Natural Selection and Causal Productivity - Mechanism and Causality in Biology and Economics - page 151

The third worry is that populations do not seem to engage in activities as a
unified entity in particular places and times. Here, it is not entirely clear what counts
as an activity or whether Glennan means to fully take on the MDC notion of activity
(which is itself not entirely clear). However, here are some candidate activities that
populations can engage in as a whole: invading (as discussed above), changing
other populations (e.g., as with predator/prey interactions), splitting, going extinct,
speciating, and changing their environments in a way that facilitates colonization
by populations of other species. Indeed, if it turns out that that these do not count as
activities, so much the worse for the requirement that entities engage in activities.
They all involve interactions (the term used in Glennan's own account of
mechanisms) between populations and other entities, including interactions
between populations and entities in the populations' environments.
Glennan does acknowledge that populations sometimes act as individuals, for
example, in migration processes. However, he says, “With respect to selection
processes, the question of whether or not populations or sub-populations should
be treated as individual entities depends upon whether or not group selection is at
work” (Glennan 2009 , p. 333). More generally, “The question of whether they are
individuals only makes sense in the context of analyzing a particular causal
process” (Glennan 2009 , p. 333). So, this raises a fourth worry, whether populations
are individuals in the natural selection process specifically.
However, it seems to me that the population is acting as an individual with
respect to the selection example that Glennan describes, even in the absence of
group selection. Again, recall that the fish form an image associated with the more
common color. This in itself is evidence that there is an interaction between the fish
and the population as a whole - the fish forms an impression of the population as a
whole, and the image is a result of the interaction. Of course, when a predator fish
kills a water bug, there is an interaction between an individual predator and an
individual bug. But that single interaction does not constitute a natural selection
process, just as the interaction of your fingers with a keyboard does not constitute
the creation of a document; that involves your interaction with the whole computer.
Or, to invoke an analogy for selection processes more generally, a single particle of
flour falling through the hole of a sifter does not constitute sifting. One sifts not a
single particle of flour, but rather a “population” of flour particles, with particles
jostling against each other, some falling through and some remaining in the sifter.
Similarly, selection occurs with respect to the whole population. Types are only
selectively favored or disfavored as compared to other types in the population; a
type that might appear reproductively successful when considered individually is
actually unsuccessful in the selection process if other types outreproduce it
(Millstein 2006 ). Thus, for selection in general, the population acts as an individual.
Finally, there is the worry that populations do not have parts that share a
common fate. However, the fact that the organisms (the “parts”) of a population
are engaging in survival and reproductive interactions implies that they do have a
shared fate, at least to some extent. For example, consider a new advantageous
variation introduced into a population. If there is interbreeding among the
organisms (one kind of reproductive interaction), then that variation may spread