Evolution, 4th Edition

■■genetic variation is produced when the genes

of two parents are mixed during sexual repro-

duction by segregation and recombination. In

asexual species, genes are mixed only by recom-

bination, and less often than in sexual species.

■■The Hardy-Weinberg equilibrium occurs under

idealized conditions in which no evolutionary

forces are acting. At this equilibrium, the three

genotypes at a locus with two alleles are in the

ratios p^2 :: 2p(1 – p) :: (1 – p)^2. Deviations from

those proportions can be used to detect selec-

tion and other evolutionary factors.

■■Linkage disequilibrium occurs when alleles at

two loci occur together more often than expect-

ed by chance. It is eroded by recombination,

and increased by selection and other evolution-

ary forces.

■■Horizontal gene transfer (HgT) is the movement

of genes between organisms by mechanisms

that do not involve meiosis, and is particularly

important to prokaryotes. It can move genes

between individuals of the same species and

of different species. HgT is important to human

health because it is the major pathway by which

bacteria evolve resistance to antibiotics.

■■mutation, which is an error that occurs when

DnA or RnA is replicated, is critical to evolu-

tion because it is the ultimate source of genetic

variation. mutations can affect anywhere from

a single base to a large piece of chromosome.

mutation rates vary greatly among species.

■■mutations in coding regions are called synony-

mous if they do not change the protein, and are

called nonsynonymous if they do. This distinction

has important evolutionary implications.

■■many mutations have no measurable effect

on survival or reproduction. Those that do are

typically deleterious. A small fraction are ad-

vantageous, and their spread leads to adaptive

evolution.

■■mutations that affect one trait virtually always

have pleiotropic effects, meaning that they also

affect other traits.

■■In species with separate somas and germ lines,

a mutation can be inherited if it alters a gene in

a cell in the germ line. mutations to somatic cells

leave no descendants to the next generation.

■■Experiments show that mutation is random with

respect to what will increase fitness.

■■There are several mechanisms of nongenetic in-

heritance. one is cultural inheritance, an essential

part of human civilization.

TERmS AnD ConCEPTS

allele

allele frequency

beneficial mutation

bp (base pair)

chromosome

coding region

codon

cultural inheritance

deleterious

mutation

deletion

DNA

(deoxyribonucleic

acid)

duplication

epigenetic

inheritance

epistasis

exon

fission

fitness

fusion

gene

gene family

genetic code

germ line

Hardy-Weinberg

equilibrium

horizontal gene

transfer (HGT)

insertion

intron

inversion

linkage

disequilibrium

locus

maternal effect

mutation

mutation rate

noncoding DNA

nonsynonymous

phenotype

pleiotropy

point mutation

polymorphic

recombination

recombination rate

RNA (ribonucleic

acid)

segregation

SNP (single

nucleotide

polymorphism)

soma

structural mutation

synonymous

tetraploidy

whole genome

duplication

SUmmARY

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