Evolution, 4th Edition

summARY

■■n ew genes are an important source of evolution-

ary novelty and adaptation. The most common

origin of new genes in eukaryotes is by gene

duplication that happens when there is an error

in dnA replication.

■■Whole genome duplication is much rarer but has

been a key event in the evolution of the genome

in many groups of organisms, especially plants.

■■n ew genes can be acquired from unrelated

species by horizontal gene transfer. HGT is par-

ticularly common in prokaryotes, in which it has

enabled the rapid evolution of traits, including

antibiotic resistance.

■■most gene duplicates degenerate into nonfunc-

tional pseudogenes. some duplicates survive,

however, and evolve to specialize in one of the

functions of the original gene, or to take on a

new function. The result is a gene family: a set of

loci that originated by gene duplication and that

typically have related biochemical roles.

■■Chromosomal deletions can eliminate function-

ing genes. natural selection can cause a deletion

to increase in frequency, for example when the

deleted gene codes for a protein that increases

the risk of infection.

■■most mutations that change the amino acid of a

protein (nonsynonymous mutations) are deleteri-

ous and are removed from the population by

purifying selection. mutations in coding se-

quences that do not change the protein’s amino

acid sequence (synonymous mutations) have only

very weak effects on fitness and evolve largely

by random drift. nonsynonymous mutations

that are beneficial are the rarest of all, but their

fixation by positive selection is the basis of much

adaptive evolution.

■■The dn/ds ratio provides a rough measure of the

relative contributions of drift and selection to the

evolution of a gene. The ratio is given by the fre-

quency of differences in the dnA sequence for

the gene in two species that are nonsynonymous

divided by the frequency of differences that

are synonymous. Typically this ratio is less than 1,

which is expected when most nonsynonymous

changes are eliminated by purifying selection

but some synonymous changes have become

fixed by drift. occasionally genes have a dn/ds

ratio greater than 1, which strongly suggests that

nonsynonymous changes have become fixed by

positive selection.

■■The fraction of protein differences among spe-

cies caused by adaptive evolution versus genetic

drift varies greatly among groups of organ-

isms. About half the differences are adaptive in

species with very large population sizes (e.g.,

Drosophila and free-living bacteria). in species

with a small effective population size, including

humans, drift is much stronger and so the frac-

tion of adaptive differences is much smaller.

■■Another important route to adaptation comes

from changes in how genes are expressed.

Recent research shows that changes in gene ex-

pression have been key to adaptation to new en-

vironments in many species, including humans.

■■i n eukaryotes, almost all genes have introns.

These allow the mRnA to be spliced in different

ways to make a variety of proteins. Chromosome

mutations that bring together exons from differ-

ent genes have yielded new genes with novel

functions.

■■ Chromosome numbers change by fusion and

fission. in some cases, fissions and fusions have

become fixed not because they increase fitness

but because they benefit from meiotic drive.

■■ Chromosome inversions can spread by several

mechanisms. one is when they bind together

beneficial combinations of alleles at two or more

loci.

■■ Genome size varies dramatically among spe-

cies. in viruses and prokaryotes, almost all of

the genome is coding sequence. in animals and

plants, most of the genome is noncoding, and

the quantity of noncoding dnA differs greatly

among species.

■■ Transposable elements, which are genetic para-

sites, are a major component of the noncoding

dnA and account for much of the variation in

genome size among species of eukaryotes.

■■ Whole genome duplication, which is particularly

common in plants, is responsible for the large

differences in gene number seen among some

closely related groups of organisms.

TERms And ConCEPTs

alternative splicing

codon bias

copy number

variation

de novo gene

dN/dS ratio

domain

exon shuffling

gene family

gene trafficking

horizontal gene

transfer (HGT)

inversion

karyotype

neofunctionalization

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