James West wrote,
Wouldn't the classic illustrative case of this be the Galapagos finches?
Geographical separation in the generally accepted sense not possible, so the
result is diversification, and eventually speciation, into econiches defined
by feeding behavior, that eventually become a kind of "micro-geographical"
separation?

The Galapagos finches (Geospizinae) are actually considered a classic case
of speciation through geographic isolation on an archipelago. The model is
that one pioneer species became established from a mainland population and
adapted to one niche. Then different subpopulations moved to other islands
and became adapted to different niches while acquiring reproductive
isolating mechanisms. (Genetic drift--the Sewall Wright effect--may come
into this process in the form of the founder effect, meaning that the small
population that establishes itself on one island may be genetically atypical
of the larger population.) Then the isolated populations gradually expanded
to the other islands, where they remained adapted to their particular niches
and remained distinct species because of the isolating mechanisms they had
acquired. This is the classical explanation for the great diversity in
small groups of birds (or any other organism for that matter) in
archipelagos; the Hawaiian honeycreepers (Drepanididae) are another example,
and Ernst Mayr based some of his arguments for geographic speciation on
other examples, such as the flycatcher Monarcha castaneoventris in the
Solomon Islands.

And isn't the diversity of Eastern American warblers a similar phenomenon?

Presumably each of these species originated in an isolated refugium,
isolated by factors such as glaciation. Look at Gene Hunn's contribution to
the other thread about "Northwestern Crows," which seems to have acquired my
name. As he points out, the Fish Crow of the southeast probably became
separate in the Caribbean and was isolated long enough to become a distinct
species, whereas the northwestern crow population was isolated for a shorter
time in an ice-age refugium and never acquired the reproductive isolating
mechanisms needed to make it a distinct species. And thanks, Gene, for
citing the Johnston reference, which I couldn't find when I sent my earlier
note.

Burt Guttman
The Evergreen State College 360-867-6755
Olympia, WA 98505 guttmanb at evergreen.edu

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