Evolution, 4th Edition

THE GENETiCAl THEoRy of NATuRAl SElECTioN 125

Selection That favors the Most Common

Balancing selection preserves genetic variation, and in most cases the population

will evolve to the same allele frequency no matter where it begins. A different pic-

ture emerges from two kinds of selection that favor whichever allele is most com-

mon. Here variation is eliminated, and which allele spreads to fixation depends

on the initial allele frequency in the population. These next two kinds of selection

therefore lead to historical contingency: the outcome of evolution is determined by

where the population begins.

Underdominance: When heterozygotes suffer

The familiar sunflower is just one of about 70 species of sunflowers found across

North America. They differ in many ways, for example in their growth forms and

the habitats where they grow. Their chromosomes also show differences: closely

related species differ in the number of chromosomes they have and in the order of

genes along the chromosomes (FIGURE 5.23). These differences are the result of

chromosomal inversions and translocations (discussed in Chapter 4) that became

fixed during the evolution of these species.

Did these chromosomal changes become established because they increased fit-

ness and spread as advantageous mutations? Surprisingly, many of them decreased

fitness when they first appeared. Sunflower species can be hybridized, which

allows measurement of the fitness of individuals with different combinations of

chromosomes. Heterozygotes for some chromosome rearrangements have lower

fertility than either homozygote because their chromosomes fail to pair correctly

during meiosis, leading to infertility [25]. When a new rearrangement is still at

low frequency, almost all of its copies are in these low fitness heterozygotes. Thus

selection acts to eliminate a new chromosome rearrangement when it is still rare.

The situation in which heterozygotes have lowest fitness is called underdomi-

nance. As its name implies, underdominance is the opposite of overdominance:

it eliminates rather than preserves genetic variation. The evolutionary outcome

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.23.ai Date 12-28-2016 01-06-17

Recommend silhouetting the sunower photo with a zoom arrow to two chromosomes.

I spelled out chromosome. Plenty of space to do that.

Chromosome 12 Chromosome 16

H. argophyllus

H. argophyllus

1 2 3 4

H. annua

FIGURE 5.23 Comparing the chromosomes of sunflower species

reveals chromosome translocations. After Helianthus argophyllus

speciated from H. annua, chromosomes 12 and 16 exchanged pieces

(translocations). Arrows show how the pieces were rearranged. The

green and blue lines join homologous genes on the chromosomes

of H. annua (above) and H. agrophyllus (below). The left-hand piece

of chromosome 12 was joined with the left-hand end of chromosome

16 (segments 1 and 2 on the H. argophyllus chromosome). The piece

from chromosome 16 also flipped end to end (segment 2). The right-

hand piece of chromosome 12 was joined with the right-hand piece

of chromosome 16 (segments 3 and 4). Experimental crosses show that

heterozygotes for these translocations have low fitness. (After [3].)

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