Species Concept Question

[Richard Pyle]

Hello all,

I have a sneaking suspicion that this is an incredibly naïve question,
because it seems to me to be so basic that it should have an obvious answer.

Consider the tropical oceans, and the pattern of coral-reef habitat
throughout the ocean.  Unlike in some terrestrial environment, there are
clear "units" of habitat (islands & island groups) separated by vast regions
of non-habitat (open ocean).  It's a patch environment on a (literally)
global scale.

Now, consider the following two patterns of distribution.  "X" refers to one
morphotype, and "O" refers to what appears to be a sister morphotype. Both
morphotypes are not subtly different from each other -- that is, they are
dramatically different enough from each other, that even the most ardent
"lumper" would be inclined to recognize them as distinct at the species
level. The lowercase "x" and "o" symbols in the patterns below refer to
individuals that are intermediate between the two morphotypes (i.e.,
hybrids).  The clusters represent island groups across a vast swath of ocean
(A, B, C, D, E).

Pattern 1:
  A       B       C       D       E
-----------------------------
XXX      XXX    Xox     OOO     OOOO
 XXX    XXXX     OxO     OOO    OOO
XXXX     XX     xoXO    OOOO    OOOO

Pattern 2:
  A       B       C       D       E
-----------------------------
XxX      XXo    Xox     OoX     OOOO
 XXX    XOxX     OxO     OOx    OOx
XoXX     XX     xoXO    OoOO    OoOO

In both patterns, there is clear evidence of gene flow between the two
morphotypes in areas of sympatry (i.e., functionally no reproductive
barriers).  In the case of Pattern 2, there is a reasonably steady cline in
relative abundance of one form to the other, with relatively smooth
transitions of intermediates as well.  This is a pattern that taxonomists
occasionally have to wrestle with, which brings up all sorts of
philosophical species-concept questions.  That's not the pattern I'm
interested in right now.

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".

My question is difficult to articulate (I understand it in my head, but am
having trouble finding the right words to communicate it). But I'll give it
a shot:

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.
- Two species epithets, nothing a zone of hybridization where sympatric.
- One species epithet and two subspecies epithets, noting a zone of
hybridization where sympatric.

???

But this is only part of my question.  A deeper level of the question has to
do with the role of distribution patterns and historical nomenclature in
making nomenclatural decisions.  For example, if in Pattern 1 population "C"
did not exist, would you be more inclined to treat them as separate species?
The gut-reaction answer is "Yes, because without evidence of gene flow
between two clearly distinct forms, one is to assume that they are distinct
species".  But if population "C" didn't exist, how would we ever know
whether the two forms were capable of freely interbreeding?  If, on the
other hand, the pattern of distribution looked like this:

  A       B       C       D       E
-----------------------------
XXX      XXX    XOX     OOO     OOOO
 XXX    XXXX     OXO     OOO    OOO
XXXX     XX     XOXO    OOOO    OOOO

That is, no evidence of gene flow between the two populations in an area of
sympatry, then most of us would have no trouble arriving at the conclusion
that these are distinct species (i.e., given the opportunity to interbreed,
they do not).

But if, as I posited above, population "C" doesn't even exist (to our
knowledge), how do we assess whether the two morphotypes should be regarded
as distinct species (old question, I know -- and mostly rhetorical.  Also,
not really at the heart of what I'm after -- as I said, I can't quite figure
out how to articulate the real question I'm after).

Now, suppose throughout most of the relevant nomenclatural history,
population "C" was not known, and the two distinct morphotypes, each with
relatively broad distributions, were consistently treated as distinct
species by all researchers over many decades.  Then, someone discovers
population "C".  Are you now tempted to disrupt nomenclatural stability and
treat them as conspecifics, or would you prefer to maintain them as
distinct, and note a zone of hybridization.  How would you approach the
situation differently if population "C" was discovered and known for many
years as a single, highly variable species, and only later was it realized
that the two endpoints of the variation spectrum were each represented as
broadly-distributed but allopatric populations?  Similarly, what if in
Pattern 2, only populations "A" and "E" were discovered initially, and named
as separate species, and then subsequent research revealed the existence of
a cline of populations?

As I re-read this note, I am still not satisfied that I have conveyed my
true question.  The true question has to do with the ways that history and
observation of geographic distribution influence and bias our nomenclatural
decisions.  Obviously our nomenclatural decisions ARE biased by both
geographic distribution *and* historical nomenclature.  We all know that (or
at least should acknowledge that).  But I'm trying to get at the heart of
the nature of how we are biased by these influences, and how those biases
and influences relate to the notion of species boundaries as defined
intrinsically by the essence of the organisms themselves; vs. species
boundaries defined only in an historical evolutionary context, vs. species
boundaries that are defined as a convenience of communication among
biologists.

Perhaps I have achieved nothing but wasted bandwidth; in which case I
sincerely apologize.  But if any of the above sparks interest among list
members, perhaps the ensuing discussion might help me understand how to
articulate my true question.

Aloha,
Rich

P.S. The distribution patterns described are not hypothetical -- they are
based on very real examples in reef fishes; some of which involve species in
my own group of interest.


=======================================================
Richard L. Pyle, PhD
Ichthyology, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
Ph: (808)848-4115, Fax: (808)847-8252
email: deepreef at bishopmuseum.org
http://www.bishopmuseum.org/bishop/HBS/pylerichard.html