Philosophy of Biology

NATURAL SELECTION

Christopher Stephens

Charles Darwin introduced the term “natural selection” in chapter IV of his
bookOn the Origin of Species[1859]

[C]an we doubt (remembering that many more individuals are born

than can possibly survive) that individuals having any advantage, how-

ever slight, over others, would have the best chance of surviving and

of procreating their kind? On the other hand, we may feel sure that

any variation in the least degree injurious would be rigidly destroyed.

This preservation of favourable variations and the rejection of injurious

variations, I call Natural Selection. (p. 80–81)

Before Darwin, the term “selection” was used by plant and animal breeders to
indicate a conscious choice to breed organisms with desired characteristics. Darwin
used artificial selection as an analogy to understand natural selection, where nature
does the picking and choosing.
In contemporary evolutionary theory, “natural selection” is usually defined as
occurring if and only if there is heritable variation in fitness [Lewontin, 1970].
For instance, if giraffes with taller necks are more likely to survive than those with
shorter necks because they can reach more food sources, then the giraffes with taller
necks arefitter. If this fitness variation is transmitted to the calves so that giraffes
with taller necks are more likely to have taller offspring and shorter giraffes more
likely to have shorter offspring, then the trait is said to beheritable. If heritable
variation in fitness exists, then we should expect,ceteris absentibus(all other forces
being absent [Joseph, 1980]), fitter traits to increase in frequency and less fit traits
to decline. The reason for theceteris absentibuscondition is that processes other
than natural selection such as random genetic drift, recombination, mutation or
migration can also affect evolutionary change. Consequently, heritable variation in
fitness (natural selection) is neither necessary nor sufficient for evolution [Endler,
1986].
Following Darwin, biologists often use the term “sexual selection” to refer to
selection that is a result of differential mating success, and reserve “natural se-
lection” to refer to other components of fitness, such as viability. This is because
some traits enhance fertility but impair viability. The peacock’s tail, for instance,
is favored by sexual selection because females prefer males with large colorful tails.
At the same time, a large colorful tail makes the peacock more likely to be a meal
for predators. When natural selection is defined as “heritable variation of fitness”
(as above), sexual selection is just a special kind of natural selection.

General editors: Dov M. Gabbay,
©c2007 Elsevier B.V. All rights reserved.

Handbook of the Philosophy of Science. Philosophy of Biology

Volume editors:

Paul Thagard and John Woods

Mohan Matthen and Christopher Stephens