|Female mallard (Anas platyrhynchos) with brood of young|
40–50, see text
Anas is a genus of dabbling ducks. It includes mallards, wigeons, teals, pintails and shovelers in a number of subgenera. Some authorities prefer to elevate the subgenera to genus rank. Indeed, as the moa-nalos are very close to this clade and may have evolved later than some of these lineages, it is rather the absence of a thorough review than lack of necessity that this genus is rather over-lumped. However, there are 42 species verified by ITIS, and the IUCN recognizes 46 species.
The phylogeny of this genus is one of the most confounded ones of all living birds. Research is hampered by the fact the radiation of the two major groups of Anas – the teals and mallard groups – took place in a very short time and fairly recently, roughly in the mid-late Pleistocene. Furthermore, hybridization may have long played a major role in Anas evolution, with within-subgenus hybrids regularly and between-subgenus hybrids not infrequently being fully fertile. The relationships between species are much obscured by this fact, and mtDNA sequence data is of dubious value in resolving their relationships; on the other hand, nuclear DNA sequences evolve too slowly to resolve the phylogeny of the subgenus Anas for example.
Some major clades can be discerned. For example, that the traditional subgenus Anas, the mallard group, forms a monophyletic (in the loose sense, i.e. non-holophyletic) group has never been seriously questioned by modern science and is as good as confirmed (but see below). On the other hand, the phylogeny of the teals is very confusing.
For these reasons, the dabbling duck lineages more distantly related to mallard group (which includes the type species of Anas) than the wigeons should arguably be separated in their own genera. These would include the Baikal teal, the Garganey, the spotted black-capped Punanetta group, and the shovelers and other blue-winged species. Whether the wigeons, which are very distinct in morphology and behavior, but much less so in mtDNA cytochrome b and NADH dehydrogenase subunit 2 sequences, should also be considered a distinct genus Mareca (including the gadwall and falcated duck) is essentially the one remaining point of dispute as regards the question which taxa should remain in this genus and which ones should not.
The following arrangement is based on current morphological, molecular, and behavioral characters and presents apparent major evolutionary groupings compared to the subgenera the species were placed in at one time or another.
Probable genus Sibirionetta – Baikal teal
- Baikal teal, Anas formosa (formerly in Nettion)
Probable genus Querquedula – Garganey (may include Punanetta)
- Garganey, Anas querquedula
Probable genus Punanetta
- Silver teal, Anas versicolor
- Puna teal, Anas puna – formerly included in Anas versicolor
- Hottentot teal, Anas hottentota
- Blue-winged teal, Anas discors
- Cinnamon teal, Anas cyanoptera
- Borrero's cinnamon teal, Anas cyanoptera borreroi – possibly extinct (late 20th century?)
- Red shoveler, Anas platalea
- Cape shoveler, Anas smithii
- Australasian shoveler, Anas rhynchotis
- Northern shoveler, Anas clypeata
Possible genus Mareca – wigeons (may include Chaulelasmus and Eunetta)
- Eurasian wigeon, Anas penelope
- Amsterdam duck, Anas marecula – tentatively placed here; extinct (ca. 1800)
- American wigeon, Anas americana
- Chiloe wigeon, Anas sibilatrix
Subgenus Chaulelasmus – gadwall
- Gadwall, Anas strepera
Subgenus Eunetta – falcated duck
- Falcated duck, Anas falcata
Subgenus Dafila – pintails
- Northern pintail, Anas acuta
- Eaton's pintail, Anas eatoni
- Kerguelen Islands pintail, Anas eatoni eatoni
- Crozet Islands pintail, Anas eatoni drygalskii
- Yellow-billed pintail, Anas georgica
- White-cheeked pintail, Anas bahamensis (formerly in Poecilonetta)
- Red-billed teal, Anas erythrorhyncha (formerly in Poecilonetta)
- Cape teal, Anas capensis (formerly in Nettion)
Subgenus Nettion – teals (paraphyletic)
- Indian Ocean clade (sometimes subgenus Virago)
- Bernier's teal, Anas bernieri
- Mascarene teal, Anas theodori – extinct (late 1690s)
- Sunda teal, Anas gibberifrons
- Rennell Island teal, Anas gibberifrons remissa – extinct (ca. 1959)
- Andaman teal, Anas albogularis – formerly included in Anas gibberifrons
- Grey teal, Anas gracilis – formerly included in Anas gibberifrons
- Chestnut teal, Anas castanea
- Atlantic/Red-and-green head clade
- New Zealand clade (Placement unresolved)
Subgenus Melananas – African black duck
- African black duck, Anas sparsa
Subgenus Anas – mallard and relatives (may include Melananas)
- American clade
- Pacific clade – the moa-nalos might be derived from this group.
