Evolution, 4th Edition

96 CHAPTER 4

environment because natural selection picks out the favorable mutations and

causes them to spread. How that happens is the key topic of the next chapter.

n ongenetic Inheritance

The vast majority of inherited changes involve alterations of the DNA (or RNA)

sequence of a genome. In some organisms, however, other mechanisms also con-

tribute to inheritance and so can play a role in evolution [2].

Epigenetic inheritance is caused by inherited changes to chromosomes that

do not alter the DNA sequence [3]. Instead, these changes affect the organism by

altering how genes are expressed. Several mechanisms are involved. Two of the

four DNA bases (cytosine and adenine) can be methylated, which is a biochemical

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_04.19.ai Date 01-12-2017

Master plate

Replica plates

Virus added

Master plate

Replica plates

Virus added

(A)

(B)

Resistance

mutations occur

in response to virus.

Resistance mutations

occur at random

before virus is added.

FIGURE 4.19 The replica plate experiment shows that mutations

in E. coli for resistance to T1 viruses occur before exposure to

the virus, rather than being induced by that exposure [14]. The

experiment begins with a master agar plate that has numer-

ous colonies of E. coli that were all derived from a single cell.

Replica agar plates are made by first pressing a velvet-covered

disk against the master plate, then against sterilized plates. This

transfers some cells from each colony to each replica plate in

the same spatial configuration as they are on the master plate.

A culture of T1 viruses is then added to the replica plates.

(A) Under the hypothesis of directed mutation, exposure to

the virus induces the bacteria to generate resistance mutations.

This hypothesis predicts that those colonies that are successful

in generating these beneficial mutations will appear at differ-

ent places on the replica plates. (B) The alternative hypothesis is

that resistance mutations occur spontaneously before the virus

is added. In this case, when the virus is added, the colonies that

already have resistance mutations will appear in the same places

on the replica plates. The experiment confirms the second

hypothesis: mutations appear at random with respect to what the

environment favors.

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