Download PoL2e Ch15 Lecture-Processes of Evolution

15
Processes of Evolution
Chapter 15 Processes of Evolution
Key Concepts
15.1 Evolution Is Both Factual and the Basis of
Broader Theory
15.2 Mutation, Selection, Gene Flow, Genetic
Drift, and Nonrandom Mating Result in
Evolution
15.3 Evolution Can Be Measured by Changes in
Allele Frequencies
15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
Chapter 15 Mechanisms of Evolution
Key Concepts
15.5 Genomes Reveal Both Neutral and
Selective Processes of Evolution
15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New
Features
15.7 Evolutionary Theory Has Practical
Applications
Chapter 15 Opening Question
How do biologists use evolutionary theory
to develop better flu vaccines?
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
Evolution is the change in genetic composition
of populations over time.
Evolutionary change is observed in laboratory
experiments, in natural populations, and in the
fossil record.
These underlying genetic changes drive the
origin and extinction of species and fuel the
diversification of life.
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
Evolutionary theory is the understanding and
application of the processes of evolutionary
change to biological problems.
Applications:
•  Study and treatment of diseases
•  Development of crops and industrial
processes
•  Understanding the diversification of life
It also allows us to make predictions about the
biological world.
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
Theory—in everyday speech, an untested
hypothesis or a guess
Evolutionary theory is not a single hypothesis
It refers to our understanding of the processes
that result in genetic changes in populations
over time and to our use of that
understanding to interpret changes we
observe in natural populations.
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
Even before Darwin, biologists had suggested
that species had changed over time, but no
one had proposed a convincing mechanism
for evolution.
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
Charles Darwin was
interested in geology
and natural history.
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
In 1831, Darwin
began a 5-year
voyage around the
world on a Navy
survey vessel, the
HMS Beagle.
Figure 15.1 The Voyage of the Beagle
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
From the observations and insights made on
the voyage and new ideas from geologists
on the age of the Earth, Darwin developed
an explanatory theory for evolutionary
change:
•  Species change over time
•  Divergent species share a common
ancestor (descent with modification)
•  The mechanism that produces change is
natural selection
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
In 1858, Darwin received a paper from Alfred
Russel Wallace with an explanation of
natural selection nearly identical to Darwin’s.
Both men are credited for the idea of natural
selection.
Darwin’s book, The Origin of Species, was
published in 1859.
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
By 1900, the fact of evolution was established,
but the genetic basis of evolution was not yet
understood.
Then the work of Gregor Mendel was
rediscovered, and during the 20th century,
work continued on the genetic basis of
evolution.
A “modern synthesis” of genetics and
evolution took place 1936–1947.
Figure 15.2 Milestones in the Development of Evolutionary Theory
Concept 15.1 Evolution Is Both Factual and the Basis of Broader
Theory
The structure of DNA was established by 1953
by Watson and Crick.
In the 1970s, technology developed for
sequencing long stretches of DNA and
amino acid sequences in proteins.
Evolutionary biologists now study gene
structure and evolutionary change using
molecular techniques.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
In biology, “evolution” refers specifically to
changes in the genetic makeup of
populations over time.
Population—a group of individuals of a single
species that live and interbreed in a
particular geographic area at the same time.
Individuals do not evolve; populations do.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
The origin of genetic variation is mutation.
Mutation—any change in nucleotide
sequences.
Mutations occur randomly with respect to an
organism’s needs; natural selection acts on
this random variation and results in
adaptation.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Mutations can be deleterious, beneficial, or
have no effect (neutral).
Mutation both creates and helps maintain
genetic variation in populations.
Mutation rates vary, but even low rates create
considerable variation.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Because of mutation, different forms of a
gene, or alleles, may exist at a locus.
Gene pool—sum of all copies of all alleles at
all loci in a population
Allele frequency—proportion of each allele in
the gene pool
Genotype frequency—proportion of each
genotype among individuals in the population
Figure 15.3 A Gene Pool
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
An experiment demonstrates how mutations
accumulate in populations:
•  Lines of E. coli were grown in the
laboratory for 20,000 generations, and
genomes were sequenced every 5,000
generations.
