Biology Now, 2e

224 ■ CHAPTER 12 Mechanisms of Evolution

EVOLUTION

population. Just by chance, some individuals

in a generation may leave behind more descen-

dants than other individuals do. In this case, the

genes of the next generation will be the genes

of the “lucky” individuals, not necessarily the

“better” individuals. Genetic drift occurs in all

populations, including mammals and bacteria,

but it is more likely to cause evolution in a small

population than in a large one.

Two ways in which strong genetic drift

may occur include genetic bottlenecks and

there is one other mechanism by which organ-

isms evolve. Although alleles that code for bene-

ficial traits like antibiotic resistance for bacteria

are usually selected for and maintained in a

population by the nonrandom action of natural

selection, in some cases, chance events can cause

allele frequencies to change from a parent gener-

ation to the next generation.

Genetic drift is a change in allele frequen-

cies caused by random differences in survival

and reproduction among the individuals in a

Time

In this small

population of

upland burrowing

crayfish, several

more blue crayfish

than orange or

green crayfish

survive after a

random catastrophe

kills off most of the

population.

In this small

population of

upland burrowing

crayfish, similar

numbers of blue,

orange, and green

crayfish survive

under normal

environmental

conditions.

After generations of

reproduction, the

rebounded

population contains

a higher frequency

of blue crayfish

than orange or

green crayfish.

After generations of

reproduction, the

population contains

frequencies of blue,

orange, and green

crayfish similar to

those of the original

population.

Figure 12.14

A genetic bottleneck is a type of genetic drift

Two small populations of upland burrowing crayfish are compared. The one on the right experiences

a genetic bottleneck event. This is genetic drift—a change in the frequency of a trait that is not

associated with natural selection. In fact, the blue crayfish could be less well adapted to the

environment than the other crayfish are.

Q1: Why do you think a genetic bottleneck is more likely to occur in a small population than in a

large population?

Q2: Genetic drift is often described as a “chance event.” Give other examples of chance events

that could cause a genetic bottleneck.

Q3: Which resulting population has more genetic diversity?