Evolution, 4th Edition

270 CHAPTER 10

Inbreeding depression also results when close relatives mate. In some species,

inbreeding depression is severe. The fitness of offspring produced by matings

between close relatives in the white campion (Silene latifolia) is reduced by up to

80 percent compared with the offspring of unrelated individuals (FIGURE 10.29).

Inbreeding depression is also seen in humans. Tay-Sachs disease is a lethal degen-

erative condition. It is caused by a recessive mutation that is relatively common

in Ashkenazi Jewish populations, where more than one-quarter of people who

suffer from the disease are children of parents who are first cousins [47]. Inbreed-

ing depression is likely the reason that many societies have social taboos against

marriage between close relatives.

Some hermaphroditic species have little inbreeding depression, and they can

evolve to reproduce almost entirely by self-fertilization (see Figure 10.1C). But

almost all of them occasionally mate with other individuals—obligate selfing is

extremely rare. The reason, once again, is selective interference [25]. A popula-

tion that is entirely selfing suffers from clonal interference for exactly the same

reasons that parthenogenetic populations do. Occasional outcrossing allows alleles

to escape their genetic backgrounds, which accelerates adaptation and stops the

evolution of a complete dependence on reproduction by self-fertilization.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_10.29.ai Date 12-15-2016

Germination success

0 0.1 0.2

Level of inbreeding

0.3 0.4

0.4

0.2

0.6

0

1.0

0.8

0.3

0.1

0.5

0.9

0.7

FIGURE 10.29 Inbreeding depression reduces the fitness of offspring

produced by matings between relatives. Experimental crosses were

made with the white campion (Silene latifolia) using plants with differ-

ent degrees of relatedness. The x-axis shows relatedness, ranging from

unrelated (at left) to 3/8 related (which is more closely related than

first cousins, but less related than full siblings). The y-axis measures the

probability that the seed germinates, which is a critical fitness compo-

nent. The fitness of the most inbred individuals is reduced by more

than 80 percent compared with offspring produced by unrelated

parents. (After [42].)

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