Species Concept Question

[Jim Croft]

>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.

This pattern is also very common on the land, especially in montane
environments, which are essentially islands isolated by a lowland sea...

>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.

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

>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...

>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

>- Two species epithets, nothing a zone of hybridization where sympatric.

yes - this is what your data shows

>- 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...

>???

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

>But this is only part of my question.

I had an horrible feeling this was the case... now why am I not surprised?

>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?

Isn't this just a long-winded way of introducing the forbidden in polite
society dinner time conversation topic ' What is a secies?'  (it is right
up there with sex and politics...)

>  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 you say the taxa are indisputably closely related this pattern
does not rule out evidence of past geneflow, followed by some form of
effective reproductive isolation...

>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.

that implies you already know the answer...  or assume that we do...  :)

>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).

if it looks like a species, smells like a species, then call it a
species...  how difficult is that?   It is just a handle to an arbitrarily
useful construct at the time...  :)

>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.

in pattern 1, why not, if that is what it look like?   In pattern 2 the
decision may not be so obvious as what you appear to be describing is a
gradual cline, and hybridism may or may not be involved or may only be part
of the story...

>  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?

Surely the historical sequence of events should not make any difference
once all the results are in?   Don't you just look at all the available
data and make the decision regardless of what people had thought before?

>As I re-read this note, I am still not satisfied that I have conveyed my
>true question.

Don't try *again*... Please!

>The true question has to do with

Damn!

>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.

I think the first is true (whether it should be or not is another matter),
but I would like to think our science was open minded in terms of prior
assertions and dispute the second...

>  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.

I guess it is impossible to escape that a persons notion of a species is
influences by the total of other peoples notions of a species.  but it
would be nice to believe that this applies in a general sense and not in a
particular, species by species...

>Perhaps I have achieved nothing but wasted bandwidth;

RP consume bandwidth!?  Never!

>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.

Nooooo!   No more with the S word... please!   :)

>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.

I can recommend a technique based on liberal use of pounded Derris
root...  effective fish problem solver that does not make as much mess or
noise as dynamite...  :)


~ Jim Croft ~ jrc at anbg.gov.au ~ 02-62465500 ~ www.anbg.gov.au/jrc/ ~