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PHIL 481
Jan. 13: Second set of readings on fitness
Note: The symbol used for fitness in relevant formulas is w.
According to Darwinian theory, ‘fitness’ (differential survival and reproductive success) is
explanatory.
At the very least causal, and one might hope predictive.
Standard models of scientific explanation (Hempel):
Deductive-nomological (when there are relevant deterministic laws):
With L symbolizing ‘law’, C symbolizing ‘(initial) conditions’, and E symbolizing that which is to
be explained:
L1, L2 … LN
C1, C2 … CN
--------------E
Inductive-statistical explanations:
LS1 … LS1
C1 … CN
--------------------Probably E
At best, fitness might explain and predict on an IS model, for there are no laws, in the
traditional sense, in biology.
Hardy-Weinberg Law:
[So long as certain conditions are met] gene frequencies and genotype ratios in a randomlybreeding population remain constant from generation to generation.
Conditions: no mutation, no genetic drift, no genetic migration, no natural selection, and
nonrandom mating!
In any event, what is ‘fitness’? And how can we tell that one organism is “fitter” than another
such that genuine explanations and predictions are possible?
A. “The circularity/tautology problem”
B. Genetic Drift: The identical-twins paradox
A. If we define ‘fitness’ in terms of actual reproductive success (say, by counting the types in
succeeding generations and deeming those most proliferate as the ‘fittest’), isn’t fitness either
an empty concept or one that only functions in a circular manner? Don’t we need a definition of
fitness that distinguishes it from reproductive rates?
B. Suppose there are identical twins, John and Joseph, whose phenotypic traits are as identical
as their DNA.
Wouldn’t we be tempted to say that, given the same set of environmental conditions (they are
not separated at birth, for example), they are equally fit (or their fitness is the same)?
And what would we mean beyond (or other than) “they are equally likely to survive and
reproduce successfully”?
But suppose that one day John and Joseph are playing golf together, and Joseph and only
Joseph is hit by lightening and killed, having not “reproduced.”
How do our intuitions about and/or definitions of ‘fitness’ run into problems here … or do they?
Simply put: a pair of organisms (or a single plant) can (for a myriad of reasons) fail to reproduce
at the rate their relative fitness (assuming we could calculate that) would predict. This will
impact the frequency of genes and/or traits in the succeeding generation and as that
generation and every other future generation is subject to genetic drift, the latter is a
significant factor in evolution and one that any account of ‘fitness’ that takes it and natural
selection to be causal should be able to accommodate.
Classical (or ecological) fitness:
a is fitter than b if and only if:
a’s traits enable it to solve the design problems set by some environment, E, more fully than b’s
traits do.
Here fitness is described as a property of individual organisms (albeit relative to some E).
Questions:
What are the design problems of E? Could we specify them? How many are there? And what is
“a design problem”?
Is there really a way of measuring the degree to which a exceeds b in solving them? Is there
really a way to identify all the relevant “problems” that affect survival and reproductive success
that would make predictions possible?
An alternative (that is popular with biologists and some philosophers – e.g., Philip Kitcher)
Population Biology’s notion of fitness
Using population genetics to identify changes in populations, groups of organisms, and/or
genes.
Defining evolution as “change in gene ratios” that are explained by natural selection.
In effect, doing away with classical or ecological ‘fitness’ altogether?
We are no longer paying attention to the biology of individual organisms.
We are paying attention to populations (or groups) and genes.
And we are noting changes in gene rations of given populations.
‘Fitness’ and ‘w’, although they appear in the theory, simply express probabilistic reproduction
rates for populations.
Kitcher and Sterlny:
Evolutionary theory, like statistical mechanics, has no use for such a fine grain of
description [as the biography of each organism]: the aim is to make clear the central
tendencies in the history of evolving populations (1988. “The Return of the Gene”.
Journal of Philosophy 85: 335-358).
But does this model explain even differences in gene frequency? Can it predict them? Can it
only describe past successes of the model, and not provide reasons for believing it will continue
to succeed?
If one is not willing to settle for an acausal account of fitness (because it would seem to mean
also giving up on a causal understanding of natural selection), are there alternatives?
The Propensity Interpretation
Viewing ‘fitness’ as a dispositional property (like ‘fragility’) that can causally intervene between
organisms and environments so that actual rates of reproduction are its effects (not its
content).
Dispositional properties and dispositions are distinguished from actual “behaviors” (e.g.,
breaking).
Those things that have a given dispositional property are distinguishable from those that do
not. (We something like “the molecular properties of the relevant matter” determine ‘fragile’
and ‘magnetic’.)
An organism can have a probabilistic disposition to have n offspring and not (as luck would have
it) actually reproduce.
Formulations of comparative fitness differences (Mills and Beatty, among others):
x is fitter than y in E = x has a probabilistic propensity >.5 to leave more offspring than y.
If fitness is a probabilistic propensity, the fitter among competing organisms will not always
leave more offspring.
Accordingly, natural selection will be understood as making the claim that fitness differences
result in reproductive differences only with some probability.
Genetic drift is one (distinct) process that explains the probabilistic nature of predictions based
on fitness and natural selection.
Mills and Beatty’s project:
Rescue ‘fitness’!
Do so in a way that is in keeping with what evolutionary theorists mean by it/how they use it
Build on Hempel’s model of explanation to provide a model of evolutionary explanations
Demonstrate that such explanations are not circular
Define natural selection in terms that presume the propensity interpretation and in a way able
to accommodate all the types of selection biologists discuss
Sober’s project:
To return to problems in Mills’ and Beatty’s original account that they themselves identified a
decade later
And to conclude, albeit for different reasons, as they did that expected number of offspring “is
not always the right way to define fitness.”
Mills and Beatty:
What, according to them, do evolutionary theorists mean by ‘fitness’?
How is a propensity interpretation of fitness in keeping with this?
What model of evolutionary explanations do they propose?
How do explanations of this form avoid circularity?
How is natural selection redefined given a propensity interpretation of fitness, and what can it
accommodate that the classic understanding cannot?
Sober
What are “the two faces” of fitness on which he focuses?
How are they related to, and what are, the challenges Sober cites as facing any account of
fitness that defines it in terms of expected number of offspring?
Long-term vs. short term thinking
Stochastic variation in numbers of offspring and problems for viewing fitness as a
“mathematical expectation”
What conclusions does Sober draw about the concept of fitness, including the propensity
interpretation thereof?