Is Natural Selection a Population-Level Causal Process? - Mechanism and Causality in Biology and Economics - page 170

Population-level: Natural selection → change in the frequency of traits
↓(supervene on)
Individual-level: The many causes → individual births and deaths
Fig. 9.2 Individual births and deaths constitute the supervenience base of the change in the
frequency of traits
To see how the epiphenomenon problem arises, one should be reminded that
each of the population-level properties supervenes on individual-level properties
(Shapiro and Sober 2007 ). Now, at the level of individual organisms, few would
deny that there are already many causes for individual births and deaths and that the
summation of such individual births and deaths constitutes the supervenience base
of the change in the frequency of traits (Fig. 9.2 ).
If many individual-level causes suffice to make a change in the frequency of
traits, then natural selection, which is supposed to be the population-level cause of
such a change, threatens to be redundant—a shadow process. Or to put it another
way, granting that a change in the frequency of traits is the summation effect of
many individual-level causes, if natural selection were the population-level cause
of such a change, then the same change would be overdetermined. To avoid
overdetermination, one cannot help but annihilate natural selection into a shadow
process. Without a satisfactory solution to the epiphenomenon and the overdeter-
mination problems, any claim that natural selection is actually a population-level
causal process would seem premature.
Shapiro and Sober ( 2007 ) have tried to debunk an argument which,
supervenience base being equal, requires natural selection to have extra causal
efficacy in order to be a population-level and separate cause (in addition to the
many individual-level causes). They hold that such a requirement is too demanding.
To meet such a requirement is, in their view, a “mission impossible.” I am not sure
that Shapiro and Sober are on the right track when they say that no extra causal
efficacy is needed to qualify natural selection as a separate cause. It is doubtful that
natural selection without extra causal efficacy qualifies as a separate cause. Extra or
not, one has to show first of all where the causal efficacy of natural selection lies.
And that is the crux of the whole issue: on the one hand, natural selection, operating
at the population level, must have causal efficacy; otherwise, it would fade into a
shadow process. On the other hand, to avoid the overdetermination problem, natural
selection should not have causal efficacy. Here lies a dilemma.
In my view, the way out of the dilemma would be a division of labor between
natural selection and individual-level causes. The question is how such a division of
labor can be envisaged. Here are some basic ideas: natural selection is part of the
cause of a change in the frequency of traits. The many individual-level causes do
not suffice for such a change. The many individual-level causes have individual
births and deaths as their effects. Although such effects suffice to sum up the
frequency and calculate the way the frequency changes, the many individual-
level causes do not suffice to explain causally why it changes in the way that it
does. Let me explain.