Table 20-3: The Dependence of the Fitness Effects of Mutations on Prior Mutations in E.    coli

 Mutation*

Number of alleles on which mean mutational effect is

Mean proportional increase

Positive

Negative

Negligible

g4205a

8

2

6

1.4

A42G

12

0

4

5.9

E104K

15

1

0

9.7

M182T

8

3

5

2.8

G238S

16

0

0

1.0 × 103

*The mutations leading to antibiotic resistance are designated by their nucleotide or amino acid position. Of the 16 possible allelic combinations of the four other sites, the positive, negative, or neutral effects of the mutation are indicated along with the mean proportional increase in fitness for mutations at the indicated site.