Question about adherence to holophyly for nomenclature

Richard Pyle deepreef at BISHOPMUSEUM.ORG
Wed Nov 17 09:34:34 CST 2004

I feel a little guilty about sending this, because I'm about to disappear on
a trip with no email access, but I would honestly like to understand
something that involves the application of scientific (Linnaean-based)
nomenclature to populations of organisms, under the paradigm that
nomenclature should reflect strict holophyly.

Suppose it is true that we have a phylogeny at this point in evolutionary
history, that looks something like this:

    Nene   Lg.CG   Sm.CG
      \      /     /
       \    /     /
        \  /     /
         \/     /
          \    /
           \  /

In the interest of having nomenclature represent holophyletic clusters, we
have three options:

1. Refer to all three forms by the same species name
2. Refer to the (Nene+Lg.CG)group by one species name,
     and the Sm.CG by another species name.
3. Refer to all three by separate species names

You could substitute "subspecies" for species if you want -- it doesn't

I suspect that, given the morphological difference, many bird enthusiasts
(taxonomists & watchers alike) would be uncomfortable with option 1.  I
suspect further that option 2 would cause no end of confusion (unless,
perhaps, you recognized the Nene & Lg.CG as distinct subspecies, belonging
to a separate species from the Sm.CG).  So, as appears to be the case in
reality, we go with option 3.

So far, so good.

Now suppose that there is some level of introgression happening between
Lg.CG & Sm.CG populations.  And, suppose that trend is continuing (e.g., as
a result of the removal of whatever barrier caused them to diverge in the
first place).  But the barrier between the Nene and both CG populations
(i.e., lotsa ocean) remains intact.  Give this trend a few thousand years,
and it's quite conceivable that the two CG populations will homogenize into
one, giving us a "true" phylogeny that looks something like this:

 Nene       Homogenized CG
   \            / \
    \          /   \
     \     Lg.CG   Sm.CG
      \      /     /
       \    /     /
        \  /     /
         \/     /
          \    /
           \  /

Now for my question:

How would someone who believes that scientific nomenclature should exactly
reflect holophyletic relationships deal with this situation, in terms of
applying names?  These are birds (ICZN rules), so no Nothotaxa options are
available.  The only truly holophyletic nomenclatural solution is to apply
only one name to the whole cluster, and treat the differences as complex
population structure (whatever that means).  But that seems to me to be a
cop-out, because the cladistic analysis (absent knowledge to the contrary)
would very likely yield something like this:

 Nene       Homogenized CG
   \            /
    \          /
     \        /
      \      /
       \    /
        \  /

...and even the strictest holophyleticist would be comfortable applying two
separate names to these populations (if consistency of distinction in form
called for it), even though one of the populations is of reticulate origin.

I don't think that it's paraphyly that is nipping at the heels of strict
holophyleticists. Rather, I think it's reticulate patterns of descent that
we/they cannot hide from. We know that it happens a lot in plants and
corals, but I am of increasing suspicion that it happens more often (perhaps
MUCH more often) than we think in other animals as well -- at least at the
level of evolutionary units that are traditionally treated as "species".
The more I allow myself to consider the reticulate derivation option in
certain reef fish taxa, the more I'm starting to see examples that could
very well fit this pattern. I think that the only reason we don't see it
happening in more organisms, is that our cladistic methodology is not
precise enough (yet?) to tease the previous two cladograms apart. Indeed,
examples of reticulate descent may eventually prove to be less of an
exception, than a rule.


P.S. I'll be spending the next couple of weeks on a boat with Charlie Veron
(among others), so I'm looking forward to discussing this more with him.  I
saw him give a presentation a couple months ago, and he seems to believe
that reticulate patterns exist in much greater frequency in other organisms
(besides corals and plants) than most taxonomists/systematists currently

> -----Original Message-----
> From: Taxacom Discussion List [mailto:TAXACOM at LISTSERV.NHM.KU.EDU]On
> Behalf Of Ken Kinman
> Sent: Tuesday, November 16, 2004 7:44 PM
> Subject: Whooper; Canadian geese split; paraphyly.
> Dear All,
>       Quivira National Wildlife Refuge remains closed to hunting
> until at least Friday due the presence of one whooping crane.  It
> is thought to be the third whooping crane "shot at" on November
> 6th, but not injured (at least not seriously so).  I have not
> found out yet if this is an adult that just happened to be with
> the two whoopers which were shot, or if it was this year's
> offspring of these two whoopers.  Whooper offspring that get
> separated from their parents sometimes end up migrating with
> sandhills to western Texas rather than Aransas (which was
> probably the case with the single whooper which I saw flying
> north with sandhills this past February---a stroke of
> serendipitous luck).  Should be interesting to see what happens
> with this one whooper still sticking around at Quivira.
>       On a related note concerning migrating birds, the Canada
> Goose has been officially split this year into two separate
> species----(1) the larger-bodied Canada Goose (Branta
> canadensis); and (2) the smaller-bodied Cackling Goose (Brant
> hutchinsii).  Even though there is sometimes some morphological
> overlap (which can make some identifications difficult), they
> apparently have slightly different mitochondrial DNA sequences.
> Should be interesting to see if increased scrutiny results in
> evidence supporting this split or not.  Intergrades between the
> two species would only have the mitochondrial sequences of the
> mother, so it is a little more complex than if they had based it
> on nuclear genes.  Sampling problems along the remote Arctic
> coast could therefore still end up making this split problematic.
>  We shall see.
>          ------ Ken Kinman
> P.S.  The same mitochondrial DNA sequences indicate that the
> Hawaiian forms (including the extant nene goose) evolved from the
> larger-bodied species (B. canadensis)-----thus rendering B.
> canadensis paraphyletic.  Does this mean strict cladists will
> insist that the nene goose be reduced to subspecies status??  Or
> will they just PRETEND that they are sister species??  [They can
> run, but they can't hide----paraphyly is nipping at their heels
> all over the place if they would just admit it].  :-)

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