Evolution, 4th Edition

mUTATIon AnD VARIATIon 93

A second general feature of mutations involves their effects on an organism’s

fitness, that is, the number of offspring it leaves to the next generation. While

many mutations have no detectable effect on survival or reproduction, most of

those that do are deleterious (that is, harmful to survival or reproduction). On

average, each human gamete carries one new deleterious mutation in addition

to many that were inherited from the previous generation [11]. (We will see in

Chapter 5 that this has important implications for health.) Much less often, muta-

tions are beneficial, meaning that they increase fitness (FIGURE 4.17). It is easy

to understand why this is so by thinking about an enzyme that catalyzes a bio-

chemical reaction. Enzymes are remarkable molecules that are able to bind to a

very specific substrate and trigger a precise biochemical reaction. Most changes

to an enzyme’s amino acid sequence alter its chemical properties, destroying the

enzyme’s ability to perform its function. In humans, mutations that change a pro-

tein typically have deleterious effects that decrease fitness by less than 1 percent,

but occasionally their effects are more severe [8]. Most deleterious mutations are

partly recessive, meaning that their negative effects are more than twice as harm-

ful when they are homozygous as when heterozygous.

Because most mutations are deleterious, natural selection favors lower muta-

tion rates (at least in organisms with sexual reproduction, like almost all eukary-

otes). Several factors apparently prevent mutation rates from evolving to even

lower levels than they already are [17]. So it is a strange and wonderful fact that life

itself, which is the glorious product of natural selection, only exists because natural

FIGURE 4.16 The actor Peter Dinklage (shown here in Game of

Thrones) has achondroplasia. This condition is caused by a mutation

in the gene for receptor protein FGFR3, which interferes with bone

formation during development. All physical dimensions of the body

are affected, particularly the long bones of the arms and legs. This

mutation is a dramatic example of pleiotropy.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_04.17.ai Date 11-07-2016 01-12-17

Q: Please conrm accuracy of content in part (A). Msp was pretty blurry and I couldn’t tell what was colored what.

I pulled the 3rd color key label from page 2 of the source pdf.

Frequency

–1 –0.1 –0.01 –0.001 –0.0001 0

Selection coefcient

0.1

0

0.2

0.3

0.4

(B)

Relative frequency

< –0.15 –0.15 –0.10 –0.05 0

Selection coefcient

(A)

Stop codons

Synonymous mutations

Nonsynonymous mutations

FIGURE 4.17 The effects of new mutations on survival and

reproduction. (A) The effects of 560 mutations in yeast (Saccharo-

myces cerevisiae). A “selection coefficient” of –0.1, for example,

means that the mutation decreases survival by 10%. These muta-

tions all lie within a small segment of the gene for the heat shock

protein Hsp90. The large majority of mutations have negative

effects, indicating they are deleterious. Mutations that generate a

stop codon destroy the protein’s function and are highly delete-

rious. Synonymous mutations have almost no effect on fitness.

Very few mutations are beneficial (with positive selection coef-

ficients). (B) Selection coefficients for nonsynonymous mutations

in humans. Again, mutations that are more deleterious are farther

to the left. Roughly 12 percent have large deleterious effects that

decrease fitness by 10 percent (that is, a selection coefficient of

–0.1) or more. The method used in this study cannot detect ben-

eficial mutations. (A after [1b]; B after [7].)

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