We all know that heritable variation in characters affecting fitness
is a prerequisite for natural selection.  Metaphorically speaking,
we can even say that heritable variation is the "driving force"
for NS.

This is a good metaphor because variation plays a force-like role.
Assuming you quantify "variation" as the variance in fitness, the
infamous "Fisher's Fundamental Theorem" assures us that the
magnitude of the response is proportional to the magnitude of the
"force".  Furthermore - (correct me if I am wrong here!) - the
direction of the response matches the direction of the "force".
If the total variance in fitness is divided among several
characters, the characters that evolve fastest are those that make
the largest contribution to the fitness variance.

My question is whether a different metaphor might not also be
good.  Suppose we were to say that heritable variation is the
"fuel" of evolution.  Can we come up with a way of quantifying
"variation" so as to make this a good metaphor.

For the "fuel" metaphor to be a good one, we need to quantify
"variation" to make it fuel-like.  That is, variation must be
inevitably "consumed" by natural selection.  Furthermore, we
want the amount of the response to be roughly proportional to
the amount of fuel consumed - not, as in the "force" case, to
the amount present.  Can this metaphor be made to work and to
work well?  (Of course, nothing rules out that the "fuel" may
be replenished by mutation or by some other process outside of
natural selection proper.)

Frankly, I don't know.  I'm pretty sure that the metaphor does
NOT work if you stick to variance in fitness as your quantification
scheme for "variation".  But does some other quantification work?
Has anything along these lines been published?

I was led to this line of thought a couple months ago while discussing
DS Wilson's use of the Price equations in support of "trait group
selection".  I made the point that the group selection process is
driven by inter-group variation, but that the process itself must
inevitably decrease ("consume?") the amount of inter-group variation.
Hence, I was led to say that the process must quickly "run out of
gas" unless the "fuel supply" is replenished by some other process -
assortive group formation, for example.  That is what I was thinking
at the time, but I still don't know whether my metaphor is a good
one and whether it applies to other situations.  What do you all
think?

Of course, the true value of a metaphor lies in the follow-up
questions that it suggests.  Is there a difference in the fuel
efficiency of various evolutionary situations?  Does the combustion
require two different fuel components (oxidizer and reductant)
with either component potentially limiting?  Etc.
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