Evolution, 4th Edition

T HE EvoluTion of GEnEs And GEnomEs 349

of the enzymes that digests the bacteria is encoded by the RNASE1B locus, which

originated by gene duplication about 4 Mya. This new enzyme rapidly evolved

nine amino acid changes (FIGURE 14.6) [67]. Those changes allow the enzyme to

work in the low-pH environment of the monkey’s gut that is needed to ferment the

leaves.

Some proteins have repeated domains that confer part of their function. The

sodium channel that is critical to the firing of vertebrate nerves has four domains

with very similar structure (FIGURE 14.7). They arose by two rounds of duplication

of a gene in a remote ancestor that coded for only a single domain [34].

Mixtures of exons duplicated from genes with different functions can generate

new genes with new functions, a process that is called exon shuffling. Exon duplica-

tion and exon shuffling have played roles in the evolution of many eukaryotic genes

[42]. An example is the jingwei locus, which is found only in the fruit flies Drosophila

teissieri and D. yakuba (FIGURE 14.8). The first three exons of this gene are duplicates

of exons in the yellow-emperor (Ymp) gene found in many Drosophila species. The

fourth exon, however, is a duplicate of the entire alcohol dehydrogenase (Adh) gene

that was retrotransposed into an intron of Ymp. The new jingwei gene shows evi-

dence of rapid evolution by positive selection after it originated about 2 Mya.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_14.06.ai Date 01-02-2017

Note: Could use photo from CH06 opener of 3e. Image here

is an FPO from David with the note

“Here's one you can put in with an FPO. If all we need is an inset

photo of a Douc langur here, I should probably look for a less expensive one”

Nonsynonymous change

Synonymous change

Human RNASE1

Rhesus monkey RNASE1

Douc langur RNASE1

Douc langur

RNASE1B

Duplication

dN/dS = 4.03

FIGURE 14.6 Duplication and adaptation of the RNASE1 gene

in the douc langur. The gene tree relates the RNASE1B duplicate

locus in the douc langur to the RNASE1 locus in the langur, rhesus

monkey, and human. Branch lengths are proportional to the

number of substitutions in the coding region. Since the duplica-

tion event, there have been 12 changes to the coding region of

the duplicate, and none to the original copy of the gene. Of the

changes in the duplicate, nine are nonsynonymous and three are

synonymous. The dN/dS ratio (see Chapter 7) is much greater than

1, which is strong evidence that the duplicate has evolved under

positive selection. (After [42], based on data from [67].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_14.07.ai Date 01-02-2017

(B)

Inside cell

I IV

Outside cell

Inside cell

Outside cell

(A)

CO 2 –

CO 2 –

II

+H 3 N +H 3 N

III

1 2 45 63 1 2 45 63 1 2 45 63 1 2 (^4563) 1 2 (^4563)
FIGURE 14.7 Vertebrate sodium channels, which are critical to firing neurons, evolved
by duplication. (A) The α-subunit of the sodium channel protein has four domains
(labeled I–IV). Each domain has six segments that traverse the cell membrane (num-
bered in each domain). The corresponding segments in each of the four domains have
very similar protein sequences. The sodium channel protein evolved by two rounds of
duplication of a locus that coded for a single domain. (B) The ancestor of the vertebrate
sodium channel had a structure similar to this bacterial sodium channel. (After [27].)
14_EVOL4E_CH14.indd 349 3/22/17 2:44 PM