112 Christopher Stephens
Trying to understand the precise scope and role of natural selection in evolu-
tionary theory generates many questions of philosophical interest. For instance,
consider a trait that has evolved by natural selection such as giraffe neck length.
Did natural selection favor longer necks because it benefited theorganismsthat
had longer necks, because it benefited thegroupsof giraffes that had long necks,
because it benefited thegenesthat are (in part) responsible for long necks, or be-
cause of some combination of these factors? This is known as the units of selection
problem, and will be discussed in a separate entry (see Wilson’s chapter on levels
of selection in this volume).
We can divide the philosophical issues raised by natural selection into two rough
categories. First, there are issues — such as the units of selection problem — that
involve understanding the process natural selection itself. Besides the units of
selection problem, philosophers have been particularly interested in issues about
how to understand and interpret fitness, as well as the question of whether natural
selection explains why individuals (as opposed to populations) have the properties
that they do. A second set of issues includes questions about therelationship
between natural selection and other evolutionary processes. Included here are the
following sorts of questions: How powerful a force is natural selection compared
to the other possible causes of evolutionary change? Are the processes of natural
selection, drift, mutation and so on properly understood as forces or causes, or are
these factors merely statistical phenomena? I will consider each of these issues in
turn.
Fitness
Herbert Spencer [1864] was the first to use the phrase “survival of the fittest”
to describe natural selection. Alfred Wallace, who independently discovered and
developed the theory of evolution by natural selection [Wallace, 1859], encouraged
Darwin to adopt Spencer’s phrase because he thought that the term “natural
selection” would mislead people into thinking that the process required conscious
thought and direction. Darwin did eventually use the phrase “survival of the
fittest” [Darwin, 1868] but refused to give up the term “natural selection”, and in
his later works used the two interchangeably.
Darwin and other biologists’ subsequent use of the phrase “survival of the
fittest” gave rise to a criticism of the theory of evolution known as the tautol-
ogy problem. If it is a matter ofdefinitionthat the fittest organisms survive, then
in what sense is it explanatory to say that the reason why one group of organisms
survived and another did not is because the first group was fitter? As a result
of this concern, some philosophers, most famously Popper [1963, later retracted],
biologists [Peters, 1976] and various creationists were led to think that this lack
of testability was a serious problem with evolutionary theory. Notice, however,
that even if somecomponentof evolutionary theory is a tautology, this does not
mean that evolutionary theoryas a wholeis “untestable” or “true by definition”.
There is much more to evolutionary theory than the definition of fitness: the mere