Evolution, 4th Edition

variation at those sites is eliminated. Polymorphism remains, however, at regions

farther away on the chromosome. This distinctive pattern is one of the telltale signs

used by evolutionary geneticists to find evidence of recent adaptation [33].

This pattern is exactly what we see in corn (maize; Zea mays mays) (FIGURE 5.16).

Starting a few thousand years ago, this crop was domesticated in Central America

from a wild plant called teosinte. Early farmers bred the teosinte plants that were

most productive and easiest to harvest, which generated strong selection on sev-

eral traits. Among them was the plant’s growth form. Teosinte is bushy, while corn

grows as a single tall stalk. This major difference is largely the result of a single

mutation that ancient farmers unconsciously selected and that is now fixed in corn

[43]. The location of the selective sweep is revealed by a region of chromosome that

has much lower heterozygosity in domesticated maize than in teosinte.

The pattern of polymorphism along the chromosomes we see in Figures 5.15

and 5.16 results when a mutation is favored by selection when it first appears. In

some situations, an allele that is present in the population is initially not favored,

but then suddenly becomes beneficial when conditions change. In this case, we

say selection is acting on standing genetic variation. Before the change, different

copies of the mutation will have had time to recombine onto chromosomes with

different combinations of alleles at other sites. As a result, when the selected allele

reaches fixation, only a very small region of the chromosome around the selected

site shows reduced polymorphism (FIGURE 5.17). Evolutionary geneticists use the

contrast between the patterns seen in Figures 5.15 and 5.17 to determine whether

or not selection acted on standing genetic variation in loci that have recently expe-

rienced adaptive evolution.

Many other methods are also used to find genes that recently experienced or

are under ongoing selection. A key to many of those methods is that they let us

distinguish the action of selection from random genetic drift. We will therefore put

off exploring those approaches to Chapter 7, where we discuss random factors and

how they interact with selection.

When Selection Preserves Variation

Chapter 4 introduced the β-hemoglobin locus. Most humans are homozygous for

the A allele. Populations in some regions of Africa and Asia also have apprecia-

ble frequencies of the S allele. Individuals who are SS homozygotes suffer from a

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.16.ai Date 12-28-2016

Exon 1

500 bp

Intron Exon 2

Teosinte

Corn

0

0.5

tb1 Locus

Heterozygosity

FIGURE 5.16 A selective sweep that occurred during the

domestication of corn. The ancestor of corn, which looked very

much like the living teosinte plant, was short and bushy. Domes-

tication in Central America roughly 10 thousand years ago se-

lected for corn plants that branched less. This favored a mutation

in a region of chromosome that regulates expression of the gene

tb1, which controls shoot branching. As the mutation spread,

heterozygosity nearby on the chromosome was drastically

reduced. Other evidence pinpoints the mutation in a noncoding

region about 60 kb upstream (to the left) of tb1 [38]. (After [43].)

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