Biology Now, 2e

214 ■ CHAPTER 12 Mechanisms of Evolution

EVOLUTION

who had come in contact with her, and then

waited anxiously to see whether the dangerous

microbe had spread.

Thankfully, they found that the vancomycin-

resistant bug had not yet spread. “We’re lucky.

aureus is a profound example of a species chang-

ing over time.

In the mid-1800s, two English biologists,

Charles Darwin and Alfred Russel Wallace,

studied the diversity of life and concluded that

species were not, as was generally thought at

that time, the unchanging result of separate

acts of creation. Instead, both men came to the

bold new conclusion that species “descend with

modification” from ancestor species; that is, new

species arise from previous species.

Descent with modification, we know today,

occurs not only in populations of large organisms

like whales and finches, but also in the tiniest

single-celled bacteria and viruses. We gener-

ally think of evolution happening over millions

of years, but some evolutionary changes, such

as adaptations for antibiotic resistance, take

place over very short time spans as particular

alleles spread rapidly through a population.

Recall from Chapter 7 that alleles are different

versions of the same gene (sequences of DNA)

produced by random mutation, and therefore

allele frequencies are the proportions of specific

alleles in a population (Figure 12.5). Evolution

corresponds to changes in the relative propor-

tions or frequencies of alleles in a population

over time.

When allele frequencies in a population

change, becoming more or less common, the

attributes or phenotypes of the population

change as well; that is, the population evolves.

As more and more staph containing the allele for

methicillin resistance survived and reproduced,

the whole population of staph evolved, becoming

new, more powerful bacteria. But how exactly

does this happen? Where do new alleles come

from, and how do the frequencies of alleles in a

population change?

Sievert and a host of other researchers and

doctors experienced the emergence of a new

allele firsthand. When the results of the foot

ulcer test came in positive for VRSA, Sievert’s

team immediately called the Centers for Disease

Control and Prevention (CDC), a government

agency that investigates disease outbreaks and

makes public health recommendations. The

Michigan Health Department and the CDC

converged on the dialysis center where VRSA

had been found. They pored over the medical

history of the patient, examined her wound, took

swabs from the nostrils and wounds of anyone

17/30 = 57%

13/30 = 43%

Because there are 15 mice, the

gene pool has 30 allele possibilities

(15 mice 3 2 alleles per mouse).

Of the 30 total alleles in this

population, 13 are white-fur-pigment

alleles, so the white-allele frequency

is 13/30 = 0.43, or 43%.

Figure 12.5

Allele frequencies are calculated as

percentages in a population

These mice have two white-fur-pigment alleles

and appear white, have two black-fur-pigment

alleles and appear gray, or have one black and

one white allele and appear gray. To calculate

the white-fur-pigment allele frequency in the

population, the number of white alleles is counted

and divided by the total number of alleles.

Q1: What would the white-fur-pigment

allele frequency be if three of the

homozygous black allele mice (having two

black alleles) were heterozygous (having

one white and one black allele) instead?

Q2: What would the white-fur-pigment

allele frequency be if all of the white mice

died and were therefore removed from the

population? Would the black-fur-pigment

allele frequency be affected? If so, how?

Q3: What would the white-fur-pigment

allele frequency be if all of the gray mice

died and were therefore removed from the

population?