Three-taxon analysis

René Zaragüeta Bagils rzb at MNHN.FR
Tue Apr 8 00:53:09 CDT 2003

P.Hovenkamp wrote:
> This is indeed the point - and it goes back to Patterson, who (thought he)
> could distinguish "taxic" and "transformational" homology. In a
> non-evolutionary world, taxic homology (the presence of a homologous
> structure in a number of taxa) is indeed possible - in an evolutionary
> world, it is not. In an evolutionary world (and models do not enter, just
> evolution) homology describes a change from one state to another, and a
> . There are no
> "concepts" that arise de novo - each character has had a state from which
> it developed.

"Models do not enter, just evolution" :
Lamarck's? Darwin's? Goldschmidt's? Neodarwinian's ? Bergson's? Teilhard de
Chardin's? Vitalism? Orthogenetics? Budhist reincarnation?...
If "just evolution" means anything, then "just evolution" is trivial.

You say that full specification of a homology statement *needs* two states
I see (at least) two problems:
- First, I do not agree, but you should have received David William's

- Second, you say that each character has had a state from which it
Then, the first one is a paraphyletic character (because it does not contain
the state that developed from it). As you say, cladistic uses character
analysis, which means that what we know about relationships of organisms is
the same as what we know about relationships of characters. As you seem to
defend paraphyletic characters (plesiomorphies, the states from which new
develop) as informative, then they may only characterize paraphyletic
groups, which do not exist in a Darwinian evolutionary world. I conclude
that your evolutionary world is one where paraphyletic groups, characterized
by paraphyletic characters, exist.
Example: Imagine that you tell me that tetrapod limbs developed from fins.
This is the same that to say that tetrapods,
characterized by limbs, arose from fishes, characterized by fins (instead
of: tetrapods are a kind of fish characterized by limbs, a kind of fin).
is a general problem, applicable also to standard parsimony, of atomistic
(transformations) versus epigenetic (differentiation) descriptions.

Concepts arise indeed de novo, in the mind of systematicians. Concepts are
ideas. The concept of paraphyly arose de novo in the mind of Hennig, for
instance, and has still to arise in lots of minds.
Are you certain that characters cannot "arise de novo"? How do you know? How
can I test your assertion?
Perhaps you think that characters are observable bits of truth, but
characters are ideas (hypothesis, theories), not observations.

> Three taxon analysis gives up on character analysis even before it's
> becoming difficult. Discarding evolution by the way.
> Peter Hovenkamp
Oops! You never practiced three-taxon analysis, didn't you? Perhaps one of
the reasons of the lack of success of three-taxon analysis is its difficulty
(conceptual and operational)!


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