Looking back on my post of two weeks ago [1], I fear I may have left the impression that Rob Dillon is smarter than Calvin Goodrich, and indeed smarter than any other malacologist who has ever waded knee-deep into the rivers of the greater Midwest before or since. That's probably not an uncommon failing of mine, but nothing could be farther from the truth. In fact, the revelation that the gastropod populations we have been calling Pleurocera acuta for 189 years are actually the same as what we've been calling Pleurocera canaliculata for 192 only dawned on me after several years of being dope-slapped by the glaringly obvious. And even then, I was only able to figure out the "Goodrichian" relationship between P. acuta and P. canaliculata backwards, through P. pyrenellum.
So to set the situation aright, I hereby offer an amendment
to my essay of 3June13. And in this
retelling I propose to lay bare for my readership the agonizingly slow process
by which this particular snail-guy's mind actually works.
The initial revelation that Goodrichian taxon shift might be
even more dramatic than Calvin Goodrich himself realized first dope-slapped me
in the back of my head eight years ago, as I scored a set of allozyme gels
comparing pleurocerid populations from the upper Powell drainage on the Virginia/Tennessee
border. I wrote this the bottom of my
data sheet late in the afternoon of October 4, 2005: "Good grief! Pleurocera unciale and Goniobasis clavaeformis
at the mouth of Indian Creek are a single random-breeding population!" John Robinson [2] and I wrote this initial
evidence of Goodrichian taxon shift into a gray literature report in 2007 [3]. I then sampled additional populations of
nominal clavaeformis, unciale, acutocarinata, curta, carinifera and vestita
across East Tennessee and North Georgia in 2008-09, ran a big mess of
additional gels, and published the formal paper subsuming the genera
Goniobasis and Elimia under Pleurocera in 2011 [4].
Meanwhile, my general survey of the freshwater gastropod
fauna of East Tennessee was moving forward.
I spent at least a week or two working in the rivers and streams between
Knoxville and Chattanooga each field season between 2007 and 2010, collecting (what
I recorded as) P. clavaeformis (in its various phenotypic forms) in essentially
every stream I visited for four years.
And incidentally, I also observed that the big river populations (historically
referred to P. unciale or curta) sort-of petered out around Knoxville, to be
replaced in the main Tennessee River by Pleurocera canaliculata.
I took a second dope-slap to the back of my noggin at
approximately 4:30 PM on August 14, 2010.
Standing knee-deep in Savannah Creek (maybe 20 km N of Chattanooga) on
that memorable day, it suddenly struck me that the snails crawling around my
feet might be Pleurocera acuta. At that
point in my career, I had collected Pleurocera acuta up in Kentucky, Ohio and
Michigan, but would not have expected a P. acuta population anywhere within 500
km of the stream where I was standing.
When I got back to my truck, I wrote this in my field notes: "(star)Wow(star) Is this Pleurocera
acuta??? My eyes are opened!"
So back in Charleston, I pulled four years of old samples
off the shelves, dug the old literature out of the file cabinets, and (over the
course of several weeks) pieced together the following line of reasoning.
Inference. The snails I collected in Savannah Creek on
14Aug10 must be what Goodrich would have called "Pleurocera pyrenellum," not
Pleurocera acuta. Goodrich (1940) gave
the range of P. pyrenellum as "tributaries of the Tennessee River in Morgan and
Lancaster Counties, Alabama, and Walker County, Georgia" [5]. Walker County wasn’t much more than 20-30 km
downstream from Savannah Creek. So "pyrenellum" my snails would seem to be.
Therefore Pleurocera
pyrenellum populations inhabit Tennessee tributaries further upstream than
Goodrich realized. Going back through my
old collections, I recognized P. pyrenellum at 10-12 additional sites,
extending up the Tennessee Valley all the way to the environs of Knoxville [6]. They were usually mixed with populations of
P. clavaeformis, and (dazzled as I had been by the phenotypic variety of
clavaeformis) I had simply missed them.