•  The lines accumulated about 45 changes
to their genomes, and these changes
appeared at a fairly constant rate.
Figure 15.4 Mutations Accumulate Continuously
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
The gene pools of nearly all populations
contain variation for many traits.
Selection that favors different traits can lead to
many different lineages that descend from
the same ancestor.
Artificial selection on different traits in a single
species of wild mustard produced many crop
plants.
Figure 15.5 Many Vegetables from One Species
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Many of Darwin’s observations of variation
and selection came from domesticated
plants and animals.
Darwin bred pigeons and recognized
similarities between selection by breeders
and selection in nature.
In both cases, selection simply increases the
frequency of the favored trait from one
generation to the next.
Figure 15.6 Artificial Selection
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Laboratory experiments also demonstrate
genetic variation in populations.
Selection for certain traits in the fruit fly
Drosophila melanogaster resulted in new
combinations of genes that were not present
in the original population.
Figure 15.7 Artificial Selection Reveals Genetic Variation
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Natural selection:
•  Far more individuals are born than survive
to reproduce.
•  Offspring tend to resemble their parents
but are not identical to their parents or to
one another.
•  Differences among individuals affect their
chances of survival and reproduction,
which will increase the frequency of
favorable traits in the next generation.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Adaptation—a favored trait that evolves
through natural selection
Adaptation also describes the process that
produces the trait.
Individuals with deleterious mutations are less
likely to survive, reproduce, and pass their
alleles on to the next generation.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Migration of individuals or movement of
gametes (e.g., pollen) between populations
results in gene flow, which can change
allele frequencies.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Genetic drift—random changes in allele
frequencies from one generation to the next
In small populations, it can change allele
frequencies.
•  Harmful alleles may increase in
frequency, or rare advantageous alleles
may be lost.
Even in large populations, genetic drift can
influence frequencies of neutral alleles.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Population bottleneck—an environmental
event results in survival of only a few
individuals
•  This can result in genetic drift and
changing allele frequencies.
Populations that go through bottlenecks loose
much of their genetic variation. This is a
problem for small populations of endangered
species.
Figure 15.8 A Population Bottleneck
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Founder effect—genetic drift changes allele
frequencies when a few individuals colonize
a new area
•  It is equivalent to a large population
reduced by a bottleneck.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Nonrandom mating:
Self-fertilization is common in plants. When
individuals prefer others of the same
genotype, homozygous genotypes will
increase in frequency, and heterozygous
genotypes will decrease.
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Sexual selection occurs when individuals of
one sex mate preferentially with particular
individuals of the opposite sex rather than at
random.
Some seemingly nonadaptive traits may make
an individual more attractive to the opposite
sex.
There may be a trade-off between attracting
mates (more likely to reproduce) and
attracting predators (less likely to survive).
Figure 15.9 What Is the Advantage?
Concept 15.2 Mutation, Selection, Gene Flow,
Genetic Drift, and Nonrandom Mating Result in Evolution
Studies of African long-tailed widowbirds
showed that females preferred males with
longer tails.
Males with artificially elongated tails attracted
four times more females than males with
artificially shortened tails.
Thus males with long tails pass on their genes
to more offspring, which leads to the
evolution of this unusual trait.
Figure 15.10 Sexual Selection in Action (Part 1)
Figure 15.10 Sexual Selection in Action (Part 2)
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Evolution can be measured by changes in
allele frequencies.
Allele frequency:
number of copies of allele in population
p=
total number of copies of all alleles in population
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
For two alleles at a locus, A and a, three
genotypes are possible: AA, Aa, and aa.
p = frequency of A; q = frequency of a
2 N AA + N Aa
p=
2N
2 N aa + N Aa
q=
2N
Figure 15.11 Calculating Allele and Genotype Frequencies
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
For each population,
p + q = 1, and q = 1 – p.
