Evolution__3rd_Edition

322 PART 3 / Adaptation and Natural Selection
Mean viability
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Mean
Variance
0 10 20 30 40
Generations of mutation accumulation
20
18
16
14
12
10
8
6
4
2
0
Genotypic variance (×10 3 )
Figure 12.5
The mutational meltdown in fruitflies protected from selection,
showing a decrease in viability. Viability is measured in flies that
are homozygous for a chromosome that has experimentally
accumulated mutations relative to flies that are heterozygous for
the same chromosome. The decline is due to the accumulated
deleterious mutations. There were 104 lines and the variance in
viability among lines increased through time (see Box 9.1, p. 233,
for the definition of variance). Redrawn, by permission of the
publisher, from Mukai et al. (1972).
Table 12.1
Mutation rates in various life forms. More complex life forms have more cell cycles per generation and more DNA. The mutation rate
per nucleotide is probably approxiamtely constant in all eukaryotes. The total number of errors, including harmful errors, increases
from bacteria to human beings. Two methods of estimating the fraction of all mutations that are deleterious give different results,
hence the columns labelled (1) and (2) for U. All numbers are approximate. After various sources; see Ridley (2001).
Number of
deleterious
mutations
Mutation rate Cell cycles per Total number
Creature per nucleotide DNA length generation of mutations 1 2
Bacteria 10 −9 –10 −10 10 6 1 1 1 1
Fruitfly 10 −9 –10 −10 3.6 × 10 8 20 4