Is Natural Selection a Population-Level Causal Process? - Mechanism and Causality in Biology and Economics - page 163

which it hitchhikes actually are numerically identical. Namely, despite the appear-
ance, they are, ontologically speaking, one and the same sampling process. It is
noteworthy that in order to show the implication of Millstein's account is problem-
atic, such a strong claim is not necessary. A weaker claim would suffice: when
selection-of and selection-for are both present, it is possible that only one single
sampling process is occurring. But is it really possible?
Recall the example of selection-of offered in Shapiro and Sober ( 2007 ). Onto-
logically speaking, selection for being green and selection of being small are not
two separate sampling processes. The distinction between them is made in the
human mind, not in the world. It is we who separate selection of being small from
selection for being green. Nature never separates the two sampling processes. Green
organisms and small organisms are selected all at once and in one shot. It is not the
case that green organisms are first selected, and then it's the turn of small organisms
to be selected. Being numerically identical, selection for being green and selection
of being small are one and the same sampling process. Since selection-of and
selection-for are, arguably, one single sampling process, an adequate account of
natural selection should be broad enough to include both selection-for and selec-
tion-of. The problem with Millstein's account is that it defines discriminate sam-
pling process (natural selection) too narrowly to include selection-of: not only is it
impossible for selection-of and selection-for to be one single sampling process, it is
also impossible that natural selection and drift, when operating together, are one
and the same sampling process. As is clear now, if a sampling process is discrimi-
nate, then it surely is not indiscriminate. Thus, if it counts as natural selection, then
it cannot count as drift, and vice versa. In short, natural selection and drift,
according to Millstein, are mutually exclusive. In my view, Millstein should
leave open the possibility that natural selection and drift coact as one single
sampling process. This includes two things: first, it is possible that natural selection
and drift act together. Second, it is possible that when they act together, only one
single sampling process is occurring.
Let's consider the first requirement. Indeed, the coexistence of natural selection
and drift, far from being unusual, makes better sense of biologists' practice. When
biologists debate whether natural selection or drift predominates in an evolutionary
event, they are presupposing that natural selection and drift may act together.
Otherwise, their debate, if not pointless, would be highly misleading. Note that
although Millstein characterizes natural selection and drift as mutually exclusive, it
does not follow that they cannot act together. To be sure, a sampling process, if it
counts as natural selection, will not be drift, and if it counts as drift, it will not be
natural selection. However, it is possible that two sampling processes act together,
one of which counts as natural selection and the other as drift. Hence, it is possible
that natural selection and drift act together. In this sense, Millstein would have no
problem with the first requirement.
As is evident now, I will argue that Millstein should meet the second require-
ment, namely, when natural selection and drift act together, it is possible that only
one single sampling process is occurring. In my view, this is a move that Millstein
should make. Although making such a move may not require Millstein to abandon