116 Christopher Stephens
population) will get to AB. In the second case, we suppose thatABis again the
fittest combination but thataB,Abandabare all equally less fit thanAB. In this
case selection doesnot play a role in explaining whyABfirst originates in the
population. In this latter case, it is only the unlikely combination of mutation and
drift that explains the origination of theABcombination. Consequently, selection
can explain the origin of biological traits if the right sort of processes occurs. It
is not enough that the process is cumulative and it doesn’t matter whether sex-
ual reproduction is involved; what is crucial is that a trait is affected by multiple
factors.
Forber argues that to resolve the second issue — whether selection can explain
why individuals have the traits they do — one must settle certain philosophical
issues about explanation that are independent of the purely scientific features of
evolutionary biology. As Matthen’s sheep example indicates, if selection helps
explain why a trait first originates, then it helps explain whysome individuals
or otherhave that trait. There is, however, a separate debate over whether this
explanation about origin also carries over to explain whyparticular individual
organisms have such traits.
Defenders of the Neander-Matthen position accept the view that part of what
explains why an individual has a particular trait depends on explaining how the
trait originated in an ancestral individual. Proponents of the Sober position, on the
other hand, argue that a developmental explanation of an individual is sufficient
to explain why individuals have the traits that they do — there is no role for
what Sober calls a variational explanation. A final resolution of this debate thus
depends on sorting out which view of explanation is the most plausible.
Adaptationism
A trait such as giraffe neck length is anadaptationfor eating leaves high in trees
if it is a result of natural selection acting for that end. If, in the relevant ancestral
population, there was selection for having longer necks because giraffes with longer
necks were more fit than those with shorter necks by virtue of being able to eat
more leaves, then we can say that the giraffe’s long-neck is an adaptation for
eating.
A few remarks about this definition of adaptation. Notice that an adaptation is
anhistoricalproperty rather than a comment about a trait’s current utility. Maybe
long necks areadaptivenow because they help giraffes see over cars or buildings,
but these are not reasons why the trait evolved. A trait can have properties that
are not part of why it evolved, such as the fact that a neck has a certain mass.
These traits are sometimes known as “spin-off” or “side effects” and there is merely
selectionof, rather thanfor, these properties [Sober, 1984]. A trait can also be
favored by natural selection for more than one reason, either simultaneously or in
sequence. Giraffe bulls engage in “necking displays” to exhibit dominance, and
so long necks might also be an adaptation for this behavior. In some cases, a
trait such as having a long neck might evolve for one reason (to reach high leaves)