Evolution__3rd_Edition

32 PART 1 / Introduction
. . . per genome ...
. . . and per gene
Table 2.2
Mutation rates in various life forms. Genome sizes are diploid figures. The “worm” is
Caenorhabditis elegans. RNA viruses do not literally have cell cycles, but the number in the
column refers to the number of times the RNA is replicated per generation. All numbers are
approximate. In the cases of RNA viruses and bacteria, there are many species with a range of
genome sizes. After various sources, see Ridley (2001).
Mutation rate Total Mutation rate
per nucleotide genome size Cell cycles per genome
Life form per replication (nucleotides) per generation per generation
RNA virus 10−4 ≈ 104 1 ≈ 1
Bacteria ≈ 2 × 106 1 ≈ 10−3 Worm
10−9 to 10−10 2 × 108 Fruitfly 3.6 × 10
10 ≈ 2
8 Human being 6.6 × 10
20 ≈ 4
9 5
i
6
i
7
200 ≈ 200
magnitude higher in bacteria than in humans (Ochman et al. 1999), but the measurements
are uncertain. The trends, if any, in the mutation rate per nucleotide copying
event within cellular life forms are unknown.
The mutation rate per genome varies between bacteria and human beings despite the
approximate constancy of the rate per nucleotide copying event because of the effects
of generation time and because we have larger genomes. In humans, for instance, the
number of cell divisions per generation in a man (from his conception to his sperm
cells, when he is an adult) goes up with age by about 23 divisions per year. The sperm of
a 20-year-old many have about 200 cell divisions behind them, the sperm of a 30-yearold
man have about 430 cell division behind them. The number of cell divisions per
generation in a woman (from her conception to her egg cells) are constant independent
of age, at about 33 cell divisions. The average number of cell divisions in a human generation
is therefore about 100–200 or more, depending on the father’s age.
Mutation rates are also sometimes expressed per gene per generation. The rate
will depend on the size of the gene and the generation length of the organism. But
with mutation rates per nucleotide copying event of 10 −9 to 10 −10 , generations ranging
from 1 to 100 cell divisions (10 0 –10 2 ), and genes ranging from 10 3 to 10 4 nucleotides,
mutation rates per gene per generation are going to range around 10 −3 to 10 −7 .
A classic memorable figure for the mutation rate per gene per generation is one in a
million (10 −6 ).
Mutation rates per year can also be useful, particularly when using the molecular
clock to date evolutionary events a which is a big theme in modern evolutionary biology
(Chapters 7 and 15 and much of Part 5 of this book). Mutation rates per nucleotide
per cell cycle can be translated into rates per year. The translation depends on the species,
particularly because species differ in generation times, as discussed in Chapter 7.
In later chapters we shall use figures per year for particular species rather than the more
general figures such as in Table 2.2.
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