216 ■ CHAPTER 12 Mechanisms of Evolution
EVOLUTION
and produce more offspring than do individuals
with other forms of that trait.
Directional selection is the most common
pattern of natural selection, in which individ-
uals at one extreme of an inherited phenotypic
trait have an advantage over other individuals
in the population. The peppered moth provides
a vivid example: Before 1959, the number
of dark-colored moths in both England and
the United States increased after industrial
pollution blackened the bark of trees, caus-
ing dark-colored moths to be harder for bird
predators to find than light-colored moths.
A reduction in air pollution following clean-
air legislation, enacted in 1956 in England
and in 1963 in the United States, caused the
bark of trees to become lighter, and suddenly
the reverse occurred: light-colored moths
became harder for predators to find than dark-
colored moths. As a result, the proportion
of dark-colored moths plummeted because
they were easily seen and eaten by predators
(Figure 12.7). Similarly, when methicillin
became widely used to fight staph, MRSA
evolved via directional selection: the bacteria
in hospital settings that were resistant to the
antibiotic survived, while those that were not
perished.
In cases of stabilizing selection, individuals
with intermediate values o f a n i n h e r i t e d p h e n o-
typic trait have an advantage over other individ-
uals in the population. Birth weight in humans
provides a classic example of this pattern of
natural selection (Figure 12.8). Historically,
light or heavy babies did not survive as well as
babies of average weight, and as a result there
was stabilizing selection for intermediate birth
weights. Today, however, this stabilizing trend
is not as strong, because advances in the care of
low-birthweight babies and an increase in the
use of cesarean deliveries for large babies have
allowed babies of all weights to thrive.
Finally, disruptive selection occurs when
individuals with either extreme of an inher-
ited trait have an advantage over individuals
with an intermediate phenotype. This type of
selection is the least commonly observed in
nature, but one example of a trait affected by
disruptive selection is the beak size within a
population of birds called African seed crack-
ers (Figure 12.9). During one dry season, birds
with large beaks survived on hard seeds andA population of Staphylococcus
aureus bacteria.Additional treatments with
methicillin do not reduce the MRSA
population.After treatment with vancomycin,
shown as a purple kitchen strainer,
the resistant bacteria (VRSA)
survive.The frequency of the VRSA
bacteria containing the resistance
allele increases dramatically. This is
evolution.After treatment with methicillin,
shown as a red kitchen strainer, the
resistant bacteria (MRSA) survive
and reproduce.Figure 12.6
Evolution happens
Imagine there is a population of Staphylococcus aureus bacteria living on your
skin. Most of them are susceptible to the antibiotic methicillin (red strainer).
A few, however, are randomly resistant to methicillin (red), like the ones
in Figure 12.2. A few of these bacteria, in turn, could also be resistant to
vancomycin (purple).
Q1: Why does the population of S. aureus bacteria not pose a life-or-
death health threat outright?Q2: Why do vancomycin-resistant bacteria have a higher frequency in
the population after treatment with vancomycin?Q3: If this figure used the mouse example of allele frequency
from Figure 12.5, and the white mice increased in numbers like the
vancomycin-resistant bacteria here did, what would happen to the
frequencies of the white-fur-pigment and black-fur-pigment alleles?