Waterfowl taxonomy is notoriously challenging, in part owing to morphological similarities between and within groups that can obscure patterns of divergence (Mowbray et al. 2020). In North America, this is best illustrated by Canada (Branta canadensis) and Cackling Goose (B. hutchinsii), which were considered conspecific for a long time, and thought to just represent the large and small extremes, respectively, of a single polytypic species (Mowbray et al. 2020). However, genetic work has shown that the two are not actually each other’s closest relatives, with Cackling Goose instead sister to Barnacle Goose (B. leucopsis; Ottenburghs et al. 2016).
In addition to morphological similarity obscuring patterns of diversity, there is also rampant hybridization within the waterfowl, further confusing the taxonomy of many species. In North America, birders have to look no further than the common and widespread Mallard (Anas platyrhynchos), which hybridizes extensively with American Black Duck (A. rubripes), Mottled Duck (A. fulvigula), and Mexican Duck (A. diazi; Lavretsky et al. 2015, 2019a,b). Around the world, Mallard also hybridizes extensively with many other species, including Hawaiian Duck (A. wyvilliana) and Pacific Black Duck (A. superciliosa), where they have been introduced and pose conservation concerns (Carboneras and Kirwin 2020, Engilis et al. 2020).
In Europe and Asia, the Bean-Goose complex includes three species, Pink-footed Goose (A. brachyrhynchus), Taiga Bean-Goose (A. fabalis), and Tundra Bean-Goose (A serrirosostris; Ruokonen et al. 2008). Like many other waterfowl groups, including the Canada/Cackling Goose complex and the Mallard complex mentioned above, the taxonomic history of this group has been complicated by morphological similarity and hybridization. Indeed, both Tundra and Taiga Bean-Goose were considered conspecific for much of their history, until research found that they differed in where they breed, in their vocalizations, are morphologically distinct, and may not actually be sister taxa (Ottenburghs et al. 2016, 2020). However, later research found that despite some morphological differences, there was much overlap in many measurements, and genetic evidence suggested there was hybridization between the two (Ottenburghs et al. 2020).
In their recent paper, which used whole-genome resequencing to investigate patterns of hybridization between the two species, Ottenburghs et al. (2020) found that while the two bean-geese can be separated, the genetic differences between them are minor, with the authors suggesting that widespread hybridization has mostly homogenized the genomes of the two species, with only a few peaks of divergence between the two. Based on demographic analyses, Ottenburghs et al. (2020) also found that most gene flow occurred from Tundra Bean-Goose into Taiga Bean-Goose.
Based on their results, Ottenburghs et al. (2020) argued that both Taiga Bean-Goose and Tundra Bean-Goose, which are currently recognized as two distinct species by most authorities, should be lumped into a single species, Bean Goose. They argue that reproductive isolation is incomplete, as evidenced by extensive hybridization, that they have very low levels of genetic divergence, and that there are no sufficiently diagnostic features to separate the two, with extensive variation in morphology. The authors do note, however, that more research is needed on the system for a more complete picture of the Bean-Goose complex, such as the inclusion of additional subspecies (notably subspecies middendorfii of Taiga Bean-Goose, which itself may represent a distinct species; Ruokonen et al. 2008), and further research of the parts of the genome that differ between the two species to learn their function and potential role in promoting speciation. Importantly, are the genetic differences identified by Ottenburghs et al. (2020) linked to morphology differences, habitat preference differences, or any other differences that could be important to reproductive isolation?
While Birds of the World currently recognize Taiga and Tundra Bean-Goose as separate species, the recent research by Ottenburghs et al. (2020) highlights how complex and confusing taxonomy can be, and shows just how much we still have to learn about different groups of birds and defining limits between species.
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