No excedrin for me. I find this topic fascinating even though I am an
occasional and casual bird watcher, and not at all a scientist. Thanks, David
Irons, for taking the time to present it clearly and concisely. Also, thanks to
Shawneen for the summary of the ring concept of differentiation.
Andrew Thomas
On Friday, March 8, 2019, 11:58:41 PM PST, David Irons <llsdirons@xxxxxxx>
wrote:
Steve Harder posed this question to me this morning: For those of us less
versed in this discussion, could you please define the criteria used to
determine when a group of individuals from a known species is or can be
construed as a subspecies? Does a certain amount of geographic isolation
matter?
In the context of this discussion, here's how I would answer Steve's question.
In the simplest terms a "subspecies" is a subset or sub-population of a species
(in this case Song Sparrow) that through reproductive isolation has developed
genetic and morphologic characteristics that differ from the rest of that
species' populations/sub-populations. The morphologic differences may or may
not be readily evident in the field to the average birder.
Some period of geographic separation or habitat fragmentation ("island effect"
if you will) typically facilitates the isolation. As an example, "Ice Ages," or
glacial maximums are believed to have precipitated divergent evolution within
many species, as sub-populations were separated from other members of their
species by continental ice sheets for long periods.
Subspecies are typically only marginally different genetically from other
populations within their species and when pairs of subspecies come into contact
on breeding grounds they may interbreed somewhat freely. Good examples of this
are Slate-colored and Oregon Dark-eyed Juncos and Yellow-shafted and
Red-shafted Northern Flickers. These two pairs of distinctive subspecies
interbreed frequently resulting in swarms of recognizable intergrades. With
Song Sparrows subspecific differences in appearance are for the most part
subtle, thus intergrades are unlikely to be as identifiable as the intergrade
juncos and flickers.
With most species, interbreeding that produces hybrid offspring is somewhat
rare, even in situations where two species within the same genus share a broad
zone of contact on their breeding grounds. Of course there are notable
exceptions to this rule. In our own region Western and Glaucous-winged Gulls,
Townsend's and Hermit Warblers and Red-breasted and Red-naped Sapsuckers
hybridize routinely. These species pairs act more like subspecies pairs on the
breeding grounds and in fact not that long ago Red-breasted, Red-naped, and
Yellow-bellied Sapsuckers were considered conspecific. I find it interesting
that Townsend's and Hermit Warblers interbreed routinely in their contact zone,
but neither species seems to hybridize with "Audubon's" Yellow-rumped Warbler,
which occurs in abundance across the breeding range of both Townsend's and
Hermits.
It can be difficult to wrap our heads around the idea of a subspecies.
Genetically speaking, the line between where subspecies and species begin and
end is a murky and subjective. It's vital to remember that 'species' and
'subspecies' are human constructs that satisfy our inherent need to sort,
categorize, define relationships and put names to things based on their
differences and similarities. How the birds sort themselves on the breeding
grounds doesn't always align with our human notions about their relationships
and the differences we humans see in their appearances may or may not be
integral to this process.
The various Song Sparrow subspecies being discussed represent sub-populations
of birds that are generally found breeding and or residing permanently within a
certain range that does not include Western Oregon. We call them subspecies
because each of these sub-populations shares some common set of morphological
characteristics that were at some point described and quantified to the
satisfaction of the ornithological community. Some Song Sparrow forms are
migratory while many others are not. The relative similarity in appearance of
many of these subspecies makes trying to determine where and how much they move
around a considerable challenge. What we do know is that the Sauvie Island bird
is clearly different in appearance from the Song Sparrow types found in Western
Oregon year around. In appearance it resembles some eastern birds, at least one
subspecies that is widespread in California and perhaps the population that
breeds in the Northern Rockies and Great Basin. Hence our thirst for trying to
understand where it may have come from.
It makes my head hurt to think about some of this stuff. I hope that I haven't
sent too many of you looking for the Excedrin after reading this.
Dave IronsBeaverton, OR
