30 EVOLUTION BY NATURAL SELECTION
Dobzhansky put forward the idea
that regularly occurring genetic
mutations are sufficient to provide
the genetic diversity—and
therefore different traits—that
makes natural selection possible.
He wrote that evolution was a
change in the frequency of an
“allele” in the gene pool, an allele
being one of the alternative forms of
a gene that arise by mutation.
A mutation is a permanent
alteration in the sequence of
deoxyribonucleic acid (DNA), the
molecule that makes up a gene
in one individual, resulting in
a sequence that differs from that
of other members of the species.
Mutations may occur as the result
of the miscopying of DNA during
cell division, or they may be caused
by environmental factors, such as
damage resulting from the sun’s
ultraviolet radiation. One mutation
might affect only the individualorganism carrying it, whereas
another might affect all its offspring
and future generations.
Inherited mutations may or may
not alter an individual’s phenotype –
its physical traits and behavior. If
mutations do affect the phenotype,
they may be to its advantage or
disadvantage, helping or hindering
an organism’s ability to survive
and reproduce successfully. If they
hinder, they are likely to disappear
from the population; if they help
an organism adapt better to itsenvironment, they become more
common over the course of
generations. Over time, they may
produce large enough divergences
from the parent population for a
new species to evolve—a process
called speciation.
Mutation rates are usually very
low, but the process is ever-present.
The changes may be beneficial,
neutral, or harmful. They do not
occur in response to an organism’s
needs, and are, in that respect,
random. However, some types of
mutations occur more frequently
than others. Scientists now know,
for example, that evolution can take
place very rapidly in bacteria
because of their frequent mutations.Different rates of evolution
The ancestors of all life on Earth
were very simple organisms.
Recent scientific research suggest
that the earliest “biogenic” rocks—
derived from early life forms—date
back nearly four billion years. In
that time, highly complex life forms
have evolved, and later fossils
of species that look more similar
to those of today reveal what hasAlbinism, as in this albino leopard
gecko, is a mutation causing a lack
of pigment. This mutation hinders the
gecko’s chances of survival, making it
lighter colored and sensitive to light.The vast majority of large
mutations are deleterious;
small mutations are both far
more frequent and more likely
to be useful.
Ronald FisherUS_024-031_Evolution_by_Natural_Selection.indd 30 12/11/18 6:24 PM