Paul Gallagher  wrote:

> In  Guy Hoelzer
 writes:
> 
> >Right.  The word "fitness" is very slippery.  I have
come to the conclusion
> >that it is merely a human construct which has been, and continues to be,
> >very useful as we struggle to understand the real process of natural
> >selection.  However, fitness does not actually exist.  It is not like we
> >will one day hit on the right definition, because there is no natural thing
> >that we are attempting to describe with the term
"fitness".  We use this
> >concept as a heuristic device.
> 
> That's an interesting point of view. I haven't given the question of
> fitness much thought, since the propensity interpretation of fitness
> seemed good enough, even if there are clear problems. John Beatty
> mentions that Mary Williams suggested treating fitness as an 
> "undefined primitive term," but she apparently did this in order
> to make "the fittest are more likely to leave more offspring" into
> an empirical claim -- so I think she's thinking along different 
> lines than you are.

Williams was axiomatising evolutionary theory into a logical structure -
fitness is a primitive (that is, uninterpreted) term in that logical
structure. I do not believe whe was trying to give a fully interpreted
model here, but just the formal structure of evolutionary explanations
(and let it be noted, she restricted this to natural selection, thus
perpetuating the myth that evolution = natural selection).

Williams, M. B. 1970. Deducing the consequences of evolution: a
mathematical model. Journal of Theoretical Biology 29:343-385.

Sober describes fitness thus in his 1984: 

"Evolutionary theory understands fitness as a probabilistic quantity. An
organism's fitness in the above model is its _chance_ of surviving. A
genotype's fitness is the average of the relevant probabilities
attaching to the organisms who have that genotype. The census
information of who lives or dies is evidence used for _estimating_
fitness; actual survival rates do not _define_ fitness." p43

and goes on to note on p49

"There seems to be no single physical proerty that varies as fitness
varies in all cases.

A property is said to _supervene_ on a set of physical properties if it
satisfies two conditions. First, the property must not itself be
physical, in that two different objects may share the property and yet
be physically quite different. Second, two systems that are physically
identical must both have or both lack the property in question. Fitness
appears to be a case in point. Fitness apparently cannot be identified
with any single physical property. And if there were two organisms that
were physically identical and lived in physicallly identical
environments, they would have to have the same fitness value."

He then goes on to quote Fisher comparing fitness to temperature in this
way.
....
-- 
John Wilkins wilkins.id.au
For long you live and high you fly, 
and smiles you'll give and tears you'll cry
and all you touch and all you see is all your life will ever be
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