Hi, Roger -
In the 1980s-1990s there was a major challenge to the use of the Biological
Species Concept (BSC) by the "Phylogenetic Species Concept" (PSC). I won't give
a detailed description of the PSC, but note that under it there would be a lot
more splitting, and hybridization would be considered less of an issue.
In about 1998 Ned Johnson of the Museum of Comparative Zoology, UC Berkeley,
and co-authors published a paper describing an updated BSC. At that time the
AOU Checklist Committee endorsed the new and improved BSC, and that has largely
informed the committee's decision-making since then. The BSC was first
formulated by Ernst Mayr and colleagues in the 1930s, and a lot has been
learned about population biology and genetics since, so an update made sense.
Under the new BSC, hybridization is considered less indicative that two
populations are the same species than previously. The important point to
understand here is that hybridization should not be treated as a True/False
question. A whole lot of grey exists between "never hybridizes" and "hybridizes
freely." Numerous examples are available of populations that hybridize
sporadically, but where most individuals mate with with "their own kind."
So under the new and improved BSC, populations that are mostly allopatric
(non-overlapping ranges) but occasionally meet, with some hybridization can
still be considered species. Also populations that overlap with quite a bit of
hybridization, but with evidence that the hybrid offspring are less viable or
selected against, can be considered species.
In general, if hybridization is not so frequent that it forms a large and
expanding "hybrid swarm" species status may be maintained. Several "kinds" of
birds have eastern and western forms that meet on the Great Plains, with at
least some hybridization. One of these, Northern Flicker, was lumped because
hybrids are very widespread, and not much evidence exists that birds in the
contact zone are very choosy. Several others, Rose-breasted/Black-headed
grosbeaks, Indigo/Lazuli buntings, etc. have been maintained as separate
species because the hybridization is fairly sporadic. Baltimore/Bullock's
orioles were first lumped into "Northern Orioles", then re-split, when more
evidence of assortative mating (discrimination, or "choosiness") was
discovered.
In a few cases (e,g, Tufted/Black-crested titmice) two wide-spread "pure"
populations are connected by a narrow band with lots of hybrids but the band
remains narrow, and the hybrids do not seem to be dispersing out into the
"pure" main populations. These are now considered separate species.
Townsend's/Hermit warblers may have a similar dynamic.
So, 2 take-home messages:
First, those who make the rules (AOU Checklist Committee) are comfortable with
giving species status to populations that sometimes hybridize, but mostly
maintain separate identities.
Second, "Species" are in part human constructs that work well to describe the
diversity of life in a majority of cases, but examples are not rare where
nature is messier, i.e. less divisible into discrete units. In those cases, a
lot of judgement is required and the best we can hope for, is that the
decisions are made somewhat consistently. The AOU Checklist Committee seems to
be trying to be consistent.
Finally, I want to say that I tend toward being a "splitter" because the more
"kinds" there are to look for, the more of a challenge it is. To me, it is an
improvement that when I am in SE Oregon, I cannot just say "Scrub Jay" and move
on. "Scrub Jay" has become a category that deserves a closer examination, and
that is a good thing.
Wayne
From: "roger freeman" <carrotguy55@xxxxxxxxx>
To: "obol" <obol@xxxxxxxxxxxxx>
Sent: Thursday, July 7, 2016 9:02:46 PM
Subject: [obol] Western Scrub-Jay split and Speciation
Cool thread about the splitting of our beloved WESTERN SCRUB-JAY into two
different species.
I am fascinated by the discussions that relate to splitting or lumping species
of plants or animals. Or about hybrids that occur between "different"
recognized species I read a lot of the manuscript that Bob Archer provided
earlier .... here it is again.
http://www.aoucospubs.org/doi/pdf/10.1642/AUK-16-77.1 ;
Several times the evidence for making the species split is given as ....
"Differences in morphology, ecology, vocalizations", etc .... But there appears
to be little/no evidence or mention of the inability to hybridize/mate and
generate normal and fertile offspring/progeny that can successfully mate again.
Even though there is some debate about what constitutes speciation, "the
biological species concept" would state simply that .... If two organisms
(plant/animal/fungi) can mate and produce fertile offspring, then they are the
same species. Granted this can be harder to "check" in animals vs plants.
Physical isolation for centuries can cause changes in morphology, but it
doesn't necessarily change speciation. With genomic technology available today,
it is quite simple to show clear significant differences in DNA sequences
between very different individuals of the same species.
The use of sub-species classification is commonly used, and provides a good
option for where there are differences in morphology, vocalizations or ecology,
but mating status is unknown. Domestic dog used to be considered a different
species than wolves, but that has recently been revised so that domestic dog is
now a sub-species of wolf due to the fact that they can successfully interbreed
and dogs are selected descendants from wolves.
Lumping or splitting is an ongoing debate in the taxonomic world.
Good birding!
Roger Freeman
