mUTATIon AnD VARIATIon 91
into two. Fissions and fusions are responsible for changes in the number of chromo-
somes in the genome. Organisms show a bewildering range of chromosome num-
bers (FIGURE 4.14). A highly venomous ant from Australia called the jack jumper
has only a single chromosome [4]. At the opposite extreme, a fern (Ophioglossum
reticulatum) has 630 pairs of chromosomes [12], while a ciliate (Oxytricha trifallax)
has about 16,000 pairs of very small chromosomes [24]! This variation results from
different histories of fissions and fusions. We still understand little about how and
why these differences evolved. Surprisingly, changes in chromosome number and
structure often have no obvious phenotypic effects.
The final and most extreme type of mutation is whole genome duplication.
Occasionally meiosis produces a gamete that carries the entire diploid genome,
rather than a haploid with just one of each pair of chromosomes. If two of these
unreduced gametes meet and fertilize each other, an offspring is produced that has
four copies of each chromosome. This genetic result, which is called tetraploidy,
happens much more frequently in plants than animals. Later rounds of genome
duplication can lead to even more complicated complements of chromosomes. One
of the interesting consequences is that the offspring typically cannot interbreed
with their parental population. Thus whole genome duplication can produce a new
species with a single mutation, as we’ll discuss further in Chapter 9.Rates and Effects of mutations
Since there are so many kinds of mutations, it is not surprising that their rates and
effects vary tremendously [7, 22].Mutation rates
DNA replication is extremely accurate in eukaryotes and bacteria.
Each time an Escherichia coli cell divides, there is roughly a chance
of only 1 out of 2 × 1010 that a given DNA base in a daughter cell
will carry a new point mutation [15]. (Is there any human action
that is so precise?) The probability that an offspring carries a new
mutation is called the mutation rate, which is symbolized by μ. The
mutation rate at a single base in E. coli is therefore μ = 1 / (2 × 1010 ).
Mutation rates vary greatly among species (FIGURE 4.15).
Each time a person makes an egg or sperm, roughly 1 out of 10^8
of the DNA bases carries a new mutation (and so μ = 10–8 per
bp) [11]. The mutation rates in RNA viruses, like those respon-
sible for AIDS, Ebola, and influenza, are thousands of times
higher (μ = 10–3 to 10–5 per bp). These extremely high muta-
tion rates may result in part from natural selection that favors
rapid evolution of viruses to evade host defenses [22]. BOX 4A
describes how mutation rates are estimated.
The concept of a mutation rate applies not just to a single DNA
base but also to an entire gene. Per-locus mutation rates are
higher than they are for single bases, simply because the locus
carries a mutation if any of its many base pairs mutates. These
rates vary greatly, both between loci within a species and between
species. Mutation rates for protein-coding loci in eukaryotes such
as humans and flies are typically in the range of μ = 10–5 to 10–7.
We can also consider the mutation rate across the entire genome.
Although the human mutation rate per DNA base is very small
(μ = 10–8), our genomes have a large number of bases (3 × 109 ). A s
a result, every time we make a gamete, it carries roughly 30 new
mutations scattered throughout the genome [11].Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_04.15_v2.ai Date 11-07-2016 01-12-17ArchaeanMutation rate (per bp)107 108 109
Genome size (bp)BacteriaUnicellular
eukaryotesInvertebratesPlantsMammals10 –1010 –910 –810 –7FIGURE 4.15 Mutation rates vary greatly. Organisms with
larger genomes tend to have higher mutation rates per base
pair per generation. (That pattern does not hold for viruses,
however.) The points for Archaea and Bacteria represent
averages of many species, and there is considerable variation
within those groups. An intriguing hypothesis to explain the
correlation between genome size and mutation rate seen
here has been proposed by Michael Lynch [16].04_EVOL4E_CH04.indd 91 3/23/17 8:55 AM