Download Genetic drift

Chapter 16: Population
Genetics and Evolution
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
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Chapter Opener
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Background: Molecular
Basis for Genetic Variation
Genetic information is encoded by DNA.
Genetic variation is caused by changes in the
nucleotide sequence of DNA.
DNA serves as a template for the manufacturing
of proteins and other nucleic acids:
each amino acid氨基酸 in a protein is encoded by a
sequence of 3 nucleotides, called a codon密码子
the genetic code contains redundancy冗余 because
only 20 amino acids need be encoded from 64
possible codons
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The source of genetic variation is
mutation and recombination.
Mutations are errors in the nucleotide
sequence of DNA:
Substitutions置换 (most common)
deletions, additions, and rearrangements重排
also may occur
Causes of mutations:
random copying errors
highly reactive chemical agents
ionizing radiation电离辐射
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Can mutations be
beneficial?
Most mutations are harmful:
the altered properties of proteins resulting
from mutations are not likely to be beneficial
natural selection weeds out most deleterious
genes, leaving only those that suit organisms
to their environments
an example is the sickle-cell mutation,
which alters the structure of the hemoglobin
血红素 molecule with deleterious effects for
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More on Mutation
Mutations are likely to be beneficial when the
relationship of the organism to its environment
changes:
selection for beneficial mutations is the basis for
evolutionary change, enabling organisms to exploit开拓
new environmental conditions
Processes that cause mutations are blind不清楚的
to selective pressures -- mutation is a random
force in evolution, producing genetic variation
independently of its fitness consequences.
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Mutation Rates
The rate of mutation for any nucleotide is low,
1 in 100 million per generation？.
Because a complex individual has a trillion or so
nucleotides, each individual is likely to sustain
支持 one or more mutations.
Rates of expressed gene mutations average
about 1 per 100,000 to 1 per million:
rates of expression of phenotypic effects are often
higher because they are controlled by many genes.
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Recombination
Variation is introduced during meiosis
when parts of the genetic material
inherited by an individual from its mother
and father recombine with each other:
recombination is the exchange of
homologous sections of maternal and
paternal chromosomes
recombination produces new genetic
variation rapidly
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Migration
 Migration of individuals within a population or between
populations can affect genetic variation in two ways.
 On one hand, high migration rates integrate populations
into larger units, which tend to retain genetic variation
just because of their size.
 On the other hand, movement of individuals between
habitats with different environmental conditions can mix
genes that have been selected under those different
conditions and increase genetic variation within the
population, both locally and as a whole.
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Environmental variation and
frequency-dependent selection
One of the most remarkable cases of such
heterozygote superiority involves the sickle-cell
allele (S) of the beta-hemoglobin molecule in
humans.
Yet, in some parts of tropical Africa, the
frequency of the S allele may reach 20% of the
gene pool. The reason for this is that in the
heterozygous state (AS), the sickle-cell allele
confers protection against malaria.
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Sources of Genetic
Variation
While mutation is the ultimate source of
genetic variation:
recombination multiplies this variation
sexual reproduction produces further novel
combinations of genetic material
the result is abundant variation upon
which natural selection can operate
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Figure 16.4
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The genotypes of all individuals
make up the gene pool基因库.
 The gene pool represents the total genetic variation
within the population.
 Not all combinations of alleles for a given gene locus will
be represented in the gene pool, especially those with
low probability.
 If a rare combination of alleles confers high fitness,
individuals with this combination will produce more
offspring, and these alleles will increase in frequency.
 Alleles : Alternative forms of the same gene, such as
the two forms of the beta-hemoglobin gene, are known
as alleles.
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Genetic markers can be used
to study population processes
Genetic markers
Allozymes
microsatellite
Minisatellites
RFLP\AFLP
EST (expressed sequence target)
SNPs
Genome wide association
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The Hardy-Weinberg Law哈文定律
In 1908, Hardy and Weinberg independently
described this fundamental law: the
frequencies of both alleles and genotypes will
remain constant from generation to
generation in a population with:
a large number of individuals
random mating
no selection
no mutation
no migration between populations
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Consequences of HardyWeinberg Law
No evolutionary change occurs through
the process of sexual reproduction itself.
