42 CHAPTER 2 | Genetics and Evolution
protein making a new biological task possible. Without
the variation brought in through random mutation,
populations could not change over time in response to
changing environments.
For sexually reproducing species like humans, the only
mutations of any evolutionary consequence are those oc-
curring in sex cells, since these cells form future genera-
tions. Mutations may arise whenever copying mistakes are
made during cell division. This may involve a change in a
single base of a DNA sequence or, at the other extreme, re-
location of large segments of DNA, including entire chro-
mosomes. As you read this page, the DNA in each cell of
your body is being damaged.^6 Fortunately, DNA repair en-
zymes constantly scan for mistakes, slicing out damaged
segments and patching up gaps. These repair mechanisms
prevent diseases like cancer and ensure that we get a faith-
ful copy of our parental inheritance. Genes controlling
DNA repair therefore form a critical part of any species’
genetic makeup.
Because no species has perfect DNA repair, new mu-
tations arise continuously, so that all species continue to
evolve. Geneticists have calculated the rate at which vari-
ous types of mutant genes appear. In human populations,
they run from a low of about five mutations per million sex
cells formed, in the case of a gene abnormality that leads to
the absence of an iris in the eye, to a high of about a hun-
dred per million, in the case of a gene involved in a form
of muscular dystrophy. The average is about thirty mu-
tants per million. Environmental factors may increase the
rate at which mutations occur. These include certain dyes,large for a statistical average to express itself; no new vari-
ants will be introduced into the population’s gene pool;
and all individuals are equally successful at surviving and
reproducing. These four conditions are equivalent to the
absence of evolution. Geographic, physiologic, and so-
cial factors may favor mating between certain individuals
over others.
Thus changes in the gene pools of populations, with-
out which there could be no evolution, can and do take
place. The mechanisms by which these changes might lead
to the formation of new species will be discussed in detail
in Chapter 6.
Evolutionary Forces
Mutation
The ultimate source of evolutionary change is mutation
of genes because mutation constantly introduces new
variation. Although some mutations may be harmful or
beneficial to individuals, most mutations are neutral.
But in an evolutionary sense, random mutation is in-
herently positive, as it provides the ultimate source of
new genetic variation. New body plans—such as walk-
ing on two legs compared to knuckle-walking like our
closest relatives, chimpanzees and gorillas—ultimately
depended on genetic mutation. A random muta-
tion might create a new allele that creates a modified
In evolutionary terms, mutations serve
as the ultimate source of all new genetic
variation. A generally positive force, most
mutations have minimal effect or are
neutral. Nevertheless, human-produced
mutagens—such as pollutants, preserva-
tives, cigarette smoke, radiation, and
even some medicines—increasingly
threaten people in industrial societies.
While the negative effects of mutation
are evident in the clear link between
cigarette smoke and cancer, the positive
side of mutation has been fictionalized
in the special talents of the X-Men.
mutation Chance alteration of genetic material that produces
new variation.(^6) Culotta, E., & Koshland, D. E., Jr. (1994). DNA repair works its way to the
top. Science 266, 1926.
20th Century Fox/The Kobal Collection/Hayes, Kerry