Evolution, 4th Edition

THE EvoluTion of GEnEs And GEnomEs 347

The Birth of a Gene

One of the most fundamental questions we can ask about evolution is: How does

biological novelty originate? The answer must lie partly with the origin of genes

that have new functions. Like much of evolutionary genetics, this research topic

has exploded recently with the arrival of large numbers of genome sequences from

diverse branches of the tree of life [12].

The human eye is a remarkable organ. It forms a high-resolution image by

focusing light through a transparent lens made of living tissue. The lens is made

up largely of proteins called crystallins. Where did they originate? It turns out that

many proteins happen to be largely transparent. Crystallins in the vertebrate lens

are derived from proteins with a variety of other functions, for example a small

heat-shock protein that protects many tissues from different types of stress [14].

Crystallins in other animals are derived from different proteins with yet other

functions (FIGURE 14.3). In some instances, crystallins continue to function in

their original roles while also serving in the lens [56].

The crystallins show how novel biochemical functions can originate by a pro-

cess central to Darwin’s theory: descent with modification. Some of them origi-

nated by gene duplication (see Chapter 4). Duplication is the most common way

that new genes arise in eukaryotes. At the molecular level, duplications are caused

by several mechanisms. One is unequal crossing over, in which recombination

happens between different positions on chromosomes that are misaligned during

meiosis. Another is replication slippage, in which the DNA polymerase loses its

place and copies a segment of chromosome twice. In either event, the resulting

chromosomes can carry a gene that is duplicated in tandem with the parental copy.

A third way that gene duplicates arise is by retrotransposition. Here the messenger

RNA from a gene is reverse-transcribed to DNA, which is then integrated into the

genome. The result is a duplicate gene that is far from the parental copy, often on

a different chromosome. Duplicates that originate this way can be distinguished

from their parental genes because they lack introns, which were spliced out of the

mRNA before it was reverse-transcribed.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_14.03.ai Date 12-30-2016

α β

γ γ

Glyceraldehyde phosphate dehydogenase

Crystallin Ancestral protein function

α Small heat-shock proteins

β/γ Related to bacterial stress protein

δ Arginosuccinate lyase

ε Lactate dehydrogenase

τ α-Enolase

ρ NADPH-dependent reductase

λ Hydroxyacyl-CoA dehydrogenase

ζ Alcohol dehydrogenase

η Aldehyde dehydrogenase

μ Similar to bacterial ornithine deaminase

π

ζ

Guinea

pig

ρ

Frog

δ τ

Turtle

δ τ π

Lizard

δ ε

Crocodile

δ ε

Bird

λ

Rabbit

ζ

Camel

μ

Kangaroo

η

Human

FIGURE 14.3 Crystallin proteins in the

lens of vertebrate eyes allow them to

focus light. Different groups of vertebrates

have different crystallins derived from a

wide variety of proteins with other func-

tions. (After [64].)

14_EVOL4E_CH14.indd 347 3/22/17 2:44 PM