166 CHAPTER 7with natural selection. The chapter then discusses how molecular differences
among species accumulate in time. It closes with a look at one of the most exciting
subjects of current research in biology: the use of DNA sequences to find evidence
for adaptive evolution in humans and other species.What Is Random genetic drift?
Genetic change between generations—that is, evolution—happens even when
selection is not at work. We have already seen that mutation changes allele fre-
quencies. Evolution also results from chance events of survival, reproduction, and
inheritance. The evolutionary process that results is called genetic drift.
The grove snail (Cepaea nemoralis) is famous for its highly variable shells (see
Fig u re 5.14). This is a terrestrial species that lives in pastures, which it often shares
with cows and sheep. No doubt thousands of snails die each day when they are
stepped on by livestock that are oblivious to what is beneath their hooves. In any
given field on any given day, some unlucky colors of snails will happen to get
crushed more often than other colors. Variation in color is determined by a small
set of loci, and so these random deaths cause changes in the allele frequencies in
that population. This scenario shows that an individual’s genotype and phenotype
are not the only factors that determine if he or she leaves genes to the next genera-
tion. Chance plays a role too.
Another opportunity for chance to influence the fate of genes comes during
meiosis. When an individual is heterozygous at a locus, only one of the two alleles
is passed to each gamete that it makes. If you have two children, there is a chance
of 1/2 that they will both inherit the same allele from you, and your other allele will
leave no descendants in the next generation. So even if all individuals survive and
leave the same number of offspring, meiosis itself causes random changes allele
frequencies.
We can discover several key features of genetic drift with an experiment. Rather
than using living organisms, the experiment simulates evolution on a computer.
The advantage of this strategy is that it can perfectly control the conditions. In
particular, the simulation is designed so that all individuals have exactly the same
chance of surviving and leaving offspring. It simulates a diploid hermaphroditic
species, and it follows the changes in the frequency of two alleles at a locus that
evolves according these rules:FIGURE 7.1 Male elephant seals
fighting for control of a harem of
females. A small number of males
obtain the large majority of matings,
contributing to the intensity of ran-
dom genetic drift.07_EVOL4E_CH07.indd 166 3/23/17 9:09 AM