Is Natural Selection a Population-Level Causal Process? - Mechanism and Causality in Biology and Economics - page 164

the idea that natural selection is characterized as a ( C -)discriminate sampling
process, it does require Millstein to revise her characterization of drift so as to
make room for its being numerically identical to natural selection when they act
jointly. I will begin my argument by pinpointing a problem with Millstein's account
of drift. That brings me to a disagreement between Brandon and Millstein with
respect to drift.
2 Why Is Brandon's Account of Natural Selection and Drift
Preferable?
According to Millstein, natural selection, as a causal process, is probabilistic. That
is why she characterizes natural selection in terms of causal relevance rather than
causal determination . When natural selection acts, it is true that physical
differences among organisms are causally relevant to differences in reproductive
success. However, the physical differences do not causally determine the
differences in reproductive success: given the same physical differences, different
differences in reproductive success are possible. Among the many possible results,
there may well be a distribution of probabilities, and every possible result is not
equally probable. The most probable result is surely the most expected when natural
selection acts, but the other results, even improbable, remain possible. That is how
natural selection, though a causal process, is nonetheless probabilistic.
Recall that Millstein characterizes drift, in terms of causal irrelevance, as an
indiscriminate sampling process. When drift occurs, physical differences among
organisms are not causally relevant to differences in reproductive success, let alone
causally determine them. Thus, drift is no less probabilistic than natural selection.
Like natural selection, drift has many possible results, among which there may well
be a distribution of probabilities, and every possible result may not be equally
probable.
Millstein claims, on the one hand, that the distinction in terms of discriminate/
indiscriminate sampling process alone suffices to separate, at least conceptually,
natural selection from drift. On the other hand, she claims that an improbable result
of natural selection might be indistinguishable from a product of drift. At first sight,
it appears paradoxical to make the two claims at once. Millstein argues, however,
that one should separate process from outcome in order to get things clear: although
natural selection as outcome might be indistinguishable from drift as outcome,
natural selection as process is conceptually distinguishable from drift as process.
And such a distinction can be made without regard to outcomes.
To Millstein's view, Brandon ( 2010 ) raises an objection:
[N]atural selection and drift are co-products of the same process, namely a probabilistic
sampling process (Brandon and Carson 1996 ; Matthen and Ariew 2002 ; Walsh et al. 2002 ).
Thus, although it is of crucial importance to separate selection and drift, one cannot do so
on the basis of process alone (contra Millstein 2002 ), one must do so on the basis of
outcomes. (Brandon 2010 )