Homology & word-"baiting"

[Curtis Clark]

At 03:02 2003-05-16 +0000, Ken Kinman wrote:
>Substitute the word "reversals" for
>"plesiomorphies", and perhaps everyone will be happy.

Reversals (if correctly interpreted) are apomorphic relative to the state
they reversed from. They are plesiomorphic only with respect to states that
subsequently evolve from them.

>     In any case, while we are being nitpicky about the term homoplasy,
>should we really consider all parallelisms to be homoplasies?

The alternative would be that some parallelisms would be homologies. It is
reasonable to infer that parallel evolution is an indirect measure of
similarity of genetic system, but by definition, parallelisms result from
independent events, and thus must be homoplasy.

>They are
>generally homologous (more so than analogous), even if they aren't
>homologous in the strictest sense as applied to synapomorphies.

Homology is similarity that is the result of common descent. Synapomorphy
is not a special kind of homology, it is merely shared, derived homology.

>If
>parallelisms are homoplasies, they are of a very "soft" sort in comparison
>to "hard" homoplasies like the convergent wings of insects and birds (with a
>continuum of homoplastic levels in between).

Homoplasy is homoplasy. Parallelism is indeed interesting evolutionarily
for different reasons than convergence, but in fact a phylogenetic
framework is necessary to distinguish the two.

>     THEREFORE, in a very real and unfortunate way, the word "homoplasy" has
>been just one more way to further indoctrinate generations of students with
>the notion that ALL paraphyletic groups are bad:     "All homoplasy is
>equally bad, and all paraphyly is equally bad.

Do you make this stuff up?

Homoplasy is false homology. It is by definition a measure of human
failure. As I tell my students, "If I said that the trunk of an elephant
and the trunk of an oak tree were homologous, when my mistake was corrected
they would be homoplastic, because of my misinterpretation. But to all of
you, it wouldn't be homoplasy, it would be stupidity." Homoplasy requires
that we incorrectly infer homology.

Paraphyly has *nothing whatsoever to do* with homoplasy; it is the result
of grouping by plesiomorphy, and plesiomorphies are by definition
homologies. Grouping by homoplasy ordinarily results in polyphyly, which
I'm sure you won't defend.

>     Sorry, but 35 years of indoctrination does not make these notions any
>less simplistic, even though decades of repetition may have given them a
>thin veneer of respectability (and an even more disproportionate edge in
>funding opportunities).

The decades of repetition have evidently had no effect in distinguishing
the meanings of homology, homoplasy, plesiomorphy, apomorphy, parallel
evolution, and convergent evolution, despite the repeated and consistent
definitions widely used in the primary literature and in textbooks.

>Sorry to remind you again, but some homoplasies are
>useful,

So you are saying it is good to be misled into inferring homology when it
doesn't exist?

>some plesiomorphies are useful,

*All* plesiomorphies are useful. It would be impossible to distinguish
apomorphies without them.

>and even some paraphyletic groups
>ARE useful.

We can continue to disagree on this one, although I'm starting to wonder
just exactly what you mean by a paraphyletic group.

>Time to give up this
>puritanical crusade of strict cladism before it does even more damage.

Holophyly aside, I hardly see insistence on agreed common definitions of
technical terms as being puritanical, and I hardly see accusations of
puritanism as anything more than an appeal to the emotions.

>I'm sure various
>strict cladists are looking forward to the end of May (actually I'll be
>leaving Taxacom around the 26th).

I'm looking forward to your being able to debate your side of the issue
from a position of understanding both sides, not just your own. I'm
reminded of a student in my speciation class a number of years ago who
insisted (parroting the opinions of another faculty member) that species
weren't real. It puzzled me that he would take a course focused on the
origin of figments of the imagination, but I pointed him to the salient
literature on the nominalist species concept, told him to read it, and
*then* I'd argue with him, because he would no longer be unarmed.


--
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department            Voice: +1 909 869 4062
California State Polytechnic University     FAX: +1 909 869 4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu