Evolution, 4th Edition

mUTATIon AnD VARIATIon 97

change to their structure (FIGURE 4.20). Likewise, the histones that bind to DNA

to form eukaryotic chromosomes can be biochemically modified. Both kinds of

changes alter how genes are expressed, and both can be transmitted across gen-

erations. Most epigenetic changes are not stable and dissipate after a few genera-

tions. Epigenetic inheritance can be important in the short term, but it does not

make major contributions to long-term evolutionary change.

Genes are not the only way that mothers affect their offspring. Maternal effects

occur when the genotype or phenotype of the mother directly influences the phe-

notype of her offspring. The direction that a snail shell coils is determined by the

genotype of the snail’s mother rather than the individual’s own genotype (FIGURE

4.21). In mammals, including humans, the amount of milk that an infant receives

from its mother has important effects on development. In turn, the amount of milk

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Standard Linaria vulgaris peloria mutant

FIGURE 4.20 The peloria mutant of toadflax (Linaria

vulgaris) results from an epigenetic mutation. One of

the five petals normally has a long nectar-bearing spur,

but in the peloria mutant all five petals have this form,

transforming the flower’s symmetry from bilateral to

radial. The mutation can result from either a change in the

DNA’s sequence or its methylation pattern. (Courtesy of

R. Grant-Downton.)

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Evolution4e_04.21.ai Date 11-03-2016

L R

dd DD

L

Dd

L

Dd

(A)

R L

R

DD dd

Dd

R

Dd

(B)

FIGURE 4.21 The direction of coiling in the snail Lymnaea peregra is determined by

the genotype of an individual’s mother, not its own genotype [23]. The shell can coil

either to the left (L) or to the right (R). The locus that affects coiling has two alleles, D

and d. (A) All offspring of a dd female with an L shell also have L shells. (B) All offspring

of a DD female with an R shell also have R shells. Although the offspring genotypes in

the two panels are the same, their shell phenotypes differ, and are determined by their

mother’s genotype. This is an example of a maternal effect.

04_EVOL4E_CH04.indd 97 3/23/17 8:55 AM