- Mariana mallard, Anas (platyrhynchos) oustaleti – sometimes considered a subspecies of Anas superciliosa; extinct (1981)
- Hawaiian duck, Anas wyvilliana – sometimes included in Anas platyrhynchos
- Philippine duck, Anas luzonica
- Laysan duck, Anas laysanensis – sometimes included in Anas platyrhynchos
- Lisianski duck, Anas cf. laysanensis – hypothetical; extinct (ca. 1845)
- Pacific black duck, Anas superciliosa
Formerly placed in Anas:
- Bronze-winged duck, Speculanas specularis
- Crested duck, Lophonetta specularioides
- Salvadori's teal, Salvadorina waigiuensis
A number of fossil species of Anas have been described. Their relationships are often undetermined:
- Anas sp. (Late Miocene of China)
- Anas sp. (mid-sized species from the Late Miocene of Rudabánya, Hungary)
- Anas greeni (Ash Hollow Late Miocene?/Early Pliocene of South Dakota, USA) – Nettion red-and-green head clade (doubtful)?
- Anas ogallalae (Ogalalla Late Miocene?/Early Pliocene of Kansas, USA) – Nettion red-and-green head clade (doubtful)?
- Anas pullulans (Juntura Late Miocene?/Early Pliocene of Juntura, Malheur County, Oregon, USA) – Punanetta?
- Anas cheuen (Early-Middle Pleistocene of Argentina) – Dafila?
- Anas bunkeri (Early -? Middle Pliocene – Early Pleistocene of WC USA) – Nettion red-and-green head clade?
- Bermuda Islands flightless duck Anas pachyscelus (Shore Hills Late Pleistocene of Bermuda, W Atlantic)
- Anas schneideri (Late Pleistocene of Little Box Elder Cave, USA)
Several prehistoric waterfowl supposedly part of the Anas assemblage are nowadays not placed in this genus anymore, at least not with certainty:
- "Anas" basaltica (Late Oligocene of "Warnsdorf", Czechia) is apparently an indeterminate heron.
- "Anas" blanchardi, "A." consobrina, "A." natator are now in Mionetta
- "Anas" creccoides (Early-mid Oligocene of Belgium), "A." risgoviensis (Late Miocene of Bavaria, Germany) and "A." skalicensis (Early Miocene of "Skalitz", Czechia), though possibly anseriform, cannot be placed with any certainty among modern birds at all.
- "Anas" albae (Late Miocene of Polgárdi, Hungary), "A." eppelsheimensis (Early Pliocene of Eppelsheim, Germany), "A." isarensis (Late Miocene of Aumeister, Germany) and "A." luederitzensis (Kalahari Early Miocene of Lüderitzbucht, Namibia) are apparently Anatidae of unclear affiliations; the first might be a seaduck.
- "Anas" integra and "A." oligocaena are now in Dendrochen.
- "Anas" lignitifila from the Late Miocene of Tuscana has been moved to its own genus, Bambolinetta, being a highly unusual marine waterfowl.
- "Anas" robusta is now tentatively placed in Anserobranta.
- "Anas" velox (Middle – Late? Miocene of C Europe) and "A." meyerii (Middle Miocene of Öhningen, Germany; possibly the same species) do not seem to belong Anas, and they may be ancestral dabbling ducks.
Highly problematic, albeit in a theoretical sense, is the placement of the moa-nalos. These are in may be derived from a common ancestor of the Pacific black duck, the Laysan duck, and the mallard, and an unknown amount of other lineages. Phylogenetically, they may even form a clade within the traditional genus Anas. However, as opposed to these species – which are well representative of dabbling ducks in general – the moa-nalos are the most radical departure from the anseriform bauplan known to science. This illustrates that in a truly evolutionary sense, a strictly phylogenetic taxonomy may be difficult to apply.
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