Monomorphic: only one allele at a locus,
frequency = 1; the allele is fixed
Polymorphic: more than one allele at a locus
Genetic structure—frequency of different
alleles and genotypes in a population
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Hardy–Weinberg equilibrium—a model in
which allele frequencies do not change
across generations; genotype frequencies
can be predicted from allele frequencies
For a population to be at Hardy–Weinberg
equilibrium, there must be random mating
and infinite population size, but no mutation,
no gene flow, and no selection of genotypes.
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
At Hardy–Weinberg equilibrium, allele
frequencies do not change.
Genotype frequencies after one generation of
random mating:
Genotype: AA
Aa
aa
Frequency: p2
2pq q2
Figure 15.12 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium (Part 1)
Figure 15.12 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium (Part 2)
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Probability of two A-gametes coming together:
p × p = p2 = (0.55)2 = 0.3025
Probability of two a-gametes coming together:
q × q = q2 = (0.45)2 = 0.2025
Overall probability of obtaining a heterozygote:
2pq = 0.495
Concept 15.3 Evolution Can Be Measured by Changes in Allele
Frequencies
Populations in nature never meet the
conditions of Hardy–Weinberg equilibrium—
all biological populations evolve.
The model is useful for predicting approximate
genotype frequencies of a population.
Specific patterns of deviation from Hardy–
Weinberg equilibrium help identify processes
of evolutionary change.
Concept 15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
Qualitative traits—influenced by alleles at
one locus; often discrete qualities (black
versus white)
Quantitative traits—influenced by alleles at
more than one locus; likely to show
continuous variation (body size of
individuals)
Concept 15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
Natural selection can act on quantitative traits
in three ways:
•  Stabilizing selection favors average
individuals.
•  Directional selection favors individuals
that vary in one direction from the mean.
•  Disruptive selection favors individuals
that vary in both directions from the mean.
Figure 15.13 Natural Selection Can Operate in Several Ways
Concept 15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
Stabilizing selection reduces variation in
populations but does not change the mean.
•  Example: Stabilizing selection operates
on human birth weight.
It is often called purifying selection, meaning
selection against any deleterious mutations
to the usual gene sequence.
Figure 15.14 Human Birth Weight Is Influenced by Stabilizing Selection
Concept 15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
In directional selection, individuals at one
extreme of a character distribution contribute
more offspring to the next generation.
For a single gene locus, directional selection
may favor a particular variant—positive
selection for that variant.
If directional selection operates over many
generations, an evolutionary trend is seen.
•  Example: Texas Longhorn cattle.
Figure 15.15 Long Horns Are the Result of Directional Selection
Concept 15.4 Selection Can Be Stabilizing, Directional, or
Disruptive
In disruptive selection, individuals at opposite
extremes of a character distribution
contribute more offspring to the next
generation.
•  Results in increased variation in the
population
•  Can result in a bimodal distribution of
traits
•  Example: bill sizes in the black-bellied
seedcracker (Pyrenestes ostrinus)
Figure 15.16 Disruptive Selection Results in a Bimodal Character Distribution
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Types of mutations:
Nucleotide substitution—change in one
nucleotide in a DNA sequence (a point
mutation)
•  Synonymous substitution (silent)—
does not change the encoded amino acid
(most amino acids are specified by more
than one codon)
•  Nonsynonymous substitution
(missense)—usually deleterious but can
be selectively neutral or advantageous
Figure 15.17 When One Nucleotide Changes
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Substitution rates are highest at positions that
do not change the amino acid being
expressed (synonymous substitutions).
Substitution rate is even higher in
pseudogenes, copies of genes that are no
longer functional.
Figure 15.18 Rates of Substitution Differ
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Types of mutations:
Insertions, deletions, and rearrangements of
DNA sequences:
•  Can have a larger effect than point
mutations
•  Can change the reading frame of proteincoding sequences
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Neutral theory—at the molecular level, the
majority of variants in most populations are
selectively neutral.
Because they confer neither advantages nor
disadvantages, neutral variants must
accumulate through genetic drift rather than
positive selection.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Rate of fixation of neutral mutations by genetic
drift is independent of population size.
