A biological continuum

Curtis Clark jcclark at CSUPOMONA.EDU
Fri Mar 31 10:58:59 CST 1995

Date sent: 31-MAR-1995

 >My point is that non-sexual reproduction is only the "rare
 >exception" you make it out to be among vertebrates.

Admittedly many terminal taxa in most groups reproduce asexually.
In my view, the evidence does not support this being an important
factor in the evolution of eukaryotic diversity.  Your focus on
reticulation, which presupposes recombination, the simplest form
of which is sex, suggests that you concur.

 >  You began
 >this thread by pointing out that continua are not found in
 >*eukaryotes* (as opposed to prokaryotes), which as a group are
 >considerably larger than vertebrates and vascular plants put
 >together.  Perhaps you, or someone else, might care to make some
 >comments about the eukaryote kingdoms of protists and fungi at
 >this point.

With regard to what?  Sexual reproduction? Reticulation? Continuous

 >  My point being that you made a sweeping
 >generalization based on your experience with a *small* subgroup
 >of organisms with a rather depauperate set of mechanisms for
 >transferring genes.

Perhaps you made a sweeping interpretation.  Perhaps you would like to
explain how examples from holarctic grasses are of more general

 >>This is exactly my point.  It seems by your post that you do not like
 >>cladistics; opinion noted.  Nevertheless, a system that by your admission

 >Don't jump to conclusions: I certainly do "like cladistics" (your
 >words), in theory;

Jumping to conclusions is something we both seem to be good at.  :-)
Nevertheless, you certainly fooled me on that one.

 >it is an interesting tool for developing
 >hypotheses (not exposing truths) but limited by the worker's
 >subjectivity and assumptions.

You might as well be describing science in general.

 >similar topologies.  What are these "countless examples" that
 >have been studied using different equivalent data sets (another
 >rather extreme generalizing statement).

Again, we are *both* skilled at extreme generalizations.  The authority
I turn to on this matter is Mark Porter (you met him at Rancho Santa
Ana Bot Gard on the same visit that you met me); he keeps up with the
relevant literature far better than I, and he has mentioned many examples
to me of congruence between molecules and morphology (I get the impression
that he thinks there are more than would be expected, considering the
vagaries of gene evolution and tree algorithms).  Of course I wrote none
of it down; I never expected to need the ammunition.  :-)

 >I still don't know what your criteria are for judging that
 >"cladistics works".

You tell me your criteria for judging whether any scientific procedure
works, and I'll let youknow whether I think cladistic methodologies

 >  I maintain that it will not show
 >reticulation because the starting premise is that there is no
 >reticulation.  Just because the results of cladistic analysis of
 >different data sets are not congruent does not mean that
 >reticulation is the culprit.

Ah! But explain to me why reticulation would be the culprit when they
*are* congruent.  Reticulation is one of many sources of confusion, but
it is an unlikely candidate for causing a *lack* of confusion in an analysis
that by both our admissions is ill-suited to deal with it

 >This was one, admittedly extreme, example, but the point is that
 >you originally said "eukaryotes", you used the term "sex" in a
 >`traditional' sense
A sweeping generalization on your part.

 > and unless each case is studied carefully in
 >depth, it is unwise to assume either a continuum or no continuum.
 >Continua exist in all groups through time and space; reticulation
 >(in many forms and many taxa at different) is real.

I am bothered by two things here.  The first is your continued implied
connection between reticulation and continuum.  You've provided no
evidence beyond your grass, just as I've provided no evidence so far
of the contrasting view beyond my sunflower.  Second, you say it is
unwise to make an assumption either way.  That was what my original post
said: if we try to shoehorn things into a continuum, we may miss important
features of variation.  But ever since then you have been pushing
continua (or is it reticulation) like they were the answer to all our
problems.  I have certainly worked with both situations, but I contend
that the better research plan is to assume discontinuity, and let the
facts falsify that or not, rather than to assume continuity, and never
look for differences.

 >Any American Kennel Club member can tell the difference between a
 >terrier and a chow chow.  Maybe even botanists can be trained to
 >do this.  :-)

Which terrier do you mean? (Or do you believe that they vary
continuously? :-) Seriously, thanks for the ammunition.  The
purpose of dog breeding is to increase the frequency of specific
phenotypes relative to others by artificial selection.  Although
I certainly don't contend that dog breeds are species, they definitely
form peaks in character space, for biological reasons that are well
understood.  Simply saying that dogs form a continuum obscures

 >Have you looked at the _Elymus trachycaulus_ complex throughout
 >the northern hemisphere?

Of course not.  Why even ask?

 > A lot of botanists would appreciate it
 >if you would point out the discontinuities in this species
 >complex after studying it (using cladistic methods or others)
 >throughout it's range.  As I said, there are hiatuses in some
 >places, but in others there are continua.

I'm sorry your group is so difficult.  I am sure that I would have
far less success than you have had in sorting it out.  Maybe
your problem is that you work on holarctic species.  :-)  But
it makes no more sense for you to generalize from Elymus than for
me to generalize from Encelia; that is not my intent, and I hope it
isn't yours either.

 >Sounds like a specialized case to me.  Probably _Encelia_ hybrids
 >are an exception rather than a rule.  :-)

Like Elymus, perhaps?  :-)  Loren Riesberg, who studies the other
type of homoploid hybrid speciation, once told me that he felt the
Encelia model was the more common.  But that isn't the point.
Alloploidy is by far the most common means of hybrid speciation,
and it does not necessarily lead to continuous variation (it almost
never leads to continuous variation in chromosome number :-).

 >From the little information you provide in this example this
 >could be interpreted in other ways.

I was providing the information as instruction, not advocation.
Excuse me for miscontruing.

 >  Cladistic methodology
 >`proved' nothing, but simply indicated a number of conflicting
 >and unresolvable hypotheses.

Never has, never will.  This is science, after all (unless I'm on
the wrong mailing list).

 >Perhaps we are taking different views on the geometry of
 >continua.  My concept of continua includes a spacial component,
 >yours does not seem to.

Another sweeping generalization, but let's explore what you mean by
spatial component.  Most organisms are spatially discrete (let's leave
fungi out of this for now), so perhaps you mean whether organisms
are clumped, random, or regular.  Or are you referring to spatial
variation in phenotypic features?  If so, it's hard for me to infer
how you could think that I didn't.

 >What are your end points (or modes) here.  Have you included all
 >eukaryotes in this graph?  Is this the graph that one gets from
 >looking at "Homoploid hybrid speciation" events.  If so it seems
 >to be one lump short of a full load :-)  Sorry, another cheap
 >shot, but I couldn't resist.  Seriously now, do you see this
 >pattern between *all* adjacent modes?

Again, I was trying to be instructive; sorry it was wasted.  I frankly
don't see what you're getting at here, other than dissing my ASCII
graphics.  Clearly you are aware of frequency plots such as histograms.

I'll tackle the rest of your post later.

Curtis Clark                                       Voice: (909) 869-4062
Biological Sciences Department                     FAX:   (909) 869-4396
California State Polytechnic University, Pomona
Pomona CA 91768-4032                               jcclark at csupomona.edu

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