Changes in allele and genotype
frequencies can result only from additional
forces外力 on the gene pool of a species.
Understanding the nature of these forces
is one of the goals of evolutionary biology.
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Deviations from HardyWeinberg Equilibrium 1
 For a gene with two alleles, A1 and A2, that occur in
proportions p and q, the proportions of the 3 possible
genotypes in the gene pool will be:
A1A1: p2
A1A2: 2pq
A2A2: q2
 Deviations from these proportions are evidence for the
presence of selection, nonrandom mating, or other
factors that influence the genetic makeup of a
population.
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Deviations from HardyWeinberg Equilibrium 2
Most natural populations deviate from
Hardy-Weinberg equilibrium.
We thus consider some of the forces
responsible for such deviations (setting
aside mutation and selection):
effects of small population size
nonrandom mating
migration
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Genetic Drift
Genetic drift is a change in allele
frequencies due to random variations in
fecundity and mortality in a a population:
genetic drift has its greatest effects in small
populations
when all but one allele for a particular gene
disappears from a population because of
genetic drift, the remaining allele is said to be
fixed 固定
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Founder Events
When a small number of individuals found
a new population, they carry only a partial
sample of the gene pool of the parent
population:
this phenomenon is called a founder event
founding of a population by ten or fewer
individuals results in a substantially reduced
sample of the total genetic variation
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Figure 16.5
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Figure 16.6
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Population Bottlenecks瓶颈
Continued existence at low population size of a
recently founded population results in further
loss of genetic variation by genetic drift,
referred to as a population bottleneck:
such a situation may have occurred in the recent
past for the population of cheetahs in East Africa
fragmentation of populations into small
subpopulations may eventually reduce their genetic
responsiveness to selective pressures of changing
environments
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Assortative Mating先择交配
Assortative mating occurs when
individuals select mates nonrandomly
with respect to their own genotypes:
positive assortative mating pairs like
with like
negative assortative mating pairs
mates that differ genetically
assortative mating does not change allele
frequencies but does affect frequencies of
genotypes (c) 2001 W.H. Freeman and
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Figure 16.7
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Positive assortative
mating leads to inbreeding.
Positive assortative mating can lead to an
overabundance of homozygotes:
one result is the unmasking of deleterious
recessive alleles not expressed in
heterozygous condition (inbreeding
depression近交衰退)
most species have mechanisms that assist
them in avoiding mating with close relatives:
dispersal, recognition of close relatives, negative
assortative mating, genetic self-incompatibility
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The inbreeding coefficient
(F)-Fixation index
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Is inbreeding always
undesirable?不适合的
Inbreeding creates genetic problems, particularly
loss of heterozygosity.
In some cases inbreeding may be beneficial:
plants that can self-pollinate are capable of
sexual reproduction even when suitable
pollinators are absent
when organisms are adapted to local
conditions, mating with distant individuals may
reduce fitness of progeny
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Optimal Outcrossing
Distance
Mating with individuals located at
intermediate distances (optimal
outcrossing distance) may be desirable:
nearby individuals are likely to be close relatives,
resulting in inbreeding
distant individuals may be adapted to different
conditions:
in controlled matings in larkspur飞燕草 plants,
crosses between individuals 10 m apart enhanced
seed set and seedling survival, compared to selfing
and mating with distant individuals
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Figure 16.8
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Genetic drift in small populations
causes loss of genetic variation
 because of the randomness of births and deaths, all the
copies of a particular gene in a population will have
descended, just by chance, from a single copy that existed
at some time in the past, referred to as the coalescence
time.
 For nuclear genes in diploid organisms, the average
coalescence time is equivalent to 4N generations, where N
is the size of the population.
