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Re: Paraphyletic groups [was Re: cladobabble...[...]]
> 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 agree that a group based solely on ancestry rather then descent
would be fairly useles; my point is that the issue between phylogenetic
and evolutionary taxonomy IS the relatively USEFULLNESS, not REALITY, of
monophyletic and paraphyletic taxa.
> 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.
As I put it previously, descent is a verb, not a noun. How
does a PROCESS "INHERENTLY" unite organisms as a group? To "group"
organisms requires them to coexist conceptually, something that never
happened (at least for most members of the group) in reality. A taxonomic
grouping is inherently artificial, whatever the basis.
> A group can only be a real, natural group in
> this context if it is united by common ancestry and descent.
Why why why? I need clarification on the cladistic definition of
"reality", and why that particular definition is favored. I prefer "that
which exists whether you think it does or not", which applies to evolution
and descent, but not to a clade or any other type of taxonomic grouping.
They are convenient illusions.
> Reality must be intrinsic to a group, not imposed by external phenomena.
I would argue that a clade IS imposed by an external phenomena, namely
> There is no
> biological phenomenon which warrants this exclusion, and no biological
> process unifies the paraphyletic group exclusive of the monophyletic
The evolutionary development of morphological similarity, even due to
convergence, is a biological phenomena. Again, a polyphyletic group based
exclusively on superficial morphological similarities regardless of
relationship is pretty useless to us as people INTERESTED in evolution,
but this is an arbitrary decision on our part. What is "warranted" is an
arbitrary decision, not inherent.
> 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.
This is assuming that the clade was there to begin with, something I
am arguing against. You have to think it up before you can prune it.
> 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.
First of all, no one is proposing that taxa be based exclusively on
morphology; any paraphyletic group can be considered as a subset of a
monophyletic group, so in essence is still exists due to the existance of
a larger monophyletic group.
Second of all, the allegation that morphology is a result of
phylogeny is a chicken and egg question; The phenotype of an organism is
not only the result of its evolution but will affect its subsequent
evolution. Morphology is the result of evolution is the result of
morphology is the result of evolution...The panda altered its wrist bone
becsause it didn't have a thumb; due to evolution. Morphology IS important
when considering evolutionary questions and issues; indeed you cannot
Even cladists refer to "basal archosaurs" and "non-avian dinosaurs".
Is using these words REALLY muddying the waters less then abbreviating to
"Thecodont" and "Dinosaur"? The groups are still being referred
to and therefore are given a place conceptually, so what has been
simplified? Everytime you define a clade, you are also recognizing a
paraphyletic group; if you were to define Aves as the most recent common
ancestor of modern birds and _Archaeopteryx_ and all its descendants for
example, you are automatically making all dinosaurs not belonging to that
clade into "non-avian dinosaurs", regardless of if you give them a
capitalized name or not.
Whether or not you recognize paraphyletic groups as formal taxa
(something even I probably wouldn't do in the case of thecodonts), the
question of what traits they retained can be SIMPLER then then considering
what triats thier descendant clades developed. For example:
Lets pretend for a minute that all "thecodonts" that didn't survive
the Rhaetian extinction went extinct for the same reason, the
PRESENCE of a morphological feature that was modified, but in DIFFERENT
WAYS, in crocodylomorphs, dinosaurs, and pterosaurs. That is an issue that
can be considered most smply by looking at them as a paraphyletic taxa:
"The Triassic thecodont extinctions were caused by the retention of trait
A" as opposed to "The Triassic extinction survivors did so through the
development of trait B in the case of dinosaurs, trait C in the case of
pterosuars, and trait D in the case of crocodylomorphs." The latter
staement is misleading; it assumes that the development of these traits
was the key to survival, when in fact it was the LOSS of the
deletrious primitive trait. It doesn't matter if they devloped B, C or D,
as long as it was done at the expense of A, the feature that caused the
extinction of a paraphyletic group.
Clades are conceptual aids, and so a paraphyletic taxa. Both are
artifical but help in seeing the big picture, and cannot be seperated from
Incidently, as intristic as cladistics is to discussions on this
list, is it really so innaproprite to discuss it, even if dinosaurs aren't
always included in the examples?