Biology Now, 2e

Battling Resistance ■ 219

only means by which new alleles are generated.

DNA mutations create new alleles at random,

thereby providing the raw material for evolution.

In this sense, all evolutionary change depends

ultimately on mutation. Mutations can stimulate

the rapid evolution of populations by providing

new genetic variation—differences in genotypes

between individuals within a population. Then,

natural selection and other mechanisms of

evolution act on the resulting phenotypes.

In sexually reproducing species, genetic

mutations that occur in an organism’s germ line

cells—the cell lineage that produces gametes

such as eggs and sperm—can contribute to

evolution. Mutations in other cells of the body,

such as skin or blood cells, can affect the indi-

vidual by causing cancer or other problems, but

those mutations are not passed to that individ-

ual’s offspring. If mutations are not passed to

offspring, they cannot contribute to evolution.

As Gilmore searched for patterns among

the cases of VRSA infection, he noticed some

peculiar things. First, most of the infections

were turning up in people with diabetes, typi-

cally in bad foot wounds. Second, in most cases,

when scientists looked closely at the samples,

they saw not only staph but also a small spher-

ical bacterium called Enterococcus, which had

evolved resistance to vancomycin years earlier

(Figure 12.11). Upon close observation, Gilmore

and others noted that the enterococci, grow-

ing cozily side by side with staph, contained a

vancomycin-resistance gene identical to the one

in the staph. The presence of this gene suggested

that staph had acquired vancomycin resistance

directly from Enterococcus, rather than via

random mutations in its own genome.

New alleles in a species emerge via mutation.

A mutation is a change in the sequence of any

segment of DNA in an organism, and it is the

Small-beaked birds fed

efficiently on soft seeds.

Large-beaked birds

could crack hard seeds.

40

60

50

30

20

10

10 12 14 16 18

0

Width of lower beak (mm)

Number of birds

No birds with beaks

of intermediate size

survived because

they fed inefficiently

on both hard and

soft seeds.

Blue bars indicate the

young birds that died.

Some small- and large-beaked young

birds (red bars) survived the dry season.

Figure 12.9

Disruptive selection for beak size

Among a group of young African seed crackers

hatched in one year, only those with small or

large beaks survived the dry season, when seeds

were scarce. Although many of the small- and

large-beaked birds did not survive, none of the

intermediate-beaked birds survived.

Q1: Almost all birds starved during the

dry season depicted here. What type of

selection would have been present if only

the intermediate-beaked birds had survived

(instead of the small- and large-beaked

birds)?

Q2: Describe a scenario in which African

seed crackers would experience directional

selection for either smaller- or larger-

beaked birds. What kind of environmental

conditions might bring about such a

situation?

Q3: Of the three patterns of natural

selection presented in this discussion,

which one always results in two different

phenotypes in the following generations?