6/5/2018

IOC World Bird List Species Concept

The IOC World Bird List adopts a modern version of the Biological Species Concept with increased emphasis on monophyletic evolutionary lineages (De Queiroz 1998, 1999; Sangster 2014).

Our improved understanding of reproductive isolation in birds and of the genetics of speciation encourages review of the species complexes of the last century. Distinct sister populations once lumped as polytypic species tend to be independent evolutionary lineages that exhibit essential reproductive isolation (Gill 2014). Recognition of these component species in classifications better portrays evolutionary status, improves sampling of bird populations for systematic studies, and facilitates the setting of priorities for the conservation of biodiversity.

Peer-reviewed publications are our primary source of potential revisions. We rely on published revisions based on rigorous integration of multiple data sets, including morphology, vocalizations and DNA relationships. Gene trees typically require supplementary data.  Multilocus samples of both mtDNA and nuclear genes are preferred, with attention to mtDNA sweeps that distort phylogenetic relationships (Rheindt and Edwards 2011).

Metrics for delimiting allopatric species are evolving. We favor the broad criteria recommended by Helbig et al. 2002 and Isler et al. (1998).  The Tobias et al (2010) numerical index has helped to standardize species ranks for international conservation, but it remains controversial as a primary basis for evolutionary classification (Collar et al 2016, Remsen 2016).

The taxonomic maturities of regional avifaunas guide our practices. Established classifications of the birds of the Western Palearctic or North America, for example, demand stronger criteria for revision than do inaugural analyses of the avifaunas of Indonesia and the Philippines. Also challenging consistency of classifications are the variable sets of tools in used in a study, e.g. genes sampled and algorithms used to reconstruct phylogenies, historical biogeography and gene flow.

New opportunities will continue to improve our classifications of world bird species. Molecular metrics provide estimates of lineage ages and thresholds of species status. Revelations about the speciation process itself will refine the nature of subspecies versus incomplete speciation. Genomics will replace classical concepts of coadapted genomes.  Coalescence models of gene flow will replace classical hypotheses of potential interbreeding. And emerging concepts, such as mitonuclear speciation, will generate productive new insights into the speciation process.

Frank Gill

June 5, 2018

REFERENCES

Collar NJ (2018) Taxonomy as tyranny. Ibis 160(2): 481–484. https://doi.org/10.1111/ ibi.12569

Collar NJ, LDC Fishpool, J del Hoyo, JD Pilgrim, N Seddon, CN Spottiswoode & JA Tobias. (2016) Toward a scoring system for species delimitation: a response to Remsen. Journal of Field Ornithology 87(1): 104–115. https://doi.org/10.1111/jofo.12141

de Queiroz K. (1998) The general lineage concept of species, species criteria,and the process of speciation: A conceptual unification and terminological recommendations. In: DJ & SH Berlocher (Eds). Endless Forms: Species and Speciation. Oxford University Press, Oxford, 57–75.

de Queiroz K. (1999) The general lineage concept of species and the defining properties of the species category. In: RA Wilson (Ed). Species: New Interdisciplinary Essays. MIT Press, Cambridge, Massachusetts, 49–89.

Donegan TM. (2018) What is a species? A new universal method to measure differentiation and assess the taxonomic rank of allopatric populations, using continuous variables. ZooKeys 757: 1–67

Gill FB. (2014) Species taxonomy of birds: Which null hypothesis? Auk: Ornithological Advances 131: 150–161. https://doi.org/10.1642/AUK-13-206.1

Isler ML, PR Isler & BM Whitney. (1998) Use of vocalizations to establish species limits in antbirds (Passeriformes; Thamnophilidae). Auk 115: 577–590. https://doi.org/10.2307/4089407

Remsen JV. (2010) Subspecies as a meaningful taxonomic rank in avian classification. Ornithological Monographs 67: 62–78. https://doi.org/10.1525/om.2010.67.1.62

Remsen JV. (2016) A “rapid assessment program” for assigning species rank? Journal of Field Ornithology 87(1): 110–115. https://doi.org/10.1111/jofo.12142

Rheindt FE & SV Edwards. (2011). Genetic introgression: An integral but neglected component of speciation in birds. Auk 128: 620–632.

Sangster G (2014) The application of species criteria in avian taxonomy and its implications for the debate over species concepts. Biological Reviews 89: 199–214. https://doi. org/10.1111/brv.12051

Tobias JA, N Seddon, CN Spottiswoode, JD Pilgrim, LDC Fishpool & NJ Collar. (2010) Quantitative criteria for species delimitation. Ibis 152: 724–746. https://doi.org/10.1111/j.1474- 919X.2010.01051.x

 

Sequence of Genera, Species and Subspecies

The sequence of genera within families and species within genera used in the IOC World Bird List is in a state of flux as we transition from a more traditional sequence to one that is based on updated molecular phylogenetic knowledge. In general, our goal is to eventually uniformly arrange the linear sequence of these taxa in a fashion similar to the principles followed by several other taxonomic authorities such as those stated in the fourth edition of the Howard & Moore Checklist and also generally followed by the two AOS Classification Committees.

Specifically, the following conventions are to be applied whenever a robust molecular phylogeny is available:

Basal clades are placed in the sequence before the increasingly next most basal clades until the terminal clades are reached in a given branch.

Where clades branch into two or more sister groups, the clades with the fewer number of species are listed before clades with greater number of species. The terminal clades with the greatest number of species are sequentially listed last.  When counting taxa within a clade for this purpose, all taxa which are presumably related to other members of that clade by more traditional taxonomic methods are included whether they were included in the paper’s phylogram or not.

Where relationships within a group of species is not clear, either by low nodal support, or if the clade terminates in two sister groups of equal species count, or a polytomy with branches of equal species count, the sister species are sequenced by listing the species with the fewer number of subspecies before those with the greater number of subspecies. If the sister species are of similar or equal subspecies diversity, they are sequenced by geographic breeding range in a northwesterly to southeasterly configuration. In that scheme, the westernmost species have precedence over the northernmost species. For groups whose breeding ranges span both hemispheres, Old World (eastern hemisphere) taxa are listed first. The northwest corner of the Old World is anchored in Iceland.

A split species is regarded as sister to its parent species unless it is known otherwise and it is listed adjacent to its parent species according to the geographical relationship of the two taxa.

Subspecies are sequenced strictly by geographic breeding range from northwest to southeast. Where possible, however, subspecies forming “subspecies groups” are initially to be grouped together.

David B. Donsker

April 29, 2023