 The process by which allele frequencies change and
genetic variation is lost due to random variations in
fecundity, mortality, and inheritance of gene copies
through male and female gametes is called genetic drift
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Effective population size
 Effective population size can be thought of as the size of
an ideal population that undergoes genetic drift at the
same rate as an observed population.
 Many factors influence effective population size, but
variation in population size and the participation of
individuals in reproduction are among the most important.
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Population growth and decline leave
different genetic traces
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Coalescence, mutation rate, and time
 all the copies of the mitochondrial genome in the
present-day human population can be traced back to a
single copy that existed roughly 140,000 years ago.
 Some people misinterpreted this result to mean that all of
us descended from a single woman alive at that time,
understandably dubbed “mitochondrial Eve.”
 The coalescence time calculated for the Y chromosome
suggests that the Y-chromosome “Adam” from whom all
our Y chromosomes descended lived only 60,000 years
ago.
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Loss of variation by genetic drift is
balanced by mutation and migration
 Mutation–drift balance
 What is the “4” doing there? Think of it this way: each
individual has two copies of each nuclear gene, and
each copy can be inherited through either the mother or
the father, so there are four possibilities for inheritance.
 In the case of mitochondrial or chloroplast genes, which
are present in a single copy inherited through only one
parent, the “4” disappears, and F ˆ mitochondrial
1/(Nμ+1).
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Migration–drift balance
Just as drift and mutation come into
balance, drift and migration also come
into balance, at which point
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Migration and Deviations from
Hardy-Weinberg Equilibrium
 Mixing individuals from subpopulations with different
allele frequencies can result in deviations from genotypic
frequencies under the Hardy-Weinberg equilibrium:
mixing results in under-representation of
heterozygotes.this phenomenon is called the
Wahlund effect
the Wahlund effect refers to reduction of
heterozygosity (that is when an organism has two
different alleles at a locus) in a population caused by
subpopulation structure.
The Wahlund effect was first documented by the
Swedish geneticist Sten Wahlund in 1928
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Genotypes vary geographically.
Differences in allelic frequencies between
populations can result from:
random changes (genetic drift, founder
events)
differences in selective factors
Such differences are particularly evident
when there are substantial geographic
barriers to gene flow.
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Ecotypes
The Swedish botanist Göte Turreson used
a common garden experiment to show
that differences among plants from
different localities had a genetic basis:
under identical conditions (in the common
garden) plants retained 保持 different forms
seen in their original habitats
Turreson called these different forms
ecotypes生态型
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Ecotypes may be close to
one another or distant.
Although ecotypes may be geographically
isolated and found some distance apart,
this is not always the case:
if selective pressures between nearby
localities are strong relative to the rate of
gene flow, ecotypic differences may arise:
plants on mine tailings and uncontaminated soils
nearby may differ greatly in their tolerance to
toxic metals (copper铜, lead铅, zinc, arsenic砷)
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Clines and Other
Geographic Patterns
Some traits may exhibit patterns of gradual
change over distance:
such patterns are referred to as clines渐变群
clinal variation usually represents adaptation to
gradually changing conditions of the environment
Other genetic patterns may be found:
geographic variation related to random founder
effects
differentiation related to abrupt geographic barriers
and spatial/temporal variation in these
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Figure 16.11
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Figure 16.12
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Figure 16.13
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Natural Selection
Natural selection occurs when genetic
factors influence survival and fecundity:
individuals with the highest reproductive
rate are said to be selected, and the
proportion of their genotypes increases
over time
Natural selection can take various forms
depending on the heterogeneity of, and rate
of change in, the environment.
