23- evolution of populations text
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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 23Chapter 23
The Evolution of Populations
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Overview: The Smallest Unit of Evolution
One common misconception about evolution is
that individual organisms evolve, in the
Darwinian sense, during their lifetimes
Natural selection acts on individuals, but
populations evolve
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Genetic variations in populations
Contribute to evolution
Figure 23.1
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Concept 23.1: Population genetics provides a
foundation for studying evolution
Microevolution
Is change in the genetic makeup of apopulation from generation to generation
Figure 23.2
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The Modern Synthesis
Population genetics
Is the study of how populations change
genetically over time
Reconciled Darwins and Mendels ideas
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The modern synthesis
Integrates Mendelian genetics with the
Darwinian theory of evolution by natural
selection
Focuses on populations as units of evolution
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Gene Pools and Allele Frequencies
A population
Is a localized group of individuals that are capable of
interbreeding and producing fertile offspring
MAP
AREA
Fairbanks
Whitehorse
Fortymile
herd range
Figure 23.3
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The Hardy-Weinberg Theorem
The Hardy-Weinberg theorem
Describes a population that is not evolving
States that the frequencies of alleles and
genotypes in a populations gene pool remain
constant from generation to generation
provided that only Mendelian segregation and
recombination of alleles are at work
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Preservation of Allele Frequencies
In a given population where gametes contribute
to the next generation randomly, allelefrequencies will not change
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Hardy-WeinbergEquilibrium
Hardy-Weinberg equilibrium
Describes a population in which random
mating occurs
Describes a population where allele
frequencies do not change
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A population in Hardy-Weinberg equilibrium
Figure 23.5
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80%CR(p = 0.8) 20% CW (q = 0.2)
SpermCR
(80%)CW
(20%)
p2
64%
CRCR16%
CRCW
16%
CRCW4%
CWCWqp
CR
(80%)
Eggs
CW
(20%)
pq
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:
q2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
64% CR from
CRCRhomozygotes
16% CR from
CRCWhomozygotes+ = 80% CR= 0.8 =p
16% CWfrom
CRCWheterozygotes+ = 20% CW= 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
64% CRCR, 32% CRCWand 4% CWCWplants
p2
4% CWfrom
CWCWhomozygotes
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Ifp and q represent the relative frequencies of
the only two possible alleles in a population ata particular locus, then
p2 + 2pq + q2 = 1
Andp2 and q2represent the frequencies of the
homozygous genotypes and 2pq represents
the frequency of the heterozygous genotype
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Conditions forHardy-WeinbergEquilibrium
The Hardy-Weinberg theorem
Describes a hypothetical population
In real populations
Allele and genotype frequencies do changeover time
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The five conditions for non-evolving
populations are rarely met in nature
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection
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Population Genetics andHuman Health
We can use the Hardy-Weinberg equation
To estimate the percentage of the human
population carrying the allele for an inherited
disease
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Concept 23.2: Mutation and sexual
recombination produce the variation thatmakes evolution possible
Two processes, mutation and sexual
recombination
Produce the variation in gene pools that
contributes to differences among individuals
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Mutation
Mutations
Are changes in the nucleotide sequence of DNA
Cause new genes and alleles to arise
Figure 23.6
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Point Mutations
A point mutation
Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptiveimpact
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Mutations That AlterGene NumberorSequence
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial
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Gene duplication
Duplicates chromosome segments
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Mutation Rates
Mutation rates
Tend to be low in animals and plants
Average about one mutation in every 100,000
genes per generation
Are more rapid in microorganisms
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Sexual Recombination
In sexually reproducing populations, sexual
recombination
Is far more important than mutation in
producing the genetic differences that make
adaptation possible
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Genetic Drift
Statistically, the smaller a sample
The greater the chance of deviation from a
predicted result
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Genetic drift
Describes how allele frequencies can fluctuate
unpredictably from one generation to the next
Tends to reduce genetic variation
Figure 23.7
CRCR
CRCW
CRCR
CWCW CRCR
CRCW
CRCW
CRCWCRCR
CRCR
Only 5 of
10 plants
leave
offspring
CWCW CRCR
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Only 2 of
10 plants
leave
offspring
CRCR
CRCR CRCR
CRCRCRCR
CRCR
CRCR
CRCR
CRCRCRCR
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0q = 0.0
Generation 1
p (frequency ofCR) = 0.7q (frequency ofCW) = 0.3
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The Bottleneck Effect
In the bottleneck effect
A sudden change in the environment may
drastically reduce the size of a population
The gene pool may no longer be reflective of
the original populations gene pool
Original
population
Bottlenecking
eventSurviving
population
Figure 23.8 A
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
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Understanding the bottleneck effect
Can increase understanding of how human
activity affects other species
Figure 23.8 B
(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.
