Hi Lars –
Ernst Mayr’s definition is more often called the “Biological Species Concept”
and abbreviated as BSC. In essence it suggests that populations of organisms
that recognize each other as “different” are species, and populations that are
BOTH willing to interbreed and able to produce fertile offspring are considered
the same species. If offspring are consistently sterile (mules) then the
parents are definitely different species, but the BSC has never treated all
cases of fertile hybrids as proof the parents are the same species. In
essence, the BSC expects species to have “isolating mechanisms” to keep them
separate. These can be “post-zygotic” like sterility and poor viability, or
“pre-zygotic” meaning behavioral or other differences that deter mixed
matings.
Mayr considered the typical method of speciation in birds to be the following:
Two populations of birds become separated (one colonizes a remote habitat, or
geological or climate changes split the range into separate sections). Over
time the two populations diverge evolutionarily as they adapt differently to
their respective local environments. When they subsequently come into
contact, they either 1) do not recognize each other as potential mates and are
therefore separate species, 2) they hybridize, the hybrids are fertile and do
well, and the populations merge – one species, or 3) they hybridize initially,
but the hybrids are inferior, so isolating mechanisms are selected for that
reduce and eventually prevent hybridization, so that they BECOME separate
species. In the latter case, post-zygotic mechanisms (inferiority of the
hybrids) led to the development of pre-zygotic mechanisms (song differences,
different courtship rituals, differences in breeding season, etc.). This is
precisely what is thought to be happening with Baltimore and Northern orioles:
introduction of agriculture, etc. to the great plains created oriole habitat
that bridged the gap between the populations, they met and hybridized, and
rather quickly the hybrids were selected against, and now hybridization is much
less common.
Since then, several other potential outcomes have been observed that do not fit
this pattern, and make species status difficult to assign. The BSC has been
modified several times over the 80-odd years since Mayr first articulated it.
The essence has stayed similar, but the modifications have changed the ways the
equivocal cases are handled – the cases where some hybridization is happening,
but less than if all the individuals were choosing mates at random. In part
these modifications have happened because new technologies allow for better
understanding of what is going on. In general, these modifications are leading
to more of the equivocal cases being treated as separate species.
Here is a quick summary of some potential situations among bird populations:
1. Breeding ranges of two populations overlap, and most individuals mate
within-populations, but a small minority mate with the other: usually treated
as separate species whether offspring are fertile or not. Bird examples:
Barrow’s X Common goldeneye, White-crowned Sparrow X Dark-eyed Junco.
2. Breeding ranges are mostly separate but have a relatively small zone
of overlap, with hybrids fairly easy to find, and fertile but not as fit:
usually treated as separate species. Bird examples: Some new info claims
that where “California” Scrub-Jays meet “Woodhouse’s” Scrub-Jays in southern
Nevada, they hybridize but the hybrids are selected against.
3. Breeding ranges are mostly separate, but meet in a narrow band, with
lots of hybridization, however hybrids seem not to disperse much out of that
band (as breeders). Formerly treated as subspecies, now tend to be called
species. Bird examples: Tufted Titmice and Black-crested Titmice in Texas,
Oak and Juniper Titmice in the Klamath and Lost River basins, Hooded and
Carrion crows in Europe.
4. Breeding ranges are largely separate but where they overlap,
hybridization is common, and the zone of interbreeding widens with time:
Treated as one species. Bird Example: Northern Flicker
5. Bird population A expands its range into that of population B, and
there is a fair amount of hybridization at the leading edge, where A is much
rarer than B, but hybridization becomes rare after population of A becomes
larger. I like to call this the Stephen Stills effect. The classic example
was the expansion of Syrian Woodpecker into Europe, through the range of the
Greater Spotted Woodpecker. I think this is happening with Eurasian Wigeons –
I believe they have colonized Alaska, and the hybrids are coming from places
where Eurasians are rare, but mixed with many American Wigeons.
6. Bird population A expands its range into that of Bird population B.
They hybridize along the leading edge, but both the hybrids and the B-type
birds disappear from the hybrid zone, leading to replacement by A-type birds.
