124 Christopher Stephens
inheritance. So this is one important difference with a Newtonian zero-force law.
Secondly, several biologists and philosophers have pointed out that fitness is not
well understood as a causal processes; rather, it is more like the actuarial notion
of overall life expectancy [Fisher, 1930; Sober, 1984; Walshet al., 2002; Matthen
and Ariew, 2002; Stephens, 2004]. Overall life expectancy is not acauseof how
long one lives, it is instead merely a statistical summary of information about how
a number of possible causes might affect one’s survival. Similarly, an organism’s
viability fitness is not a cause of its survival (or death); rather, it is a statistical
summary of the possible causes that might affect an organism’s viability, weighted
by their chances of occurring. The fact that a particular cockroach (or a trait type
in a population) has a .8 chance of surviving to adulthood is not what causes the
cockroach to survive (if it does). It is a reflection of a weighted summary of possible
factors — many of which never come to pass - that might kill the cockroach.
Critics of the traditional view think that natural selection is like the notion
of fitness or life expectancy — it is not a force or even a cause; rather, it is
merely a statistical summary of various life and death events that occur in a
population. Endler [1986, 29], for example, compares natural selection to erosion.
He claims that natural selection is theresultof heritable variation in fitness, rather
than identical to it or a cause of it. He compares natural selection to erosion,
which is a result of variance in resistance to weathering. Endler also objects that
natural selection is disanalogous to force in physics, since it cannot be decomposed
into something analogous to a mass and acceleration. Endler allows that natural
selection can be a cause — it is, however, merely an effect of heritable variation
of fitness, but it is a cause that can lead to genetic change in populations.
Defenders of the traditional view often reply by arguing that there is an impor-
tant distinction between fitness and natural selection. Fitness may not cause an
organism to survive or reproduce, but to say that one organism is fitter than an-
other is not to say why it is fitter. Selection is supposed to be a causal notion that
explainswhyone trait is fitter than another. Furthermore, terms such as ‘natural
selection’ and ‘drift’ can be used in two ways — to talk about a process, or to talk
about the result of a process. The traditionalists maintain, while the critics deny,
that the process notions of natural selection and drift can be understood causally.
Another major source of doubt comes from thinking about drift as a force.
First, it should be noted that it is common ground in this debate that it does not
make sense to talk about how much drift and selection contribute in an individual
case. This makes no more sense that it does to decompose the effects of the bias
of a coin and the number of times it is tossed in determining why a particular flip
of the coin came up heads or tails. Drift, if it is to be understood as a force or
cause, is, as Sober says “a force of a different color” [Sober, 1984, 117], and can
only be understood as a population level force or cause.
Critics of the traditional view point out that it is puzzling to think about drift
as a force or cause by thinking about the following kind of example. Imagine that
there are two scenarios, each with two organisms, one having traitT 1 which is fitter
than the other organism, which has traitT 2. In the first scenario, the organism