If you are reading this on the web sometimes URLs get stripped out.
Try looking up some of the terms I use below in wikipedia for some
background information.

Inline.

On May 18, 2009, at 3:10 PM, Larry Schwitters wrote:

> National Geographic describes a Lawrence's Warbler as a backcross
> (most often produced by crossing a first-generation hybrid with one
> of the parent species.)

That would be more generally called an intergrade in one species when
you get a hybrid with any fraction of "the original" mating with any
other hybrid with a different fraction.

http://en.wikipedia.org/wiki/Intergrade

Which leads to the whole interesting ideas of clines

> In biology, a cline is a gradual change of phenotype (trait,
> character or feature) in a species over a geographical area, often
> as a result of environmental heterogeneity.

i.e. a gradual change in say the appearance of a bird over a
geographical area (think about the plumage of the Song Sparrow, the
House Finch, the Fox Sparrow and the like). I think you've noticed
the problem this causes to both field guides and birders trying to do
an ID. Or just birders moving around in the USA ("That's a Song
Sparrow? Really?").

And then to ring species.

I've mentioned ring species before on this list. They are sort of a
"dirty little secret" of listing because they show that sometimes you
really can't assign "a species" to bird e.g. the northern circumpolar
ring of Larid species (gulls)

<http://en.wikipedia.org/wiki/Ring_species>

> The Herring Gull, which lives primarily in Great Britain and
> Ireland, can hybridize with the American Herring Gull (living in
> North America), which can also hybridize with the Vega or East
> Siberian Herring Gull, the western subspecies of which, Birula's
> Gull, can hybridize with Heuglin's gull, which in turn can
> hybridize with the Siberian Lesser Black-backed Gull. All four of
> these live across the north of Siberia. The last is the eastern
> representative of the Lesser Black-backed Gulls back in north-
> western Europe, including Great Britain.
>
> The Lesser Black-backed Gulls and Herring Gulls are sufficiently
> different that they do not normally hybridize; thus the group of
> gulls forms a continuum except where the two lineages meet in Europe.
>

And you though it was just the Glaucous-winged x Western hybrids
(intergrades) that were problematic! The trick is not having many
birders in Northern Siberia :-)



It seems the birding approach to this is one of either not hearing
about ring species or ignoring the fact that they exist. Until you
see something like the bird described on this thread then the
explaination of "oh, that's a hybrid" is usually the end of it. Which
is not a bad way out but it minimizes the interesting biology hidden
in those species that do interbreed.

Richard Dawkins coined the phrase "the tyranny of the discontinuous
mind" in the Salamander's Tale in his book "The Ancestor's Tale" to
describe this need to draw arbitrary boundaries (e.g. with species)
that have no real meaning. It's a problem with which box to check on
your list if you really have to check a box.

http://www.americanscientist.org/bookshelf/pub/richard-dawkins


> Lets see if we have this straight. A Blue-winged Warbler mates out
> of its species with a Golden-winged Warbler to produce a nest full
> of little hybrid Brewster's Warblers, which by old-school
> definition are sterile. One of these Brewster Warbler hybrids
> becomes involved with a Blue-winged or Golden-winged Warbler member
> of the opposite sex, and nine months later a clutch of Lawrence's
> Warblers hatch.

You assuming that all hybrids are sterile i.e that two separate
species always can't breed which is the biological species concept
but that's not always case. For example, ring species wouldn't be
possible if hybrids were sterile. And there are other ways to define
a species.

http://en.wikipedia.org/wiki/Biological_species_concept

In fact it's probably the case that the Hermit and Townsend Warbler
species diverged when separate populations of a common ancestor were
separated by an ice sheet lobe. They diverged in appearance and got
on with their life's but not to the extent that can't interbreed. The
lobe went away and now they meet up halfway and now do interbreed but
they're separated most of the time to keep their mostly separate
identities.

In some cases diverging species can breed but don't. A classic
example of allopatric speculation (diverging species that can exist
side by side but don't interbreed so they keep their "separateness")
is in grasshoppers where two identical looking but different sounding
species won't breed together but if you but a male of the same
species as the female in a cage adjacent to the mating pair he makes
the right sound and she'll mate with another male of a different
species who is in her case _and_ produce offspring.

http://en.wikipedia.org/wiki/Allopatric_speciation

Of course this ends up coming down to "what is a species" which is a
very interesting rathole to fall into. Biologists call it the
"Species Problem"

BTW, Darwin said in Origin of Species:

> "... I was much struck how entirely vague and arbitrary is the
> distinction between species and varieties" Darwin 1859 (p. 48)
>

In someways it hasn't changed.

http://en.wikipedia.org/wiki/Species_problem

> Anyone know how all this was figured out?

Today you can do it with DNA fingerprinting with samples from wild
birds.

In the olden days you would do it by collecting (shooting) birds and
comparing phenotypes. There is a huge collection of Hermit X Townsend
Warblers at the Burke for example (and a UW Prof to go with them
whose name I'm sadly forgetting ...).

It's possible some lab breeding experiments have been done too though
they can be a challenge.
--
Kevin Purcell
kevinpurcell at pobox.com