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Re: Willow Flycatchers and DNA tests (long)




On Sun, 8 Jul 2001, mjwestphal wrote:

> Just a comment on the Willow/Alder Flycatcher discussion.  
[snip]

> I don't know about DNA tests.  Maybe someone else can answer that.

As someone who uses the tools of molecular biology (that is, "DNA tests")
to study birds, I'll offer a little perspective, not only on the
willow/alder situation, but on the general power of these tools to answer
questions like "how many Empidonax flycatcher species are there?"  I'll
admit upfront that I'm not sure if molecular work has yet been published
on North American Empidonax.

I commonly see sentiments from birders that imply that once the "DNA
tests" are done, all the thorny problems will go away.  It just ain't so.
Molecular techniques are a set of new tools, and sometimes the insights
they give are unique and powerful.  But they certainly don't provide all
the answers, and the answers they do provide are sometimes not clearcut.
Nature herself isn't always clear cut (insert logging joke here).  

What matters is the biology of the bird populations - are these two groups
of birds reproductively isolated, for instance?  The actual DNA sequences
of the genes are one way of studying that (groups of birds that don't
interbreed will differ more than groups that do interbreed).  In some
circumstances, DNA tests may be the best way to measure population
differences, but at in other situations, behavioral or plumage data
could be more informative.

After all, plumage is controlled by genes, song is controlled by genes
(especially in flycatchers like Empidonax, which don't learn their songs
but sing them "innately"), and behavior is influenced by genes.  Knowing
the sequences of bases in the DNA of some particular gene in a flycatcher
is another piece of information to add to the puzzle, but in birds like
the Willow and Alder Flycatchers, which are so well known, it's only one
piece.

The reason Willow and Alder Flycatchers were separated into two species
was because the two have different habitat preferences, different ranges,
different nest placement (usually 1-4 feet off the ground for Alder, 5-20
off the ground for Willow) and nest construction (much soft lining in
Willow, little in Alder) and different songs and calls. There are minor
(trivial?) plumage differences.  All these differences persist even when
the two types nest in the same swamp, and the differences are presumably
maintained because the two types do not interbreed - they're full species.  
They just happen to look very similar, which is why it took so long to
recognize that they were two species.

I'm not sure what exactly a "DNA test" would add to that picture.

There are certainly questions to which DNA tests have supplied clear and
surprising answers.  We now know that more than three quarters of
apparently monogamous birds engage in extra-pair copulations, leading to
mixed paternity in nests.  The birds were very sneaky about this -
*nobody* suspected the extent of avian infidelity before molecular tools
showed the truth.  Also, some other mysteries such as Cox's Sandpiper, a
fantastically rare bird with an odd distribution was proven by molecular
methods to by a hybrid.  If I remember right, one parent was a Pectoral
Sandpiper and the other was a Dunlin, and the researchers could even tell
which species was the mother and which the father.

But quite often, the answers from the DNA tests are equivocal.  The
"timberline" sparrow, which used to be considered a disjunct northern
subspecies of Brewer's Sparrow, has several behavioral differences leading
some to propose recently to elevate it separate species status.  However,
"DNA tests" show that it's not very different genetically from Brewer's
sparrow.  Does this mean that it's *not* a good species after all?  Not
necessarily.  If Timberline Sparrows have become reproductively isolated
from other Brewer's sparrows, it might take hundreds of thousands of years
for genetic differences to accumulate to the point where there was a clear
detectable difference.  Nonetheless, they would be still be valid species
during that time by most definitions.  It will take more than a DNA test
to settle this question.

Well, I've muddied the waters enough.  I think molecular work, combined
with field work, will answer all sorts of new exciting questions -
otherwise I wouldn't be doing it myself.  I just wanted to warn against
seeing a "DNA test" as a foolproof solution to all these problems.

And as an afterthought, recording and analyzing bird songs for ten years
has taught me that verbal descriptions are no substitute for sound
recordings, either.  Claiming that a single bird sang both Willow and
Alder songs is sort of like claiming to have seen a Shy Albatross on the
back 40.  No matter how experienced the observer, we want to see the
photos.

Chris Hill
Coastal Carolina University
Conway, SC