Biology Now, 2e

Battling Resistance ■ 223

the human immune system. These proteins

could make it easier for staph and other bacte-

ria to grow in a wound because the host immune

system would be less able to fight them off.

The lack of the antibiotic genes and the pres-

ence of new genes create the ideal conditions for

a mixed infection, in which different species of

pathogens mingle in a festering soup of contam-

ination. Mixed infections are breeding grounds

for antibiotic resistance because they are sites

of gene flow among different organisms. In this

explanation, the CC5 staph evolved via the three

usual mechanisms—mutation, natural selec-

tion, and gene flow (via horizontal transfer)—to

be more susceptible to take up the vancomycin-

resistance allele from Enterococcus.

In addition to natural selection (and its

intriguing permutation called sexual selection;

see “Sex and Selection”), mutation, and gene flow,

evolved to effectively pick up the allele for vanco-

mycin resistance.

First, all the vancomycin-resistant CC5

staph bacteria have the same mutation in a

gene called DprA. DprA appears to be involved

in preventing horizontal gene transfer. A muta-

tion in this gene might make it easier for the

staph to take up DNA from other bacteria, such

as Enterococcus.

Second, the CC5 strains lack a set of genes

that encode an antibiotic that kills other bacte-

ria. Perhaps this antibiotic normally kills Entero-

coccus near the staph, which would explain why

horizontal transfer between the two species does

not typically occur.

Finally, Gilmore and his team found that in

place of that missing set of antibiotic genes, the

vancomycin-resistant CC5 staph have a unique

cluster of genes encoding proteins that confuse

S

taphylococcus and other bacteria replicate asexually

by copying their DNA and dividing in two. But adding

sex to the equation complicates reproduction. Another

mechanism by which species evolve is called sexual

selection. In sexual selection, nature selects a trait that

increases an individual’s chance of mating—even if that

trait decreases the individual’s chance of survival.

Sexual selection favors individuals that are good at

getting mates, and it often helps explain differences

between males and females in size, courtship behavior,

and other traits. Species whose males and females are

distinctly different in appearance, such as peacocks,

lions, and ducks, are said to exhibit sexual dimorphism. In

many species, the members of one sex—often females—

are choosy about whether to mate. In birds, for example,

brightly colored males may perform elaborate displays

in their attempts to woo a mate. In other species, males

may attract attention by other means, such as calling

vigorously. Females then select as their mates the males

with the loudest calls.

Yet, some characteristics that increase an individual’s

chance of mating can decrease its chance of survival. For

example, male túngara frogs perform a complex mating call

that may or may not end in one or more “chucks.” Females

prefer to mate with males that emit chucks, but frog-eating

bats use that same sound to help them locate their prey.

As a result, a frog’s attempt to locate a mate can end in

disaster.

Sex and Selection

A male peacock

performs an elaborate

mating display to a

potential mate.

Male túngara frogs face an

ecological trade-off: the same type

of call that is most successful at

attracting females for mating also

attracts predatory bats to dinner.