Hi, Roger -
As you noted there are multiple species concepts in play. I mentioned the 2,
BSC and PSC, because of the number of committed and sometimes strident
advocates they had during the 1980s-90s, and because PSC advocates were pushing
its adoption onto the AOU Checklist Committee.
Two more points of interest:
First, the BSC is more of a "lumper's" species concept than the PSC or many of
the others.
Second, the story played out the other way in Europe, with Phylogenetic Species
Concept advocates becoming dominant. This is why European sources split the
large white-headed gulls into many more species. This is also why our Mongolian
Plovers became "Lesser Sand-plovers," etc.
You commented about assortative mating. Assortative mating in some sense may be
in play in play in many species, but it plays a particularly important role in
places where closely related populations overlap geographically. If mating is
perfectly assortative, i.e. if all members of Population A only mate with
members of Population A, and B only with B, then of course the BSC and most
other species concepts require that they be considered separate species. If
mating is completely random Then BSC and some other species concepts (not all)
would call them a single species. But again there is a lot of gray area between
these extremes: They can overlap and mate with their own kinds, with rare to
occasional "mistakes." The single documented occurrence in northern California
a few years ago of a Black Phoebe X Say's Phoebe hybrid should not be enough to
make us lump these usually distinct species, for example. Or they can
interbreed more commonly but with the offspring selected against. This seems to
be the case in the one well-studied California Scrub X Woodhouse's Jay overlap
zone. In cases like Tufted X Black-crested Titmice (or Carrion X Hooded Crows
in Europe) they can interbreed freely in a very narrow contact zone but without
intermediates expanding out of this zone. The BSC interpretation here is that
the main populations (back from the contact zone) have some as yet poorly
understood mechanism for maintaining their integrity (i.e. for preventing
spread of "foreign" genes. There are tougher cases where multiple contact zones
occur and the birds behave differently in different places.
One situation that leads to some mixed mating between separate species is where
one of the two is very rare, and individuals have difficulty conspecific mates
but the other species is available. The only wild Smew I ever saw was a drake
in Kodiak in 1976, and it was enthusiastically displaying to hen goldeneyes. I
do not know that any were tempted, but had mating occurred, that would hardly
be reason to lump the two species. Similarly, the Steller's Sea Eagle that
lived at Taku River near Juneau for years was reported to have nested with a
Bald Eagle. I like to call this the Stephen Stills effect after his song "Love
the one You're With."
In the Oriole situation, a study done in the 1940s or '50s collected birds on
transects across the Great Plains and found lots of hybridization in the
central areas, leading to the lumping. Then the transects were repeated several
decades later, and fewer hybrids and more "pure" birds were found throughout.
The authors concluded that isolating mechanisms were evolving rapidly, and the
two forms were re-asserting their separate identities, hence the re-splitting.
Having lived in Kansas for a while between the two studies I am not totally
sold by the conclusion of rapid evolution: it was clear to me that the type and
abundance of oriole habitat was also changing rapidly, and I think that could
explain some of the differences between the two sets of samples.
Wayne
From: "roger freeman" <carrotguy55@xxxxxxxxx>
To: whoffman@xxxxxxxx, "obol" <obol@xxxxxxxxxxxxx>
Sent: Friday, July 8, 2016 8:10:46 PM
Subject: Re: [obol] Western Scrub-Jay split and Speciation
Hi Wayne,
Thank you for the insightful, thoughtful and clear discussion regarding
alternatives to the Biological Species Concept (BCS) such as the Phylogenetic
Species Concept (PSC) that you describe. Well said. As you are likely aware,
there are other options such as Isolation species Concept, Ecological Species
Concept, Cohesion Species Concept, etc. It is messy but as Lars stated
...."real progress is being made in taxonomy, and at an accelerating rate". The
world of genomics and DNA is shining the light in many dark corners.
For sure, this is a debatable and mushy subject, defining what is a species,
and I respect the job and challenge the AOU Checklist committee has in trying
to sort this out. These are intelligent and experienced people making judgement
calls based on the best information available. Depending on the species concept
you prefer, there remains some room for debate about the decisions made on
splitting and lumping. This is by no means a perfect and indisputable science.
Charles Darwin stated ...."Those who make many species are the splitters, and
those who make few are the lumpers". As you admitted, you tend to be a splitter
and I would admit I tend to be a lumper. There is remarkable genetic variation
within every species and selection pressures create diversity. Assortative
mating is a type of selection pressure and is likely present in all species.
Mating choices are often made related to the most colorful, the strongest, the
loudest, the biggest, etc. But why would assortative mating be a reason to
split the Northern Oriole again? If a collection of female Baltimore and male
Bullock's orioles were placed in a large aviary, would they readily breed and
hybridize? And subsequently, would they have progeny that continued to
successfully hybridize and blend their respective gene pools. Or would the
original individuals just eventually die of old age, with a few if any
infertile offspring? BCS would provide a species answer here. Of course, it
becomes a debate again of one's preference of species concept.
I wish there was another taxonomic layer below species that wasn't
"sub-species", which sounds so ... ho-hum. Morphological or vocalization
differences just doesn't feel like enough evidence to split species, but
inability to successfully hybridize does. And believe me, I have no clue
whether California and Woodhouse's Scrub-Jays successfully hybridize out on the
open range. Probably they don't!
Anyway, good stuff! Please enjoy birding whether you are a splitter or lumper.
Roger Freeman
On Fri, Jul 8, 2016 at 9:29 AM, < whoffman@xxxxxxxx > wrote:
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