1
m = 2 Nµ
2N
N = population size
µ = neutral mutation rate
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
The rate of evolution of particular genes and
proteins is often relatively constant over time
and can be used as a “molecular clock” to
calculate evolutionary divergence times
between species.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Relative rates of synonymous and
nonsynonymous substitutions:
•  The rates should be similar if an amino
acid can be one of many alternatives
without changing the protein’s function—
amino acid replacement is neutral with
respect to fitness of the organism.
•  If an amino acid position is under positive
selection, the rate of nonsynonymous
substitutions should exceed the rate of
synonymous substitutions.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
•  If an amino acid position is under purifying
selection, the rate of synonymous
substitutions is expected to be much
higher than nonsynonymous substitutions.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Particular codons in a gene sequence can be
under different modes of selection.
Evolution of lysozyme:
Lysozyme digests bacterial cell walls. It is
found in almost all animals as a defense
mechanism.
Some mammals have foregut fermentation,
which has evolved twice—in ruminants and
langurs. Lysozyme in these lineages has
been modified to rupture only some bacteria
in the foregut to release nutrients.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Lysozyme-coding sequences were compared
in foregut fermenters and their
nonfermenting relatives, and rates of
substitutions were determined.
For many amino acid positions, the rate of
synonymous substitution in the lysozyme
gene was much higher than
nonsynonymous, indicating that many of the
amino acids are evolving under purifying
selection.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
But at other positions, the rates were similar.
Amino acid replacements happened at a much
higher rate in the langur lineage.
Lysozyme went through a period of rapid
change in adapting to the stomachs of
langurs.
Lysozymes of langurs and cattle share five
convergent amino acid replacements, which
make the protein more resistant to
degradation by the stomach enzyme pepsin.
Figure 15.19 Convergent Molecular Evolution of Lysozyme
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
The hoatzin is the only foregut-fermenting
bird.
It has independently evolved modifications to
lysozyme similar to those found in cattle and
langurs.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Heterozygosity can be advantageous as
environmental conditions change; because
of the advantage, polymorphic loci are
maintained.
Colias butterflies live in an environment with
temperature extremes. The population is
polymorphic for an enzyme that influences
flight at different temperatures.
Heterozygotes are favored because they can
fly over a larger temperature range.
Figure 15.20 A Heterozygote Mating Advantage (Part 1)
Figure 15.20 A Heterozygote Mating Advantage (Part 2)
Figure 15.20 A Heterozygote Mating Advantage (Part 3)
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Genome size varies greatly.
There is some correlation between genome
size and organismal complexity, but not
always.
If only the protein and RNA coding portions of
genomes are considered, there is much less
variation in size.
Figure 15.21 Evolution of Gene Number
Figure 15.22 A Large Proportion of DNA Is Noncoding
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
Some of the noncoding DNA can alter the
expression of surrounding genes.
Some noncoding DNA consists of
pseudogenes, which are nonfunctional but
occasionally develop novel functions.
Other noncoding sequences help maintain
chromosome structure, and some consist of
parasitic transposable elements.
Concept 15.5 Genomes Reveal Both Neutral and Selective
Processes of Evolution
The amount of nonconding DNA may be
related to population size.
Noncoding sequences that are only slightly
deleterious are likely to be purged by
selection most efficiently in species with
large population sizes.
In small populations genetic drift may
overwhelm selection against these
sequences.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Sexual reproduction results in new gene
combinations and produces genetic variety
that increases evolutionary potential.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
In the short term, sexual reproduction has
disadvantages:
•  Recombination can break up adaptive
combinations of genes
•  Reduced rate at which females pass
genes to offspring
•  Dividing offspring into genders reduces
the overall reproductive rate
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Why did sexual reproduction evolve? Possible
advantages:
•  Facilitates repair of damaged DNA—
damage on one chromosome can be
repaired by copying intact sequences on
the other chromosome
•  Elimination of deleterious mutations
through recombination followed by
selection
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
•  In asexually reproducing species,
deleterious mutations can accumulate;
only death of the lineage can eliminate
them
§  Muller called this the genetic ratchet—
mutations accumulate or “ratchet up”
at each replication; known as Muller’s
ratchet.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
•  The variety of genetic combinations in
each generation can be advantageous
(e.g., as defense against pathogens and
parasites)
Sexual recombination does not directly
influence the frequencies of alleles,
it generates new combinations of alleles on
which natural selection can act.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Lateral gene transfer—individual genes,
organelles, or genome fragments move
horizontally from one lineage to another
•  Species may pick up DNA fragments
directly from the environment.
