I can't directly answer your question, "How common is hybridization across genera?" I think it's more useful to address the underlying question, "What (or who) defines genera?" In nature, there are just species, more strictly or less strictly separated from other species by lack of interbreeding. Genera, tribes, families, etc. are all human inventions, to suggest degrees of relatedness among species. I don't think we know just what factors might induce a male of one kind and a female of another kind to mate; we can imagine all kinds of behavioral factors, maybe including the degree to which an individual has been imprinted on its own species or on individuals of other species early in development, maybe its social status within its own flock, maybe some subtle genetic factors. But the result, in any case, is that occasional hybrids occur between species that aren't particularly closely related; the example I like to use comes from the orioles, where hybridization between the Baltimore and Bullock's led to them being lumped as one species, the Northern, a few years ago--a lumping now reversed. Yet they aren't even closest relatives; the Baltimore is most closely related to Altamira, with which it does not hybridize (as far as I know), while the Bullock's is somewhat removed and more closely related to Streak-backed. (This information is a few years old now and may be improved by newer data.) Those of us who grew up with sort of classical thinking about the nature of speciation have probably thought of interbreeding as a remnant of incomplete speciation, but apparently that just ain't so. Two taxa can apparently be quite well separated as species and yet still serve up an occasional hybrid. A couple of odd geese looking at each other with romance in their hearts don't know if Mssrs. Sibley and Monroe have put them into the same genus or not.

Does that help at all?

Burt Guttman
The Evergreen State College
Olympia, WA 98505 guttmanb at evergreen.edu
Home: 7334 Holmes Island Road S. E., Olympia, 98503

________________________________

From: tweeters-bounces at mailman1.u.washington.edu on behalf of pslott
Sent: Wed 4/13/2005 11:15 AM
To: TWEETERS
Subject: [Tweeters] Hybridization question



Hybridization question

Tweeters,

Yesterday, I saw my first Greater White-fronted X Canada Goose hybrid at
Nisqually. It surprised me because these species don't seem terribly
similar to me and I spent some time trying to make it into a Greater
White-fronted X Bean Goose, Anser, before I had to accept the similarity
to the Canadas, Branta, that were with it. Hybrids I can think of at the
moment seem to be from the same genus, so I was more surprised when I
realized that D. Sibley gives them a separate genus (and another
separate genus for Snow Goose, Chen, that, apparently, also hybridizes
with GWFG). So I thought I'd see what C. Sibley and Monroe had to say
about the genus of these birds. Their world list put Snow Goose into
Anser with GWFG, but left Canada in Branta.

So, my question is, How common is hybridization across genera?

Patricia Lott
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