Re: Popper and Palaeontology (was: Re: "running" elephants - locomotary analoges)

[John Conway <john_conway@mac.com>]

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 
arrangement.

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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