NATuRAl SElECTioN ANd AdAPTATioN 61
the “entities” that differ in survival and reproduction in most discourse about evo-
lution are individual organisms with different phenotypes (individual selection).
Evolution by natural selection in sexually reproducing populations entails changes
in the frequencies of alleles at the locus (or loci) that underlies variation in the
phenotypic characteristic that influences fitness.
We will almost always discuss natural selection among genes and among heri-
table individual phenotypes because selection has no lasting evolutionary effect
without inheritance. Most of our discussion will assume that inheritance of a trait
is based on genes. However, many of the principles of evolution by natural selec-
tion also apply if inheritance is epigenetic (based on, for example, differences in
DNA methylation; see Chapter 4) or is based on cultural transmission, especially
from parents to offspring. Culture has been defined as “information capable of
affecting individuals’ behavior that they acquire from other members of their spe-
cies through teaching, imitation, and other forms of social transmission” [44].
We must be very careful to understand that natural selection is not an agent or
active power, and certainly not a purposeful one, even though the language we use
often seems to personify it, or suggest that it is an agent. Darwin coined the term
“natural selection” to parallel the selection that breeders of crops and domestic
animals use to improve desirable characteristics. In later editions of The Origin of
Species, he wrote that “it has been said that I speak of natural selection as an active
power or Deity; but who objects to an author speaking of the attraction of gravity
as ruling the movements of the planets? Every one knows what is meant and is
implied by such metaphorical expressions; and they are almost necessary for brev-
ity.” Likewise, evolutionary biologists often say that selection “favors” a certain
characteristic, or they refer to selection as a “force.” This is metaphorical language,
used for brevity. Natural selection is a name for statistical differences in reproductive
success among genes, organisms, or populations—and nothing more.Natural selection and chance
“Natural selection” is not synonymous with “evolution.” Natural selection can occur
without any evolutionary change, as when natural selection maintains the status
quo by eliminating deviants from the optimal phenotype. And processes other than
natural selection can cause evolution.
One of those processes is genetic drift: random fluctuations in the frequencies of
genotypes within a population. (Genetic drift is the subject of Chapter 7.) Neutral
alleles are those that do not alter fitness: the average reproductive success does not
differ between individuals that carry one neutral allele or the other. The frequen-
cies of these neutral alleles may change in a population by genetic drift. If this
occurs, the bearers of one allele have had a greater rate of increase than the bearers
of the other allele, but natural selection has not occurred, because the genotypes
do not differ consistently in fitness: the alternative allele could just as well have
been the one to increase. There is no average difference between the alleles, no bias
toward the increase of one relative to the other. Fitness differences, in contrast,
are average differences, biases, differences in the probability of reproductive suc-
cess. Natural selection is the antithesis of chance. In practice, we can ascribe genetic
changes to natural selection rather than random genetic drift only if we measure
numerous individuals of each genotype or phenotype, and find an average differ-
ence in reproductive success.The effective environment depends on the organism
The environmental factors that impose natural selection on a species are greatly
influenced by the characteristics of the species itself: the evolutionary history of a
species affects its relationship to the environment [31]. The branching structure of03_EVOL4E_CH03.indd 61 3/22/17 1:19 PM