the continuum continues

[Curtis Clark]

Date sent: 30-MAR-1995
Darbyshire continues:
 >     A) Curtis, you're still stuck on vertebrates.

A strange "accusation".  While I once was a zoologist (for as long as
it took to get a B.S.), I am now known among my colleagues for
criticizing "vertebrate-centric" evolutionary thinking.  I base
my statements on evidence from plants. (Or were you implying that my
plants have backbones?)

[Animal stuff deleted; I am obviously not expert enough to comment on it.]

 >     B) I do not know what you mean by "cladistics...works well".
 >     Cladistics as practiced today often produces one (or many
 >     more) *hypothetical* phylogenies that "in many cases"
 >     appeals to the subjective conceptions of the worker.  Even
 >     if one accepts the theory of cladistics (presumably we are
 >     talking about the Hennig derivative procedures here, which
 >     do not permit the consideration of the continuous nature of
 >     organismal phylogeny through time)

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
does not (see quote above) is nevertheless capable of producing congruent
phylogenies of many groups from different sorts of information.  The
cases where cladistic methodology doesn't work in my view provide an
estimate of the extent to which evolution has not taken a branching
topology.  Cladistics (to me at least) is a tool, neither God nor Satan,
and it shows me countless examples of groups without reticulation, and
groups in which reticulation is a minor feature.

 >     one sees what one wants in the results.  The presence of
 >     reticulation cannot be determined by popular cladistic
 >     procedures, in fact one starts with the assumption that
 >     reticulation does not exist.

Reticulation causes cladistic techniques to fail.  When they work, this
is evidence for the reduced importance of reticulation.

 >     C) Reticulation is very real, even in vertebrates; to
 >     mention two cases: the complex of salamanders in the
 >     genus _Ambystoma_  from eastern North America.  This group
 >     has been studied intensively since the early 1960s only to
 >     reveal more and more complexity to the reticulation.  Four
 >     diploid species are known to be hybridizing and exchanging
 >     genes (A. laterale, A. jeffersonianum, A. texanum, A.
 >     tigrinum) and other species are suspected to be involved.
 >     Diploid, triploid, tetraploid and even pentaploids are
 >     known; polyploids being mostly female.

More zoology stuff, but I've left this in because it supports my point.
This is a notable example *precisely because it is notable*.  If
reticulation were the rule and branching phylogeny the exception,
this would not merit mention, and we would focus on the unusual
groups that *don't* show reticulation.

 >  Also among the most
 >     heavily studied organisms are cereal grasses and their
 >     relatives.  Wheat, barley and rye belong to the Triticeae
 >     tribe, in which a vast array of `intergeneric' hybrids have
 >     been documented where two or more distinct genomes have
 >     combined in hybrids to give an incredible taxonomic tangle
 >     of intergrading species and genera.

Sure.  I lecture about this all the time.  But any subsistence farmer
from central Asia can tell the difference between einkorn and emmer.
Just because there is reticulation here, that is not evidence of continuous
variation.  And in the example of cultivated wheat, the chromosome
numbers of the polyploid series are discrete, with no true-breeding
intermediates.

 >Are these plants asexual F1 populations; no segregation at F2
 >generation, no chance for backcrossing?  Are these plants which
 >normally only reproduce through chasmogamous, outcrossing
 >flowers?

No, no, no, yes.  "Homoploid hybrid speciation" = Origin of new diploid
true-breeding species from hybrids between other species.  The group
I work with is Encelia of the Asteraceae; these are obligate outcrossers,
F1s are fully fertile.  Backcrossing in cultivation is simple and backcrosses
are fertile and vigorous, but backcrossing in nature is rare for ecological
reasons that we are still trying to figure out.  Hybrid species appear to
have resulted from crossing among F1s, F2s, and so on, with selective
elimination of plants tending toward either parent's morphology.  You
might be surprised to know that one of the lines of evidence supporting
the hybrid origin is cladistic: co-ocurrence of autapomorphies of the
putative parents.

 >Introgression does not result in new species, agreed, it results
 >in continua.

No it does not.  The mere fact that we can recognize a situation as
introgression is evidence that the parent species are distinct in at
least part of their range.  Yes, there are intermediates, but the bulk
of variation follows the parental means.

 >OK, in some cases intermediates are not important "statistically
 >or biologically", but do not restricting sampling to specific
 >cases or in time or in space or in numbers, so that few
 >intermediates are detected.  To say it again, the extent of
 >variation between forms is continuously variable, from uniform
 >continua to wide hiatuses.  To deny the existence/significance of
 >continua is to deny an important aspect of biological variation.

Let me try my hand at ASCII graphics:


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You stress the presence of intermediates.  I stress the bimodality.
I do not deny the presence of intermediates.  You seemingly deny the
importance of the bimodality.

 >Maybe the difficulty is that your work is done on California
 >plants (?).

Cheap shot.

 > I suggest that you might try to examine a
 >circumboreal species complex (animal or plant) *throughout it's
 >distribution*.

I could suggest that *you* examine a case that supports *my* views,
but instead I'll agree to disagree.


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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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