Biology Now, 2e

222 ■ CHAPTER 12 Mechanisms of Evolution

EVOLUTION

cluster 5 strains that was leading to this vanco-

mycin resistance bubbling up in these strains,”

says Gilmore. Compared to other strains of staph,

CC5 strains appeared to have evolved the ability

to readily take up and use vancomycin-resistance

alleles from Enterococcus.

Primed for Pickup

In 2012, Gilmore and his team at Harvard

analyzed the DNA of 11 of the 12 known cases

of VRSA at the time (samples from one of the 12

weren’t available, and the thirteenth and four-

teenth cases had yet to occur). In the genome of

every sample of VRSA, they found three traits

that demonstrate how the CC5 strains of staph

become more alike. A mutual exchange of alleles

through gene flow can counteract the effects of

the other mechanisms, such as mutation, that

tend to make populations more different from

one another.

Gene flow appeared responsible for the

emergence of VRSA; staph picked up an allele

for vancomycin resistance from Enterococcus.

But Gilmore made another observation that

suggested why this horizontal gene trans-

fer was happening now, and not back in the

1980s when vancomycin had f irst begun to be

widely used.

Gilmore noted that all the VRSA samples

taken from patients were clonal cluster 5 (CC5)

strains of S. aureus. “There were implications

that there was something special about clonal

Population 1 Population 2

Population 1 is a large

population containing

birds of genotypes

AA, Aa, and aa.

Population 2 is located

far from population 1

and initially has only

birds of genotype AA.

A bird of genotype aa migrates

from population 1 to population 2,

thereby introducing the a allele to

population 2.

aa

aa

AA

AA

Aa

Aa

aa

aa

AA

AA

AA

Figure 12.13

Migrants can move alleles from one population to another

As a result of gene flow between these two genetically disparate populations of geese, the populations will eventually become

more similar to each other. Mating between a migrant aa genotype goose and a population 2 goose of AA genotype will result in Aa

genotype offspring. Continued mating of the Aa genotypes will result in all three possible genotypes (AA, Aa, and aa). The resulting

population 2 gene pool will look much more similar to the gene pool of population 1.

Q1: If a goose with genotype AA had migrated instead of the goose with genotype aa, would the scenario described here

still be considered gene flow? Why or why not?

Q2: If a goose with genotype Aa had migrated instead of the goose with genotype aa, would the scenario still be

considered gene flow? Why or why not?

Q3: If the goose with genotype aa had migrated to population 2 as shown but had failed to mate with any of the AA

individuals, would the scenario still be considered gene flow? Why or why not?