siskin (Spinus spinus) was inconsistent among all the trees produced in Nguembock et al. (2009) and in strong conflict with more recent, multi-locus studies (Zuccon et al., 2012; Beckman and Witt, 2015). As discussed regarding the SACC proposal 488 to resurrect the genus Sporagra, studies of fringillid phylogeny prior to 2012 were not sufficiently robust to justify taxonomic changes.

New Information:

SACC proposal 488 united the South American siskins in the genus Sporagra, based on the previous finding that the siskin species that occur in South America form a monophyletic clade (Nguembock et al., 2009), a conclusion that is bolstered by the newest phylogenetic evidence (Beckman and Witt, 2015). A key question is whether the use of Sporagra would render the non-South American siskins paraphyletic. The non-South American siskins, including the Central American species, notatus, are currently included in the genus Spinus by the NACC. Our recent study using three mtDNA genes and two nuclear genes decisively confirmed that notatus is sister to all South American siskins (Arnaiz-Villena et al., 1998, 2007; Nguembock et al., 2009; Zuccon et al., 2012; Beckman and Witt, 2015), which indicates a conflict between the South American and North American checklists. We argue that the best solution is to include all of the New World siskins in a single genus, Spinus. One critical reason for this is that the position of Spinus dominicensis, the Antillean Siskin, varied depending on gene (or linkage group, to be precise). The mtDNA supported S. dominicensis as sister to the South American siskin clade including Spinus notatus. In the Z-chromosome-linked MUSK phylogeny, S. dominicensis formed a clade with the North American, certain Central American and Eurasian siskin species. The autosomal myoglobin gene supported a different topology that we judged to be unlikely to reflect the true species tree (see Discussion in Beckman and Witt, 2015). Given that each of three independent loci supports a different topology, only one of which supports reciprocal monophyly of the South American (plus S. notatus) and the non-South American siskins, we suggest that the true species tree is close to a polytomy, reflecting a rapid radiation that resulted in gene-tree conflict. In our judgment, even if the true species tree can be resolved by genomic-scale data in the future, the level of genetic differentiation between the two clades is not sufficient to warrant generic recognition. On the contrary, the morphological and ecological uniformity of New World siskins as a whole is striking.

Recommendation:

Use of the genus Spinus for all of the New World siskin species would be appropriate in light of the robust monophyly of the clade, its overall recency (common ancestor estimated at ~2 Ma), its phenotypic uniformity, and the conflicting gene trees with respect to the affinities of the Caribbean species.

References:

Arnaiz-Villena, A., Álvarez-Tejado, M., Ruíz-del-Valle, V., García-de-la-Torre, C., Varela, P., Recio, M.J., Ferre, S., Martínez-Laso, J., 1998. Phylogeny and rapid Northern and Southern Hemisphere speciation of goldfinches during the Miocene and Pliocene Epochs. Cell. Mol. Life Sci. 54:1031-1041.

Arnaiz-Villena, A., Ruiz-del-Valle, V., Moscoso, J., Serrano-Vela, J.I., Zamora, J. 2007. MtDNA phylogeny of North American Carduelis pinus group. Ardeola 54:1-14.

Beckman, E. J. and Witt, C. C. 2015. Phylogeny and biogeography of the New World siskins and goldfinches: Rapid, recent diversification in the Central Andes. Mol. Phylogenet. Evol. 87:28-45.

Chesser, R. T., Banks, R. C., Barker, F. K., Cicero, C. Dunn, J. L., Kratter, A. W., Lovette, I. J., Rasmussen, P. C., Remsen Jr., J. V., Rising, J. D., Stotz, D. F. and Winker, K. 2009. Fiftieth Supplement to the American Ornithologists' Union Check-List of North American Birds. Auk 126:705-714.

Nguembock, B., Fjeldså, J., Pasquet, E. 2009. Molecular phylogeny of Carduelinae (Aves, Passeriformes, Fringillidae) proves polyphyletic origin of the genera Serinus and Carduelis and suggests redefined generic limits. Mol. Phylogenet. Evol. 51:169-181.

Zamora J., Lowy, E., Ruiz-del-Valle, V., Moscoso, J., Serrano-Vela, J. I., Rivero-de-Aguilar, J., Arnaiz-Villena, A. 2006. Rhodopechys obsoleta (desert finch): a pale ancestor of greenfinches (Carduelis spp.) according to molecular phylogeny. J. Ornithol. 147: 448-456.

Zuccon, D., Prys-Jones, R., Rasmussen, P.C., Ericson, P.J.P. 2012. The phylogenetic relationships and generic limits of finches (Fringillidae). Mol. Phylogenet. Evol. 62:581-596.

E. J. Beckman and C. C. Witt, July 2015

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Comments from Zimmer: “YES. This group does seem to have undergone a rapid and recent radiation, and, taken as a whole, the New World siskins do appear to form a morphologically and ecologically cohesive group. I think treating them in a single genus (Spinus) is probably the way to go, in spite of the conflicting gene trees.”

Comments from Stiles: “YES. This is the most thorough treatment of the siskins to date (and to agree with NACC).”

Comments from Pacheco: “YES The recent results of Beckman and Witt demonstrate that there is no more reason to treat siskins in two genera in the lists of SACC and NACC.”

Comments from Jaramillo: “YES. It is a bit complex it seems, with no clear resolution, at least the answer varies depending on the analysis and data used. This suggests a rapid radiation (polytomy) in this group. As such, putting all of them in Spinus looks like the appropriate course of action.”