Evolution, 4th Edition

88 CHAPTER 4

locus to evolve if the two loci are in linkage disequilibrium. A second reason that

linkage disequilibrium is important is that it allows us to find genes that affect

traits of interest. We will see in Chapter 6 how this idea is used to study the genetic

basis of traits as diverse as the size of a tomato and adaptation to high elevation in

humans.

Gene mixing with asexual inheritance

Almost all eukaryotes reproduce sexually. A small fraction of eukaryotes, all pro-

karyotes, and all viruses reproduce asexually (without meiosis). But even these

organisms have mechanisms that mix their genes. While the mechanisms are very

different, their evolutionary consequences are largely the same.

Horizontal gene transfer (HGT) is the movement of DNA between different indi-

viduals without help from sexual reproduction. It is particularly common in pro-

karyotes (see Chapter 2), and several mechanisms are involved [5, 19, 20]. HGT can

move genes between individuals of the same species and sometimes even between

different species. Some bacteria exchange DNA by conjugation, in which two cells

exchange DNA while in direct contact. Other bacteria simply take up naked DNA

that was left behind by bacteria that died. When a virus replicates inside a bacte-

rium, its offspring viruses sometimes integrate a piece of the host’s genome into

their own. When those viruses then infect another bacterium (or less commonly, a

eukaryote), the virus can insert the gene from the previous host into the DNA of the

new host. As we saw in Chapter 2, HGT is important to the evolution of antibiotic

resistance in bacteria that are responsible for important human diseases.

mutation: The Ultimate Source of Variation

The replication of DNA is an exquisitely precise affair, but it is not perfect—errors

are made. These mutations are the ultimate source of genetic variation in all

organisms. Without these errors, there would be no variation, no evolution, and

no life. Fundamentally, mutation can be thought of as an inevitable consequence

of the Second Law of Thermodynamics, which (among other things) implies that

no process can occur without error.

Mutations come in a variety of forms that differ in how much of the genome they

affect. We begin with point mutations, which affect only a single DNA base, and

end with whole genome duplication, which affects every DNA base in the genome.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_0412.ai Date 11-02-2016 01-12-17

Rare gA/ga form

Thrum

GA/ga

Pin

ga/ga

Anthers

Stigma

Style

The height of the anthers is

controlled by alleles a and A.

The length of the style is

controlled by alleles g and G.

Rare individuals with

the gA/ga genotype

have both anthers

and stigma high.

FIGURE 4.12 Variation in flowers of the prim-

rose (Primula vulgaris) illustrates how linkage

disequilibrium can result from selection. Allele

A, which produces anthers that are high in

the flower, is most often with allele G, which

produces a stigma that is low. Allele a, which

produces low anthers, and allele g, which

produces a high stigma, are together most fre-

quently. Gametes with the Ag or the aG allele

combinations give rise to plants that self-fer-

tilize, producing offspring that survive poorly.

This maintains the linkage disequilibrium: the

ag and AG allele combinations are much more

common than Ag and aG. (After [10].)

04_EVOL4E_CH04.indd 88 3/23/17 8:55 AM