[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Paraphyletic groups [was Re: cladobabble...[...]]
I know we're straying from dinosaurs, but I crave your indulgence
that I might respond on this topic, which has been bugging me all weekend.
At 09:26 AM 11/21/97 -0700, LN Jeff wrote:
> Why not define it as "John and Sue Wagner and all thier ancestors?"
>Why would that be less real? Such a group would be paraphyletic; it would
>exclude your cousins, aunts, uncles, etc...
I don't know a "JoHn" Wagner (I am sure several exist, though :).
Excluding some descendants obviates the natural process of ancestry and
decent. However,as a group based solely on ancestry, this may indeed be a
real, natural group (I have neither the time nor the free brainpower to
address this fully). It is clearly a different sort of thing from a clade.
It may even be biologically meaningful, but would be useless for
systematics. The group you define would exclude some of the descendants, and
thus would not tell you much about the evolution of a group as a whole.
>I am simply disturbed by the a priori
A priori is probably not the concept you are going for here. "A
priori" implies that analysis of utility will demonstrate the existance of
these two types of taxa. :)
>assumption on the part of cladists that monophyletic groups EXIST and
>paraphyletic groups do NOT shutting down the issue before we get to
>determining which is more useful.
A paraphyletic group would be "real" if my earlier analogy of a
"phase in the system of life" held true always. For example, if a group were
defined as "The most recent common ancestor of A and B and all of its
descendants, excluding the most recent common ancestor of C and D and all of
its descendants" it would produce a mechanically separable phase. [However,
this definition leads to more problems. What if the group falls out in
analysis as (C, (D, (A, B)))? What is the group then?]
However, recall that reality is not based on whether we can
seperate clades from the tree of life, although this is a manifestation of
that reality. Clades are real groups because they are united by the real
process of ancestry and descent. This is an inherent link among all animals,
and groups them together irrespective of our interpretations of their
morphology or the evolutionary history of the clade. This process is what
would allows us (if we were omnipotent and omnicient) to mechanically
seperate the phases from the tree of life. Clades cannot "give" reality to
non-clades by superimposition. A group can only be a real, natural group in
this context if it is united by common ancestry and descent. Reality must be
intrinsic to a group, not imposed by external phenomena.
Paraphyletic groups have no independent existance. You might define
them by seperating out a clade, and removing an internal clade. There is no
biological phenomenon which warrants this exclusion, and no biological
process unifies the paraphyletic group exclusive of the monophyletic
group. This represents an interruption in the process of common ancestry and
descent, and thus obviates the unifying process which breaths existance (if
you will) into clades. In order to say that the paraphyletic group has some
unifying property which gives lends it existance, there must be something
inherent in the process of removing the monphyletic group which "gives" it
that intrinsic reality. However, removing the internal clade does not change
the other animals at all. The remaining animals show no particular
phylogenetic affinity for each other as a separate group sans the excluded
clade. The grouping of taxa into the paraphyletic group is entirely
dependant on the exclusion of one real group from another. It has no
inherent existance, even if the layfolk of Peoria can percieve its essence.
Removing an internal clade removes the ties which bind the more inclusive
clade together, and cannot create real groups where they did not already exist.
No part of the genome or history of a member of a paraphyletic group
tells you that it should expressely be a part of such a group, although it
may tell you that it is not a part of the clade excluded. There is no unique
property of non-avian dinosaurs which unites them exclusive of avian
dinosaurs, although there is a process which unites avian dinosaurs
exclusive of non-avian dinosaurs.
This is not a contraditiction, it is a consequence of natural
processes. It is the tangible difference which exposes the artificiality of
paraphyetic groups. Confusion in this regard stems from a subjective
perception that existance is dialectical in nature. You might think that if
there are two subgroupings within a group, the same properties which apply
to one must perforce apply to the other. This may work in cases where the
groups are equivalent concepts (such as sister groups). A common fallacy
linked to traditional taxonomic methodology is that two taxa of the same
rank represent a true dichtomy. They often do not. In the case of a
paraphyletic and a monophyletic taxon, the two are certainly *not* diametric
opposites. More inclusive versus less inclusive sets do not necessarily have
equal status as entities, and properties pertaining to the one do not
necessarily pertain to the other.
A good analogy is light and darkness. Darkness is an absence of light,
and we tend to think of the two phenomena in an antithetical relationship.
However, darkness is not a real phenomenon in and of itself. There can be
no "dark beams" (at least not outside of AD&D). The only real phenomenon
here is light. Darkness is merely the absence (or more properly, the
reduction below percievable levels) of ambient light. It is not itself a
Look at it another way: To recognize a monophyletic group, The
Watcher, The Beyonder, or The Maximortal merely wades into the river of
time and prunes a clade off at the root. However, to recognize a
parpahyletic group, the supreme entity in question must first prune a
clade, then prune one or more clades from that clade, and accept what is
left over. A paraphyletic group cannot be taken from the tree of life as a
unit. It cannot even be removed from a clade, something else must be removed
before it can be identified. It owes its "existance" not to a unifying
process (ancestry and descent), but to a secondary effect of that process.
It is like the darkness: what you get when the light is gone.
>The relative USEFULLNESS of
>monophyletic and paraphyletic taxa in terms of conveneience and enabling
>us to understand the big picture is what your three points relate to.
>THIS is a valid point of contention;
That certainly is a point of contention, and I hardly think you will
convince any practitioner of PT that paraphyletic taxa are useful enough to
justify the extra levels of complexity, confusion, and obfuscation which
they add to systematics. Add to that the philosophical difficulties outlined
above, and it seems clear that parpahyletic taxa are best left in the
wastebasket of systematics.
"...standing between the Darkness and the Light..."
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"Chimp here does the killing." - Doug Mackenzie