Albatrosses

Royal Albatross; photo Lex van Groningen

Albatrosses are the 'largest' birds in terms of wingspan. Royal Abatrosses, for instance, may reach a wing span of almost 3.5m, which make them look like feathered sail plaines. They are also the largest members of the tubenose family. Only the smallest albatross species are equalled in size by the Giant Petrels (Macronectes). Albatrosses occur in all oceans, except the northern part of the Atlantic. In ancient times they were also present in that part of the world, but nowadays only an occasional straggler find its way to the North Atlantic. Most of the 24 species are Southern Hemisphere breeders, only three actually breed north of the Equator in the Pacific Ocean.

Taxonomy

Albatross taxonomy is subject of discussion for a long time, and has been at times rather chaotic. Based on external characters: plumage patterns, tail shapes, bill structure (size, organization of the plates and coloration) albatrosses were, until recently, divided in 13-14 species in four 'natural groups': the Great Albatrosses, the Mollymawks, the North Pacific Albatrosses, grouped in the genus Diomedea and the Sooty Albatrosses Phoebastria. More recently DNA-analyses supports the division in four distinct groups but the were elevated (Nunn et al 1996) to a generic status and has led to a splitting into 24 species (Robertson & Nunn 1998): Great Albatrosses Diomedea (7 species), the Northern (Pacific) Albatrosses Phoebastria (4 species), the southern Mollymawks Thalassarche (11 species) and the Sooty Albatrosses Phoebetria (2 species). Recently this taxonomy is challenged by Penhallurick and Wink (2004) who proposed to lump some of the 'species' again based on their molecular analysis. Since then the discussion flared up and has not ended yet. Brooke (2004) lists six species of Great Albatrosses, including two subspecies of Antipodian Albatross.
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Evolution

The splitting of the ancient tubenoses (albatrosses and petrels) from a lineage of aquatic species assumingly started during the Cretaceaous. The first evidence of an albatross-like bird dates back from the Oligocene (Diomedoides minimus) and from South Carolina (Plotornis delfortii). This is a clear indication that albatrosses once had a worldwide distribution. 'True' albatrosses occurred during the Miocene and Pliocene, fossil records are mostly from the Northern Hemisphere, from the Atlantic as well as from the Pacific. Some taxa (Diomedea milleri) were smaller than extant species, other (Phoebastria californica) similar to the Short-tailed Albatross P. albatrus. Larger northern fossils are from P. anglica (Suffolk, England) with almost the dimensions of present day Diomedea. Southern fossils are much scarcer but known from Australia: the small Mollymawk-like D. thyridata and a larger from Argentina and South Africa. (Robertson & Nunn 1998)

In their study on the taxonomy on extant taxa of the Procellariiformes Penhallurick & Wink (2004) suggest that the earliest split in the albatross lineage took place about 44.4 My ago and produced the ancestors of the present Great and North Pacific Albatrosses Diomedea-Phoebastria group and the Mollymawks and Sooties Thalassarche-Phoebetria.
Diomedea and Phoebastria splitted up around 25.8 My ago and and subsequently Phoebastria diverged into the Waved Albatross P. irrorata (25.8 My ago) and Short-tailed P. albatrus (10.6 My ago) and Laysan (P. immutabilis) and Black-footed (P. nigripes) around 7.9 My ago.
Within the Diomedea-group exulans and epomophora splitted up 13.2 My ago. The several Wandering albatross species, as well as the two Royals diverged probably less than 1 My ago. Speciation within the Wandering Albatross complex is likely the result of  fragmentation and expansion of the breeding range, combined with a restricted gene flow due to strong natal philopatry. According to Rains et al. (2011) this speciation took place about 225-265 Ky ago during the late Pleistocene.
The other group, Thalassarche-Phoebetria, diverged about 24 My ago. The two Sooties diverged from each other about 5.3 My ago.
Within Thalassarche the first split (8.0 My ago) concerned the Yellow-nosed Albatross T. chlororhynchos and the rest which subsquently splitted into the lineage Black-browed-Grey Headed melanophris-chrysostoma (8 My ago), followed by the diverging of Buller's and Shy bulleri-cauta (7.9 My ago). Melanophris and chrysostoma diverged 4.0 My ago. Bulleri and cauta diverged earlier (7.0 My ago). Both black-browed types (T. melanophris and T. impavida) diverged 2.6 My ago. Both Yellow-nosed (T. chlororhynchos and T. carteri) less than 1 My ago. T. bulleri divided into Buller's T. bulleri and Pacific Albatross T. (b.) platei.
Links to several papers on taxonomy and evolution of albatrosses can be found and downloaded from the references page.

References