RE: "Common ancestor" in cladistics

[Nick Pharris <npharris@umich.edu>]

Quoting "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>:

> A real case of this is _Branta canadensis_ (Canadian geese), which have had
> multiple insular isolate populations which have diverged from the ancestral
> form (some of the Hawaiian forms being the most interesting and most
> divergent), even while the good old-fashioned ones are doing just fine.

Indeed, the species hitherto recognized as the Canada Goose turns out to be
based on plesiomorphic plumage patterns (gray-brown body, black neck and head,
white chin patch).  The large-bodied forms are actually related to a Hawaiian
radiation containing at least three species (including the nene, _B.
sandvicensis_), whereas the small-bodied "Canada" Goose is closer to the
Barnacle Goose, _B. leucopsis_.

The American Ornithologists' Union has accordingly just recognized the
small-bodied form as a separate species, the Cackling Goose (_Branta
hutchinsii_).  Further splits may be in the offing.

References:

Banks, R.C., C. Cicero, J.L. Dunn, A.W. Kratter, P.C. Rasmussen, J.V. Remsen,
Jr., J.D. Rising, and D.F. Stotz.  2004.  Forty-fifth supplement to the
American Ornithologists' Union _Check-list of North American birds_.  The Auk
121:985-995.

Paxinos, E.E., H.F. James, S.L. Olson, M. Sorenson, J. Jackson, and R.C.
Fleischer.  2002.  MtDNA from fossils reveals a radiation of Hawaiian geese
recently derived from the Canada Goose (_Branta canadensis_).  Proceedings of
the National Academy of Sciences USA 99:1399-1404.

Nick Pharris
Department of Linguistics
University of Michigan