Evolution__3rd_Edition

..
Mutation rates are problematic to
measure ...
. . . and can be expressed per
nucleotide ...
Mutations can also delete, or duplicate, a whole chromosome. Mutations on the
largest scale can duplicate all the chromosomes in the genome. The duplication of the
whole genome is called polyploidy. For example, suppose that the members of a normal
diploid species have 20 chromosomes (10 from each parent). If all 20 are duplicated
in a mutation, the offspring has 40 chromosomes. Polyploidy has been an important
process in evolution, particularly plant evolution (Chapters 3, 14, and 19).
That concludes our review of the main types of mutation. It is not a complete list of
all known mutation mechanisms, but is enough for an understanding of the evolutionary
events described later in this book.
2.6 Rates of mutation can be measured
CHAPTER 2 / Molecular and Mendelian Genetics 31
What is the rate of mutation? The mutation rate can be estimated from the rate at
which detectable new genetic variants arise in laboratory populations. Novel genetic
variants used to be detectable only if they influenced the organism’s visible phenotype.
Now we also have rapid DNA sequencing methods, and these can be used to detect
nucleotide differences between parent and offspring.
The measuring conditions must be such as to minimize, and ideally to eliminate, the
action of natural selection. The reason is as follows. Mutations may be advantageous (if
they increase the survival of the mutant bearer), neutral (if they have no effect on the
survival of the mutant bearer), or disadvantageous (if they decrease the survival of the
mutant bearer). In natural conditions, many mutations are disadvantageous and their
bearers die before the mutation can be detected. The mutation rate will then be underestimated.
Thus the measuring conditions need to be such as to neutralize the damage
done by disadvantageous mutations. Then all mutant bearers will survive and the
underlying mutation rate can be detected. Natural selection usually cannot be completely
neutralized, however, and the estimates that we have for mutation rates are only
approximate.
Mutation rates can be expressed per nucleotide, or per gene, or per genome. Also,
they can be expressed per molecular replication, or per organismic generation, or per
year. Table 2.2 gives some numbers. The mutation rate per nucleotide per molecular
replication is the most basic number, and it depends on the hereditary molecule and
the enzymatic machinery used by the organism. RNA viruses such as HIV use RNA as
their hereditary molecule; they have a replicase enzyme (called reverse transcriptase)
but lack proof-reading and repair enzymes. RNA viruses have a relatively high mutation
rate, of about 10 −4 per nucleotide. All cellular life forms, including bacteria and
human beings, have a similar set of proof-reading and repair enzymes, and use DNA as
their hereditary molecule. DNA is less mutable than RNA, partly because DNA is a
double-stranded molecule, and the proof-reading and repair enzymes further reduce
the mutation rate. Bacteria, and humans, have a mutation rate of about 10 −9 to 10 −10 per
nucleotide per molecular replication (or per cell cycle, in these cellular life forms). The
mutation rate per nucleotide per cell cycle seems to be approximately constant in
cellular life forms, at least relative to the much higher figure in RNA viruses, but it may
not be exactly constant. Some evidence suggests that the mutation rate is an order of