Natural Selection 123which the adaptationism debate is about how nature restricts the space of options
for natural selection to act on as opposed to how other sorts of constraints (such
as genetic ones) restrict natural selection once it has options to act on.
Finally, it is also worth mentioning that empirical adaptationism is best thought
of as a research programme (see [Orzack and Sober, 1994; Sober, 2000]). It is
possible to test both particular adaptationist (and non-adaptationist) hypotheses
as well as general claims about the long run. Particular adaptationist claims are
testable if they are stated with enough precision, and whether or not most traits
in most populations are primarily a result of natural selection is something that
can be tested by accumulating data about many individual cases.
Forces and Causes
Biologists [Roughgarden, 1979; Gillespie, 1998] and philosophers [Sober, 1984]
often talk about natural selection, random genetic drift, mutation and other evo-
lutionary processes as forces and causes. Sober, for example, uses the Hardy-
Weinberg equation to formulate one of two versions of a “zero-force’ law that
states what will happen to gene and genotype frequencies if none of the standard
processes such as natural selection, mutation, migration and drift are at work in
a population. We can speak of natural selectioncausing the trait of having a
long neck to evolve in the giraffe lineage. We can talk about how mutation and
drift are forces that areequal in strength, so that theycounteractthe effect of one
another [Gillespie, 1998], or we can say that in small populations drift is amore
importantforce than selection. Population genetics has models in which selection
opposesmutation and models in which mutation operates in thesame direction
as selection, so it looks like these evolutionary processes can have directions and
strengths, just like forces.
Other biologists and philosophers have expressed doubts about understanding
these processes as forces [Endler, 1986, Walshet al., 2002] or as either forces or
causes [Matthen and Ariew, 2002]. They raise several sorts of doubts about the
Newtonian analogy, and prefer instead to think of natural selection as a purely
statistical theory. We will consider a few of their objections here, focusing on the
processes of natural selection and random genetic drift. They argue that notions
such as fitness appear to be statistical summaries rather than causes, and argue
that the traditional view exaggerates the degree to which evolutionary theory is
analogous to Newtonian physics. They also argue that thinking of drift as a force
or a cause has some peculiar consequences. Let us take each of these issues in
turn.
To what extent can evolutionary theory be understood as a theory of forces
analogous to those in Newtonian physics? First of all, it is important to recognize
that the possible evolutionary processes of selection, drift, mutation and so on
occur against various background theories about, e.g., how heredity works [Sober,
1984; 1996; Matthen and Ariew, 2002; Stephens, 2004]. It does not make sense
to talk about natural selection operating alone, in the absence of a system of