Janet Hardin asked,

> What is the definition of a "superspecies?"
>
> By asking this, I'm hoping any one of several Tweets who are truer
> ornithologists than I can refresh my brain. My general impression of the
> term has been that it covers situations similar to the one under
> discussion, where there is perhaps a cline between recognized forms and a
> whole series of interbreeding forms in between.

Mike Patterson then replied:

> Generally speaking, a superspecies consists of two or more sibling species
> that are very closely related. The Western Flycatcher is now considered a
> superspecies consisting of Pacific-slope and Cordilleran Flycatcher for
> example. The siblings within a superspecies may or may not interbreed
> (it's not a requirement or anything).

Sorry, Mike--there's some confusion here among several terms. First,
let's dismiss the matter of _sibling species_, which is not closely
related to the superspecies question. Sibling species are species whose
members are morphologically so similar that they are very difficult to
separate. The Empidonax flycatchers are probably our best examples. Over
the last few years there have been several cases in which what was once
considered a single species of empid has been divided in two, such as the
division of the Western into Cordilleran and Pacific-slope Flycatchers;
some recent discussions (on Birdchat or Tweeters--I can't remember)
suggest that if they are legitimately two species, they are damned hard to
distinguish from each other.

Now, as to _superspecies_: Remember that the criterion for species
status according to the biological species concept (and I hope we can
avoid a repeat of _that_ discussion) is a lack of interbreeding. But one
can only test interbreeding between two populations if their ranges
overlap--in other words, if they are _sympatric_. If the populations are
_allopatric_, with disjoint ranges, they never have the opportunity to
interbreed and we can't ask whether they are distinct species or not.
Such allopatric but closely related populations are presumably in the
process of speciation by means of geographic isolation, and they are
likely to be at some intermediate state genetically between being
subspecies and distinct species. Ernst Mayr (in 1940) therefore called
these populations _semispecies_, and a whole group of two or more
semispecies is a _superspecies_. Dean Amadon, I believe, substituted
_allospecies_ for "semispecies," but I don't know why. From what I've
read, it seems unlikely that Western and Glaucous-winged Gulls can be
called semispecies of a superspecies; since they do meet and do hybridize
regularly, the situation looks much more like the Northern Flicker
situation, where it is more realistic to consider the two to be
subspecies of one large species. (But I'm not pretending to know the
gull situation well enough to say this with any authority.)

Now, regarding _cline_: A cline exists within a species when there is a
gradient of form across a geographic range. For instance, there seems to
be a cline in the Song Sparrows along the Pacific Northwest coast, as the
birds get larger and darker from south to north. The species is formally
divided into a few named subspecies across that range, and I'd leave it
to someone who's an expert on the species to say whether that division is
realistic or whether the morphological changes are quite continuous
across that range.

Burt Guttman guttmanb at elwha.evergreen.edu
The Evergreen State College Voice: 360-866-6000, x. 6755
Olympia, WA 98505 FAX: 360-866-6794

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