Revelation. Travelling down the main Tennessee valley
from Knoxville to Chattanooga, Pleurocera canaliculata begins to appear in the
main river (red above) as P. pyrenellum begins to appear in the tributaries
(pink). This was the third major dope-slap
I took during my long bewilderment with the East Tennessee Pleuroceridae, although
I didn't record the date and time of the whacking. The relationship between pyrenellum and
canaliculata looks exactly like the Goodrichian relationship between
unciale/curta and clavaeformis (blue above).
Hypothesis. But the range of P. canaliculata extends
throughout the greater Midwest, up the Ohio River all the way to Pittsburgh,
while nominal pyrenellum populations are restricted to drainages of the Tennessee. So might the small-stream form upstream from
all those populations of canaliculata in Ohio, Kentucky, Indiana and Illinois
be what everybody calls P. acuta in Yankeeland?
The point of this confession is that my colleagues and I
were no more able to see the Goodrichian relationship between acuta and
canaliculata standing on the shores of the Wabash River than Calvin Goodrich [7]. I flipped the whole story around when I told
it two weeks ago, so that it "made sense," following the conventional practices
of science. I made the process of
discovery look logical, even inevitable, when it most certainly was not. I'm really a bonehead. Ask my wife.
Allow me to close with another of my philosophical
peregrinations. My regular day-job is
teaching majors-level genetics at a very ordinary liberal arts college of
regional reputation. It’s a big chore –
there are three or four other members of my department doing the same
thing. All younger than I, of course.
So I organize the subject matter of genetics historically,
the way I myself learned it, starting with Mendel, then Morgan, then Beadle
then Avery and McLeod and McCarty and Watson and Crick and so forth. And just when it begins to look like we might
get anywhere near the present day, I go back to Hardy and Weinberg and finish
with the "Modern Synthesis" of the 1930s and 40s.
Some of my colleagues tell me that undergraduate students "don’t get this." They prefer to start their
majors-level genetics sections with our modern understanding of the molecular
basis of inheritance, because given the underlying mechanism, Mendel's 1866 results
become a lot easier to explain. And the
kids prefer to focus on today's science, in any case. They don't care about peas.
That line of argument is so self-evidently horrific to me
that in my first draft of this essay, I left it dangling in the wind, without
comment. Like Stede Bonnet on the
Charleston battery.
But I shall close with the simple observation that the K-12 teaching
of science and the profession of science in higher education are two entirely
different things. In the former we are
building palaces, and in the latter we are laying roads. I trust that my readership has enjoyed this
month’s brief foray into road building.
Next month, it’s back to the masonry.
Notes
[2] John D. Robinson was working on his MS in Marine Biology
with me at the time. He went on to earn
his PhD at the University of Georgia and has recently moved to the Cornell
area.
[4] Dillon, R. T., Jr. (2011) Robust shell phenotype is a local response to
stream size in the genus Pleurocera.
Malacologia 53: 265-277. [pdf]
- Goodbye Goniobasis, Farewell Elimia [23Mar11]
[5] Goodrich, C. (1940) The Pleuroceridae of the Ohio River drainage
system. Occas. Pprs. Mus. Zool. Univ.
Mich. 417: 1 -21.
[6] By the 8/2011 debut of the FWGTN website, I had
documented 8 populations of P. canaliculata in the big rivers below Knoxville
and 24 populations of nominal "P. pyrenellum" in the tributaries. Since I've decided to save the nomen "pyrenellum" as a subspecies (more about that in a future post) the
distribution of both shell forms is still apparent on the pdf map available from the P. canaliculata page on
the FWGNA site.
[7] In fact, the Indiana survey of Pyron et al. (2008)
retained both P. canaliculata and P. acuta as quite distinct species. For a pdf of Pyron’s survey, see:
- The Freshwater Gastropods of Indiana [23Jan09]
Posts
No comments:
Post a Comment