After DQP and Ockham, two more principles (from the birds)

[Michael Gochfeld]

I have had various parts of my anatomy kicked on this list, but I have
not as yet been chastised for invoking avian examples to illustrate
points.  So here I am again using two short communications on  two
rather drab sparrows which sing two uninspired songs---as a basis for
examining views on speciation.  This is a very long (and rambling post)
so hit delete now.  Otherwise stay tuned.

In 1999 Klicka et al. (a group of five authors from UNLV, Bell Museum
(Minn), Royal Ontario Mus, Alberta Mus (Edmonton), and a National
Wildlife Refuge) proposed that an isolated population of sparrows from
Yukon and BC, previously considered to be a subspecies of Brewer’s
Sparrow (Spizella breweri), was actually a distinct species (S.
taverneri) which they called the Timberline Sparrow.  I remember reading
at the time that the habitat and altitude were distinctive, the voice
different, and the molecular data supported divergence sufficient to
convince them (and the referees), but not me,  that it was a species.

Ernst Mayr and Ned Johnson (MVZ-Berkeley) wrote a rebuttal to The
Condor, arguing that the data supported only subspecies designation.
They note in their abstract “For several reasons we disagree with their
conclusion: lack of evidence for the reproductive isolation
.loss of
information on the close relationship and allopatric distribution of the
two taxa that would accompany their elevation to species, and violation
of the principle of taxonomic balance.  Until the demonstration of vocal
or display differences relevant to pair formation and maintenance,
taverneri and breweri are best regarded as reproductively compatible
subspecies.”

This caught my eye for two reasons.  Felix has identified two
“principles”, the DQP or duck-quack principle and Ockham’s Razor. Mayr
and Johnson highlight two more: the “information principle” and the
“taxonomic balance principle”, the latter of which I don’t think we have
discussed.

Also they seem to exemplify conservative taxonomy.   My first thought
was that since the 1998 AOU Checklist treated the two as subspecies, M&J
would argue don’t change their status without definitive evidence
(obviously more definitive than they feel Klicka provided).  That would
be conservative.

But is it conservative.  The AOU Checklist (1998) shows that Swarth and
Brooks published S.taverneri originally as a separate species back in
1925.  Of course this was a time when subspecies concept was a novelty
and most new birds were described as species. I think that is beside the
point.

The 1931 AOU Checklist (which has sat unopened on my shelf for many
years) shows that the AOU treated the two as subspecies.  The checklist
doesn’t dwell on criteria or how this decision was reached, and I can’t
find reference to a revision between 1925 and 1931 that sank taverneri
to subspecies level.

Ergo: it would have been conservative back then to have kept them as
different species. But by that time the lumping craze (or trend) had set
in.
And the new data provided by Klicka et al. in 1999 would have validated
their original status----i.e. conservative.

So who or which of these authors is conservative?

Mayr and Johnson note that the two populations are separated by habitat
and by 150 miles. They say that S.taverneri is an incipient speices “as
are all geographically isolated populations and we must infer whether or
not it has already reached species level”.

They look at reproductive isolation as the chief criterion (no
surprise), and require information on pairing behavior.  Therein lies
the rub. Allopatric forms cannot test the hypothesis, and in captivity
birds may often pair inappropriately, particularly if they don’t have
the choice.  Does this doom all allopatric forms to subspecies status.
Couldn’t be.

Ironically, yesterday I gave a lecture on bird song and communication in
my wife’s Animal Behavior class, and I used species-specific song
learning and copying among sparrows as one example.  Obviously no one
has tested these two species, but my colleagues in Millbrook worked for
years with Song and Swamp Sparrows and showed that Song Sparrows have
limited ability to learn Swamp Sparrow syllables, but not vice versa,
while White-crowned Sparrows can learn many different songs from
unrelated species.

So this conflicting background information doesn’t help interpret the
vocal differences that Klicka et al published. They could be local
dialects, or species specific.  That would be a lot easier to test in
the laboratory.  If species A responds to A song much more than B song
and species B responds more to B than A song, that strengthens the
notion of separate species.  That’s a big advantage birds offer over
butterflies.  Working with tactile and olfactory cues is much more
difficult in fact if not principle, than working with acoustic signals.

Mayr and Johnson elaborate an “information principle”.  If  A and B are
listed as separate species their relatedness is obscured and in fact one
might assume that A and B are just as different as A vs C and B vs C.
On the other hand, they argue that if A and B are listed as subspecies
their identify is maintained while  their relatedness emphasized.

 But it is the principle of balance that is the punch line of this
posting.   “All entities (taxa) within a Linneaean category should be as
equally different from each other as possible. For instance, one should
not raise a genus to family rank when this family would be far less
distinct than the other related families are from each other.”

M&J then provide a clincher based on mitochondrial DNA.   A and B
(taverneri and breweri) differ by only 0.13% of base pairs while the
other species of Spizella differ by about 6%.  Avise notes that many
subspecies pairs differ by 2-3%.  But the water is muddied by Johnson’s
own work with Sapsuckers where species that show assortative mating,
have very small mtDNA differences.

Klicka et al. responded to M&J: “We concluded that taverneri represents
a newly evolved phylogenetic species and probably a biological species
as well.”   “We believe that the arguments presented by Mayr and
Johnson
.are equivocal and show why the biological species concept (BSC
) HAS BEEN LOSING FAVOR IN MANY DISCIPLINES” (EMPHASIS MINE).

Well clearly if they are discarding the BSC they can’t be conservative.

I see the lingering problem that two taxa may not show assortative
mating (yet) but there could still be selection against their offspring
(hybrids).  That’s not evolutionarily stable, but it must be happening
to many taxa at any one time.

So now it becomes apparent that Klicka et al. treat them as separate
species, whether or not they are really biological species.  They rebut
the information argument suggesting that keeping them separate provides
information that they are separately evolving taxa.  They rebut the
Balance principle:  “Nothing in the formal taxonomic code mandates that
taxonomists follow this so-called ‘principle’ of balance.” And they
conclude that taxonomic systems should reveal evolutionary patterns, not
degrees of sequence divergence.  “We know of no molecular studies that
find all congeners equally related.”

At the center of the discussion is when does a taxon become a species.

And finally “How we recognize species depends on the species concept
employed”.

And there I am finally shocked. How can we have a system that emphasizes
species without agreeing on a species concept.

So two groups of very thoughtful people reach opposing conclusions
regarding two very similar taxa in North America, and taking up several
pages of a scientific journal, to boot. Good luck in following the
arguments on the other 10,000 bird species and their relatives much less
on LEPS.

Anyway, I’m not impressed by the lack of mtDNA divergence in these two
taxa, and wouldn’t have thought it supported separation. So probably it
is more conservative not to split them.

Mike Gochfeld

PS A word from the amateur (in my). As a bird watcher, splitting these
two would CREATE a new species to add to my life list.  Or it creates a
need for those who want to get them all, to go out after a bird they
might well have ignored in the past.




 
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