•  Genes may be transferred to a new host
in a viral genome.
•  Hybridization results in the transfer of
many genes.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Lateral gene transfer can be advantageous; it
increases genetic variation.
•  Most common in bacteria; genes that
confer antibiotic resistance are often
transferred among species
•  Relatively uncommon in eukaryotes, but
hybridization in plants leads to gene
exchange
The endosymbiosis events that gave rise to
mitochondria and chloroplasts were lateral
gene transfers.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Gene duplication—genomes can gain new
functions
Gene copies may have different fates:
•  Both copies retain original function, which
can increase the amount of gene product.
•  Gene expression may diverge in different
tissues or at different times in
development.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
•  One copy may accumulate deleterious
mutations and become a functionless
pseudogene.
•  One copy retains original function, the
other changes and evolves a new
function.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Sometimes entire genomes may be
duplicated, providing massive opportunities
for new functions to evolve.
In vertebrate evolution, genomes of the jawed
vertebrates have four diploid sets of many
genes.
Two genome-wide duplication events occurred
in the ancestor of these species. This
allowed specialization of individual vertebrate
genes.
Concept 15.6 Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New Features
Successive rounds of duplication and
sequence evolution may result in a gene
family, a group of homologous genes with
related functions.
The globin gene family probably arose via
gene duplications.
Figure 15.23 A Globin Family Gene Tree
Concept 15.7 Evolutionary Theory Has Practical Applications
Molecular evolutionary principles can be used
to understand protein structure and function.
Puffer fish produce a toxin (TTX) that blocks
Na+ channels and prevents nerve and
muscle function.
But Na+ channels in the puffer fish itself are
not blocked by the toxin.
Nucleotide substitutions in puffer fish genes
result in changes in the channel proteins that
prevent TTX from binding.
Concept 15.7 Evolutionary Theory Has Practical Applications
Mutations in human Na+ channel genes cause
several neurological diseases.
Study of these gene substitutions aids in
understanding how Na+ channels function.
Biologists compare rates of synonymous and
nonsynonymous substitutions across Na+
channel genes in various animals that have
evolved TTX resistance.
Concept 15.7 Evolutionary Theory Has Practical Applications
Living organisms produce many compounds
useful to humans. The search for such
compounds is called “bioprospecting.”
These molecules result from millions of years
of evolution.
But biologists can imagine molecules that
have not yet evolved.
In vitro evolution—new molecules are
produced in the laboratory to perform novel
functions
Figure 15.24 In Vitro Evolution
Concept 15.7 Evolutionary Theory Has Practical Applications
In agriculture, breeding programs have
benefited from evolutionary principles,
including incorporation of beneficial genes
from wild species.
An understanding of how pest species evolve
resistance to pesticides has resulted in more
effective pesticide application and rotation
schemes.
Concept 15.7 Evolutionary Theory Has Practical Applications
Molecular evolution is also used to study
disease organisms.
All new viral diseases have been identified by
evolutionary comparison of their genomes
with those of known viruses.
Studies of the origins, timing of emergence,
and global diversity of human pathogens
(including HIV) depend on evolutionary
principles and methods, as do efforts to
develop effective vaccines.
Answer to Opening Question
Changes in surface proteins make influenza
virus strains undetectable to the host’s
immune system (positive selection for
changes in surface proteins).
By comparing ratios of synonymous to
nonsynonymous substitutions, biologists can
detect which mutations are under positive
selection.
Answer to Opening Question
They then assess which current flu strains
show the greatest number of changes in
these positively selected codons.
These flu strains are most likely to survive and
lead to flu epidemics of the future, so they
are the best targets for new vaccines.
Figure 15.25 Evolutionary Analysis of Surface Proteins Leads to Improved Flu Vaccines