Biology Now, 2e

218 ■ CHAPTER 12 Mechanisms of Evolution

EVOLUTION

another example, sharks and dolphins are only

distantly related (sharks are fish, and dolphins

are marine mammals), yet they both evolved

for success as predators in the ocean and share

common characteristics, such as a streamlined

body (Figure 12.10). When species share char-

acteristics due to convergent evolution and not

because of modification by descent from a recent

common ancestor, those characteristics are

called analogous traits (instead of homologous

traits).

Enter Enterococcus

Intent on determining how VRSA was evolv-

ing after the first Michigan infection, Gilmore

began to track the appearance of the bug. He

and others wanted to know where the allele for

vancomycin resistance came from, and how

staph had acquired it 14 different times.

birds with small beaks survived on soft seeds,

but birds with intermediate-sized beaks fed

inefficiently on both types of seeds. As a result,

the birds with intermediate-sized beaks died,

and the birds with large and small beaks lived.

Therefore, natural selection favored both large-

beaked and small-beaked birds over birds with

intermediate beak sizes.

Any of these patterns of natural selection can

cause distantly related organisms to evolve simi-

lar structures because they survive and repro-

duce under similar environmental pressures.

This type of evolution, called convergent evolu-

tion, results in organisms that appear very much

alike despite vastly dissimilar genetics. Cacti

found in North American deserts and distantly

related plants (euphorbias) found in African

and Asian deserts offer an excellent example of

convergent evolution. These two types of desert

plants have very different genetics, but they look

similar and function in a similar manner. In

Babies that weighed 8 pounds at

birth survived at a higher rate

than did lighter or heavier babies.

0 1 2 3 4 5 6 7 8 9 10

Birth weight (lb)

99

10

20

30

40

50

60

70

80

90

95

98

Percentage of babies that survived to age 4 weeks

Figure 12.8

Stabilizing selection and human birth

weight from 1935 to 1946

This graph is based on data for 13,700 babies

born between 1935 and 1946 in a hospital in

London. In countries that can afford intensive

medical care for newborns, the strength of

stabilizing selection has been greatly reduced in

recent years. However, even with improved care of

low-birthweight babies and cesarean deliveries of

large babies, babies at the extremes of newborn

weight still survive at a lower rate than those

closer to the median weight.

Q1: Think of another example of stabilizing

selection in human biology. Has modern

technology or medicine changed its impact

on the resulting phenotypes?

Q2: How do you think a graph of birth

weight versus survival for a developing

country with little health care would

compare to the graph shown here?

Q3: How do you think a graph of birth

weight versus survival for an affluent area

of the United States today would compare

to the graph shown here?