Species Concept Question

[Richard Pyle]

Part 1"

> Actually Pattern 2 is the more interesting and the more difficult to
> explain.

Agreed -- but nevertheless not the focus of my current query.

> What do you do if it looks like a hybrid, smells like a hybrid,
> but one of the putative parents is no where to be found in the
> vicinity.   The immediate doubt arises, maybe the supposed hybrid isn't,
> and if it isn't what is it, and what is going on?

That sort of thing happens *occassionally* in fishes -- but usually the case
is one species is common, the other species is extremely rare, and the
hybrid is extremely rare.  That both one of the species is rare and the
hybrid is rare suggests that the rare parent species "B" cannot find a
conspecific mate, and thus opts for gamete exchage with the heterospecific.
In some cases (one that comes to mind), one of the parent species has never
been reported from a locality, but several of the hybrids have.  This
suggests to me that one or a very few of the rare parent spawned multiple
hybrid progeny.  This sort of patterns seems not to be a stable situation,
but rather a momentary situation (on evolutionary time scales), that we just
happened to capture in our tiny temporal window of observation.

> >The first pattern represents a case where there are two clearly
> two distinct
> >morphotypes -- as I said, so distinct that even the most dedicated lumper
> >would treat them as separate species.  The two morphotypes are
> clearly each
> >other's closest relative, and have a generally parapatric
> distribution with
> >respect to each other. Throughout most of the ranges of each of the
> >morphotypes, there is tremendous consistency of form (i.e., very little
> >geographic variation within each of the respective ranges);
> except for the
> >zone of sympatry.  In that zone, you find a complete spectrum of
> >individuals, ranging from essentially pure "X" to pure "O", with every
> >imaginable intermediate in-between.  A classic "hybrid swarm".
>
> This situation, as illustrated is a no-brainer...  where the two taxa
> coexist, they hybridize and appear to integrade...  mildly
> interesting and
> end of story...

Hmmm...were it only that straightforward in the minds of all taxonomists....

> >When confronted with Pattern 1, which of the three nomenclatural
> solutions
> >do you feel is the one that best facilitates communication among
> biologists:
> >
> >- One species epithet, noting two distinct geographic variants.
>
> no - you implied the taxa were indisputably different morphologically

Yes, but male humans and female humans are indisputably different
morphologically; as are the major human races; as are breeds of dogs; etc.
Maybe these are exceptional cases, and don't represent the "natural" world.
But even still, a failure to allow *any* polymorphism would dramatically
(and nonsensically) explode the number of species on Earth.  Even if we
restricted it to geographically segregated polymorphism we'd run into
trouble, I think.  One of the examples I had in mind when making the
original post is:

Centropyge flavissima
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=1774961360

and

C. vrolikii
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=388967542

Throughout each of their (broad) distributions, they are essentially
indistinguishable from island group to island group.  In the (narrow)
regions of sympatry, hybrids outnumber either parent form, and come in a
wide spectrum of shades:

http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=1690520359
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=1773380990

Looking at the photos, any taxonomist who knows the group wouldn't hesitate
to treat them as distinct species (indeed, in the 150 years both species
have been known, they have NEVER been treated as conspecific, despite
HUNDREDS of published treatments).

But...perhaps flavissima is simply a xanthic morph of vrolikii, which
somehow managed to dominate the central tropical Pacific; separate from its
parent stock vrolikii, which dominates the western Pacific.  In this case,
the genetic dissimilarity between the two may be positively trivial --
potentially much smaller than the differences among geographically disparate
populations which we would not hesitate to regard as conspecific (if in the
latter case, the genetic divergence did not translate into obvious
phenotypic character differences).

So, in my mind "indisputably different" is a very subjective state.

> >- Two species epithets, nothing a zone of hybridization where sympatric.
>
> yes - this is what your data shows

And that is how I, and every other taxonomist before me, have treated it --
in this case.  But there are other cases that are less obvious.  The case
that prompted me to make the post involves:

Centropyge acanthops
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-982590340

and

C. fisheri
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-1339602635

and they overlap in the Maldives:
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-1547254375
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-1923267964

Actually, the case with C. fisheri is more complicated.  Until my recent
revision of the group, it has been thought of as an Hawaiian endemic, with
C. flavicauda occupying most of the Indo-Pacific.  However, as it turns out,
fisheri/flavicauda is more along the lines of my "Pattern 2", with a wide
spectrum of variation, and no place to draw a line.  So "fisheri" varies a
lot, as follows:

http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-1377454029
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=1117729528
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-2001194522
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=1189968978
http://www2.bishopmuseum.org/PBS/images/JER/detail.asp?ID=-1339602635

This genus is not, in any way, particular to this sort of pattern -- it just
happens to be a genus I recently dealt with in a comprehensive taxonomic
way.  There are other known examples, in a wide variety of reef-fish
familes; and undoubtedly many more as-yet unknown examples.

> >- One species epithet and two subspecies epithets, noting a zone of
> >hybridization where sympatric.
>
> maybe...  but you you have to revise your initially assertion if
> undisputed
> difference...

Not necessarily -- I might just have to choose a definition of "subspecies"
that reflects how I think that taxonomic rank should be used (i.e., two or
more clearly distinct geographic variants, worthy of nomenclatural
recognition, but which show evidence that evolutionary lineages may only be
temporary -- an island in the river, as opposed to a fork in the river).

> yep - the safe option - call it a complex, and move on...  :)

That's essentially exactly what I've done -- but as I said, the REAL
question is not so striaghtforward, and I still haven't figured out how to
articulate it.

> >But this is only part of my question.
>
> I had an horrible feeling this was the case... now why am I not surprised?

Not horrible, Jim!  It's a good thing!  A GOOOODDD thing!!! :-)

Aloha,
Rich