Evolution, 4th Edition

mUTATIon AnD VARIATIon 89

Point mutations

The simplest type of mutations are point mutations, which occur when a single

DNA base is changed from one to another of its four possible states (A, G, C, or T).

We have seen that changes to some codons are synonymous (they do not alter an

amino acid in a protein) while others are nonsynonymous (they do alter an amino

acid). A nonsynonymous mutation in the β-hemoglobin gene results in the S allele

that appeared earlier in the discussion of the Hardy-Weinberg equilibrium (see

Figure 4.3). When homozygous, this single microscopic change to the genome has

sweeping effects on development and physiology, causing a medical condition called

sickle-cell anemia.

As with the S allele in the β-hemoglobin gene, any change to the second position

of a codon is nonsynonymous. Most (but not all) changes to the first position are

also nonsynonymous. In contrast, most changes to the third base of a codon are

synonymous. (You can verify this from Figure 4.2.) We will see in C hapter 14 that

as a result, the three positions of codons evolve quite differently from one another.

Much of the eukaryotic genome and some of the prokaryotic genome does not

code for any gene product. Point mutations in some noncoding DNA can, however,

affect an organism by altering how genes are expressed (see Chapter 15). When a

species evolves by natural selection, the genetic changes can involve coding DNA,

noncoding DNA, or both.

Structural mutations

Some kinds of mutations affect more than one DNA base. These are structural

mutations, which can be as small as a few bases or as large as billions of bases.

Most happen as errors when chromosomes are replicated. Different kinds of struc-

tural mutations are illustrated in FIGURE 4.13.

Deletions are a common type of structural mutation that occurs when a seg-

ment of a chromosome is left out during replication. A deletion of only three base

pairs in a sodium channel gene causes cystic fibrosis, which is one of the most

common human genetic disorders in Europe and North America. Other deletions

are much larger. The first genome-wide survey of deletions in humans made the

surprising discovery that individuals typically have about a dozen deletions that

average 465,000 bp in size [21]. While some of those deletions are in noncod-

ing regions, many of them eliminate several genes. Most (but not all) deletions

that knock out genes are harmful. Insertions are the opposite situation, in which

a segment of DNA is added to a chromosome, either from nearby on the same

chromosome or elsewhere in the genome. Some insertions cause genetic disease.

Huntington’s disease is a neurological disorder whose most famous victim was the

American folk singer Woody Guthrie. The disease is caused by multiple insertions

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_04.13.ai Date 11-04-2016

Deletion Duplication Inversion Fission Fusion

FIGURE 4.13 Five types of structural mu-

tations that alter chromosomes.

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