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Figure 16.14
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Stabilizing Selection
Stabilizing selection occurs when
individuals with intermediate, or average,
phenotypes have higher reproductive success
than those with extreme phenotypes:
favors an optimum最优 or intermediate
phenotype, counteracting抵消 tendency of
phenotypic variation to increase from mutation
and gene flow
this is the prevailing流行 mode of selection in
unchanging environments
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Directional Selection
Under directional selection, the fittest
individuals have more extreme phenotypes than
the average for the population:
individuals producing the most progeny are to one
extreme of the population’s distribution of
phenotypes
the distribution of phenotypes in succeeding
generations shifts toward a new optimum
runaway sexual selection失控性选择 is an excellent
example of this phenomenon
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Disruptive Selection
 When individuals at either extreme of the range of
phenotypic variation have greater fitness than those
near the mean, disruptive selection can take place:
tends to increase phenotypic variation in the
population
may lead to bimodal distribution双峰分布 of
phenotypes
uncommon, but could result from availability of
diverse resources, benefits associated with
alternative life histories, or strong competition for a
preferred resource
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Figure 16.15
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Directional selection changes
allele frequencies.
Selection changes the makeup of the gene pool.
Selection has several important aspects:
directional selection against a deleterious allele results
in a decrease in frequency of that allele, coupled with
an increase in frequencies of favorable alleles
the rate of change in the frequencies of alleles is
proportional成正比 to the selective pressure
evolution stops only when there is no longer any
genetic variation to act upon; directional selection
thus removes genetic variation from
populations
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Maintenance of Genetic
Variation 1
A paradox:
natural selection cannot produce
evolutionary change without genetic variation
however, both stabilizing and directional
selection tend to reduce genetic variation:
how does evolution continue under such
circumstances?
does availability of genetic variation ever limit the
rate of evolutionary change?
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Maintenance of Genetic
Variation 2
 Mutation and migration supply populations with new
genetic variation.
 Spatial and temporal variation tend to maintain
variation by favoring different alleles at different times
and places.
 When heterozygotes have a higher fitness than
homozygotes, the relative fitness of each allele depends
on its frequency in the population (frequencydependent selection频率依赖的选择):
alleles are selected for when at low frequency and against when
at high frequency
heterozygote superiority is also called heterosis杂种优势
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Figure 16.16
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Figure 16.17
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How much genetic
variation?
About 1/3 of genes that encode enzymes
involved in cellular metabolism show variation in
most species:
about 10% of these are heterozygous in any given
individual
however, most genetic variation is apparently neutral
or has negative effects when expressed
thus most variation has no fitness consequences or is
subject to stabilizing selection
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Genetic Variation is
Important
Under changing environmental conditions,
the reserve of genetic variation may take
on positive survival value.
There seems to be enough genetic
variation in most populations so that
evolutionary change is a constant
presence.
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Evolutionary Changes in
Natural Populations
Evolutionary changes have been widely
documented, particularly in species that have
evolved rapidly in the face of environmental
changes caused by humans:
Cyanide氰化物 resistance in scale insects (Chapter 9)
pesticide and herbicide resistance among agricultural
pests and disease vectors
increasing resistance of bacteria to antibiotics抗生素
In each case, genetic variation in the gene pool
allowed these populations to respond to
changed conditions.
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黄花茅
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Figure 16.18
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Figure 16.19
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Useful Conclusions from
Population Genetics Studies
Every population harbors some genetic variation
that influences fitness.
Changes in selective factors in the environment
are usually met by evolutionary responses.
Rapid environmental changes caused by
humans will often exceed the capacity of a
population to respond by evolution; the
consequence is extinction.
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Summary 1
 Mutations are the ultimate source of genetic
variability.
 Recombination and sexual reproduction result in novel
genetic combinations.
 The Hardy-Weinberg law predicts stable allelic and
genotypic frequencies in certain conditions.
 Deviations from Hardy-Weinberg equilibrium are
caused by mutation, migration, nonrandom mating,
small population size, and selection.
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Summary 2
Selection pressures may vary geographically,
giving rise to variation in gene frequencies
within the geographic range of a species.
Selection may be stabilizing, directional, or
disruptive.
Selection tends to remove genetic variation, but
mutation, gene flow, and varying selective
pressures maintain it.
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