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Hybridization of Bird Species

Stanley Friesen sarima at tdatirv.UUCP
Wed Apr 29 14:34:46 EST 1992

In article <1992Apr27.072849.17251 at yang.earlham.edu> allens at yang.earlham.edu (Allen Smith) writes:
|        Actually, from what I know of the subject, differentiation by
|species via the hybridization success test is sometimes a matter of
|definition. There's one "species" ranging from someplace around Florida to
|Washington State, in which the ends can't interbreed. But the middle
|sections can do so, and the hybrids are quite successful.

Indeed.  This type of thing is called a ring species.  There are several
known examples (a salamander from the mountains of California, a circum-
boreal duck, ...).

I did not say that the biological species definition was always unambiguously
applicable!  Biology is seldom that clean.  There are a number of known
borderline cases, including introgressive hybridization zones (where the
hybrids successfully backcross only to *one* of the parent 'species').

In the case of ring species, the more or less official decision has been to
treat them as a single species because a well-defined chain of interbreeding
intermediates exists.  This is mostly a matter of making an arbitrary
decision for the purposes of convenience.

However, the existance of a few small areas of extensive hybridization
between two otherwise distinct forms, where the two forms coexist over
a much wider range than the hybridization zones, is *not* considered
sufficient to make the two forms a single species (otherwise we would have
to lump the domestic dog, the coyote, and the wolf as a single species).

|        Also, how does one decide where the species divisions are in the
|past? One can't exactly do a breeding experiment. Sometimes, enough DNA is
|isolatable to run a DNA hybridization check, but where does one draw the
|line? Admittedly, one way would be to look for hybrids. But what if the
|two species in question are seperated by time?

This is indeed a difficult problem!  One can only collect the best available
evidence and then make an educated guess.

This problem is not unique to extinct forms though.  Many times two closely
related living forms are seperated by a large geographic gap, so the check
for wild hybrids is impossible, and zoo hybrids are irrelevent to determining
species identity (lions and tigers can cross in the zoo).

A good example of this allopatric species problem is the matter of the
Eastern and Western Redbud. Are they one or two species?  They are extremely
similar, and in 'common garden' experiments they can be interbred freely.
But there is no common ground in nature, the Eastern is restricted to the
eastern deciduous forests, and the Western is restricted to the far western
mountain states - with the nearly treeless Great Planes between.  An argument
could be made for either decision (it depends on whether you emphasize the
morphologicasl similarity or the ecological differences).

Much can be done though. Studies of similar living forms can give clues
as to how different two forms can be and still successfully interbreed
in the wild.  One can pay close attention to mass death sites, since they
are a good way of telling if a set of forms belonged to the same breeding
population, and were thus part of a single species.  The recent discovery
of several such mono-specific bone-beds for several types of horned dinosaurs
(relatives of Triceratops) have already greatly clarified the species level
taxonomy of these forms.  It is now even possible to reliably determine
the sex of adult specimens of about half the species. (The upcoming detail
studies of some of the other bone beds will probably do the same for the
other half of the species).

The presence of more than one distinct morphology in a single time-place
is sure indication of either sexual dimorphism or two or more species.
(Note, a reasonable sample is needed to be sure one is not just getting
the two extremes of a single highly variable population - that is one
needs to be able to check for bimodal, or multi-modal, distributions).

In short, careful application of the principle of population biology
can give one reasonable evidence for the species boundries *within*
a single geographic formation.  Then we just apply the allopatric
species criterion we use for living allopatric species for the remainder.
[That is you guess].
uunet!tdatirv!sarima				(Stanley Friesen)

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