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process is natural selection. Nonetheless, when natural selection and drift operate
together, they are one single sampling process, and if this sampling process is
representative, then it surely is natural selection or otherwise drift. It follows that if
this sampling process is more likely to be representative, then it is less likely to be
unrepresentative, and vice versa. So when natural selection and drift operate together,
if there is any change in terms of strength, there surely is a trade-off between each
other's strengths. In this sense, Brandon's account meets the requirement (
T
).
As shown above in my argument, the trouble with Millstein's causalist account
turns out to be the view that natural selection and drift are, ontologically speaking,
two separate sampling processes. A question arises: could Millstein avoid the
trouble without undermining her causalist account? In my view, it suffices for
Millstein to revise her characterization of drift in such a way that drift ceases to
be equivalent to an indiscriminate sampling process. To this end, it suffices that
unrepresentative sampling processes count as drift. Note that an indiscriminate
sampling process could still count as drift, provided that when it operates together
with natural selection, the requirement (
T
) is satisfied. As is evident, if unrepresen-
tative and indiscriminate sampling processes both count as drift, then it is
not
the
case that drift and natural selection are necessarily mutually exclusive. And the
trouble with Millstein's account would be avoided. Such a revised concept of drift
is
de-unified
to the extent that it seems to cover heterogeneous cases. I would say
that such a de-unification, instead of showing the revised concept is inappropriate,
serves to show what is characteristic of drift. Indeed, drift is an umbrella concept
supposed to cover all cases where chance plays a role in a sampling process. Given
that chance might play a role in a variety of ways and in a variety of situations, it is
no wonder why the concept of drift is un-unified and even un-unifiable. Thus,
Brandon's account of drift as unrepresentative sampling process turns out to be
helpful to Millstein's account of drift as indiscriminate sampling process, and both
could be incorporated into an un-unified account of drift.
3 Could Natural Selection Be a Population-Level Causal
Process?
According to Millstein, natural selection is by nature comparative: whenever it acts
as a cause of evolution, it impinges on comparative and thus population-level
properties (e.g., variation in fitness, frequency of traits) rather than on individual-
level properties (e.g., fitness, traits). On this view, natural selection is a population-
level causal process. One problem with such a view is that it threatens to make
natural selection into a shadow process. Indeed, this problem has been identified by
Shapiro and Sober (
2007
) as follows:
Natural selection is not a population-level causal process. If it were a sampling process
operating at the population-level, it would be, so to speak, a shadow process, an epiphe-
nomenon of individual-level causal processes. (Epiphenomenon problem)
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