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Re: Popper and Palaeontology (was: Re: "running" elephants - locomotary analoges)
On Wednesday, April 23, 2003, at 03:16 AM, Jonathan R. Wagner wrote:
I have never been happy with this characterization of parsimony,
and I think a number of others share my displeasure. The assertion
that parsimony analysis requires that "evolution is parsimonious" is
like saying that using newtonian mechanics presupposes that the
universe works in a strictly newtonian fashion. I do not believe any
scientist who uses newtonian mechanics would, if pressed, agree to
that statement. Newtonian mechanics is an approximation, a model; it
can give us reasonable approximations of the true values under a given
set of assumptions without having a 1:1 correspondance with the
phenomena it models.
I don't think your example really supports your point. One could easily
turn this around, and say that a scientist using newtonian mechanics
presupposes that the universe (or least the part of it they are
modelling) is "newtonian enough", just as people say that parsimony
analysis presupposes evolution to be "parsimonious enough".
In the same way, parsimony, likelihood, or any other optimality
criterion, is never expected to accurately reconstruct the way
evolution "works" (if such a thing CAN be accurately reconstructed).
It is (depending upon your point of view) either a model intended to
give the closest approximation of evolution that we can get on
theoretical grounds, or an imperfect model that represents the only
reasonable approach to solving the problem currently available (but
see my post on likelihood and Bayesian methods), or it is a logical
procedure for reconciling conflicting character data that has no
pretensions to being an overt model of evolution. None of these
applications requires that "evolution [be] parsimonious."
But it is meant to model the >pattern< of evolution - otherwise no-one
would bother doing it. By pattern of evolution, I mean the arrangement
of the real characters in respect to clades. That's to say, the pattern
of evolution must be parsimonious enough to be approximately recovered
by a parsimonious arrangement of data. I don't see how the parsimonious
arrangement of data could possibly reflect a non-parsimonious real
Who'd have thought simplicity could be so complex? ;)
To paraphrase Paul Sereno (Pete, you may wish to note the date and
time): parimony does not imply that homoplaisy is minimal, it merely
minimizes assumed homoplaisies.
And why would you want to minimise assumed homoplasies, if you don't
think they are minimal (i.e. unlikely)?
And another thing: exactly HOW could evolution be parsimonious?
What does that mean? No one has ever answered this to my satisfaction.
The possible interpretations I have arrived at almost always lead to
some pretty wild assumptions (e.g., evolution is an active process,
morphological characters are objectively real, etc.). I'm open to a
good explanation, but I've never heard one. Yet people seem really
comfortable bandying this statement around as if we all KNOW what it
means. I'm waiting...
Although the statement that "Evolution is [...] at least [...]
parsimonious enough" was not mine, I think it can be clarified by
replacing "evolution" with "pattern of evolution" (as above). It could
be argued that in the long run, through random acquisition, character
states do form a parsimonious pattern.
I don't miss the Clade Wars. :)
And I missed the Clade Wars. :)
John Conway, Palaeoartist & Protophilosopher
"Do I contradict myself? Very well, then I contradict myself, I am
large, I contain multitudes." - Walt Whitman
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