These tend to be treated as separate species. The best-studied example is
Blue-winged Warblers displacing Golden-winged Warblers. The same thing seems
to be happening at a slower pace with Townsend’s Warblers displacing Hermit
Warblers.
7. Two closely related forms meet in multiple places, and in some they
overlap without (much) hybridization, and in others they do hybridize. These
are some of the most difficult cases for imposing our static species labels on
dynamic nature. This seems to be the case with Pacific-Slope and Cordilleran
Flycatchers. In California and the states to the east, they act like separate
species. In Washington, British Columbia, and the northern Rockies, they seem
to merge pretty thoroughly.
Hope this helps
Wayne
From: obol-bounce@xxxxxxxxxxxxx [mailto:obol-bounce@xxxxxxxxxxxxx] On Behalf Of ;
Lars Norgren
Sent: Thursday, March 10, 2016 7:12 AM
To: dmarc-noreply@xxxxxxxxxxxxx
Cc: David Lauten; obol
Subject: [obol] Re: The continuing Sapsucker Dilemma
This discussion has come up regularly on Obol and hopefully will continue to do
so. The "biological definition of a species" (BDS) is closely associated with
an Austrian academic named Ernst Mayr . It dominated biological education in
mid-twentieth century America. I got it more from my parents, who went to
college in the fifties, than from textbooks or in the classroom. According to
the BDS if the offspring of two species is fertile, the two parents must be
con-specific. I presume this is why the AOU recognized only two species of
sapsuckers back in the sixties when I started birding. When I saw my first
Red-breasted Sapsucker on Country Club Hill in Corvallis back in 1968 I opened
the Golden Field Guide to discover it was "only" a subspecies, and promptly
lost interest. I had already seen a Yellow-bellied Sapsucker in my own yard,
also in Corvallis.
A few years later the AOU lumped a bunch of common species, resulting in
"Northern Flicker", "Yellow-rumped Warbler", and "Northern Oriole". This was
all due to field studies that had documented hybrid zones of
Yellow-shafted/Red-shafted Flickers,etc and the offspring were fertile. From
what I have read, Ernst Mayr would not have been welcome on Obol. He had no use
for common courtesy and viciously defended his BDS throughout his life. All too
often the history of science is strongly affected by personality. One of my
sons told me that science advances one obituary at a time . I failed to closely
scrutinize the "Yellow-bellied" Sapsucker that overwintered at my house for
three years and will never know if it was a Red-naped or Yellow-bellied, Both
were considered subspecies so the lister in me didn't care.
As mentioned earlier in this thread, the AOU did split the sapsuckers.
They resplit the Northern Oriole and elevated Sooty and Dusky Grouse to full
species. The AOU will probably resplit the Gray Jay. "Oregon" Jay was a full
species back in the thirties. Ironically, another concept of Ernst Mayr, the
"Rasenkreits", was based on his armchair study of large, white-headed gulls.
The advent of DNA testing revealed a messier relationship among the Herring
Gull complex than Mayr would have liked. The pink-footed Glaucous-winged Gull
is more closely related to the yellow-footed California Gull than to the
pink-footed Western Gull. Despite genetic distance, the Western and
Glaucous-winged Gulls readily hybridize. On Destruction Island, Washington, 60%
of breeding gull pairs are ascribable to neither species. This "Olympic Gull"
poses the question of definition of a hybrid.
I guess a true hybrid is when a "pure" Western Gull mates with a
"pure"Glaucous-winged Gull. As I recall, such pairings were only documented 7%
of the time when someone took the trouble to go into the field. Olympic Gulls
are certainly fertile and generally mate with other Olympic Gulls.
On Wed, Mar 9, 2016 at 11:11 PM, Andrew Thomas <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
Thanks, Dave Lauten, for your thorough response. Point taken. (Maybe we need
canofworms@xxxxxxxxxxxxx.) I am not a scientist, but I understand from your
post, as well as from my desultory reading over the years, that evolution
theory presents life as a continuum. So, back to birds; no more worms.
sent from iPad
On Mar 9, 2016, at 7:06 PM, DJ Lauten and KACasteleinPOST: Send your post to obol@xxxxxxxxxxxxx
<deweysage@xxxxxxxxxxxx> wrote:
Well this opens a can of worms....
