Guy Hoelzer  wrote in
news:ce4oh8$1mgt$1{at}darwin.ediacara.org: 

> I have a simple suggestion about the validities of considering
> absolute vs. relative fitnesses.  IMHO, focusing on absolute fitnesses
> is ideal in the absence of dynamical feedback among individuals (more
> generally, elements of your dynamical system).  On the other hand,
> feedback is often generated through social and ecological interactions
> such as competition.  I have often argued in favor of using relative
> fitness because I am comfortable assuming that growing populations
> will always run into resource limitation leading to within population
> competition, which elevates the importance of relative fitness
> compared with absolute fitness.  JE can correctly argue that fitness
> interactions, such as competition, can manifest evolutionary effects
> by changing absolute fitness tallies, but I don't find that argument 
> to be a persuasive reason to stick with an absolute fitness model.  A
> fundamental difference between the two approaches is that the relative
> fitness approach anticipates and integrates the effects of factors
> that limit population size (e.g., competition).  The absolute fitness
> approach assumes that population sizes are not constrained, or that we
> can safely ignore any effects of population regulation.  Neither
> approach is perfect. I think that one's choice of approach ought to be
> determined by the extent to which he/she thinks that mechanisms of
> population size limitation (specifically, negative frequency or
> density-dependent effects, where population members are essentially
> acting as indistinguishable parts of the population) will occur in the
> desired window of predictive power.  It is a trivial point to say that
> the absolute fitness approach is a completely sufficient model of the
> past, because all forms of fitness interaction have had their effects
> and the accumulation and subtraction of abundances of alternative
> forms must add up perfectly from the past to the present. However,
> this retrospective model is merely bean counting without offering a 
> deep understanding of how evolution works.  IMHO, a deeper
> understanding requires appreciation of evolutionary processes at many
> levels of organization and their covariances (as emphasized in the
> Price equation). Sean Rice's recent book seems to be an excellent
> pointer to the future of evolutionary theory.

I agree that absolute fitness is a useful concept. We have often debated 
whether there is a direction to evolution, and to a certain extent I 
would argue that there is a general overall increase in absolute fitness. 
Things such as thermoregulation, variable dentition, ability to modify 
coloration, and compound eyes, to name a few, might all qualify as 
examples of increases in absolute fitness for large animals occupying 
similar niches as their older ancestors. But as you point out, there are 
many constraints to the concept. Is it better to be larger or smaller in 
size? Is it better to be a generalist or a specialist? Is it better to be 
complex or simple? The answer is always relative. 

The further problem with absolute fitness is that it has very little 
value as a metric. One can hope to measure relative fitness over a 
limited number of generations in the laboratory, and one can guesstimate 
it over a larger number of generations in the field, but measuring 
absolute fitness would require absolute knowledge - otherwise one is only 
really measuring relative fitness.

Yours,

Bill Morse
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