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The FounderEffect
The founder effect
Occurs when a few individuals become
isolated from a larger population
Can affect allele frequencies in a population
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Gene Flow
Gene flow
Causes a population to gain or lose alleles
Results from the movement of fertile
individuals or gametes
Tends to reduce differences between
populations over time
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Concept 23.4: Natural selection is the primary
mechanism of adaptive evolution
Natural selection
Accumulates and maintains favorable
genotypes in a population
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Genetic Variation
Genetic variation
Occurs in individuals in populations of all
species
Is not always heritable
Figure 23.9 A, B
(a) Map butterflies that
emerge in spring:
orange and brown
(b) Map butterflies that
emerge in late summer:
black and white
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Variation Within a Population
Both discrete and quantitative characters
Contribute to variation within a population
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Discrete characters
Can be classified on an either-or basis
Quantitative characters
Vary along a continuum within a population
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Polymorphism
Phenotypic polymorphism
Describes a population in which two or more
distinct morphs for a character are each
represented in high enough frequencies to be
readily noticeable
Genetic polymorphisms
Are the heritable components of characters
that occur along a continuum in a population
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Measuring Genetic Variation
Population geneticists
Measure the number of polymorphisms in a
population by determining the amount of
heterozygosity at the gene level and the
molecular level
Average heterozygosity
Measures the average percent of loci that are
heterozygous in a population
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Variation Between Populations
Most species exhibit geographic variation
Differences between gene pools of separate
populations or population subgroups
1 2.4 3.14 5.18 6 7.15
XX1913.1710.169.128.11
1 2.19 3.8 4.16 5.14 6.7
XX15.1813.1711.129.10Figure 23.10
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Some examples of geographic variation occur
as a cline, which is a graded change in a traitalong a geographic axis
Figure 23.11
EXPERIMENT Researchers observed that the average size
of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing
elevation. To eliminate the effect of environmental differencesat different elevations, researchers collected seeds
from various altitudes and planted them in a commongarden. They then measured the heights of the
resulting plants.
RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at
which the seeds were collected, although the heightdifferences were less than in the plants natural
environments.
CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates the
role of genetic as well as environmental differences.
Meanheight(cm)
Atitude(m)
Heights of yarrow plants grown in common garden
Seed collection sites
Sierra NevadaRange
Great BasinPlateau
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A Closer Look at Natural Selection
From the range ofvariations available in a
population
Natural selection increases the frequencies of
certain genotypes, fitting organisms to their
environment o
ver generations
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Fitness
Is the contribution an individual makes to the
gene pool of the next generation, relative to
the contributions of other individuals
Relative fitness
Is the contribution of a genotype to the next
generation as compared to the contributions of
alternative genotypes for the same locus
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Directional, Disruptive, and StabilizingSelection
Selection
Favors certain genotypes by acting on the
phenotypes of certain organisms
Three modes of selection are
Directional
Disruptive
Stabilizing
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The three modes of selection
Fig 23.12 AC
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
(c) Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
Phenotypes (fur color)
Original population
Originalpopulation
Evolved
population
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The Preservation ofGenetic Variation
Various mechanisms help to preserve genetic
variation in a population
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Diploidy
Diploidy
Maintains genetic variation in the form of
hidden recessive alleles
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alancing Selection
Balancing selection
Occurs when natural selection maintains
stable frequencies of two or more phenotypic
forms in a population
Leads to a state called balanced polymorphism
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Heterozygote Advantage
Some individuals who are heterozygous at a
particular locus
Have greater fitness than homozygotes
Natural selection
Will tend to maintain two or more alleles at that
locus
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The sickle-cell allele
Causes mutations in hemoglobin but also
confers malaria resistance
Exemplifies the heterozygote advantage
Figure 23.13
Frequencies of thesickle-cell allele
02.5%
2.55.0%
5.07.5%
7.510.0%
10.012.5%
>12.5%
Distribution ofmalaria caused by
Plasmodium falciparum
(a protozoan)
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Frequency-Dependent Selection
In frequency-dependent selection
The fitness of any morph declines if it becomes
too common in the population
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An example of frequency-dependent selection
Phenotypicd
iversity
Figure 23.14
Parental population sample
Experimental group sample
Plain background Patterned background
On pecking a moth image
the blue jay receives afood reward. If the bird
does not detect a moth
on either screen, it pecks
the green circle to continue
to a new set of images (a
new feeding opportunity).
0.06
0.05
0.04
0.03
0.02
0 20 40 60 80 100Generation number
Frequency-
independent control
N l V i i
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NeutralVariation
Neutral variation
Is genetic variation that appears to confer no
selective advantage
S l S l ti
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Sexual Selection
Sexual selection
Is natural selection for mating success
Can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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Intrasexual selection
Is a direct competition among individuals of
one sex for mates of the opposite sex
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Intersexual selection
Occurs when individuals of one sex (usually
females) are choosy in selecting their mates
from individuals of the other sex
May depend on the showiness of the malesappearance
Figure 23.15
Th E l ti E i f S l R d ti
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The Evolutionary Enigma of Sexual Reproduction
Sexual reproduction
Produces fewer reproductive offspring than asexual
reproduction, a so-called reproductive handicap
Figure 23.16
Asexual reproduction
Female
Sexual reproduction
Female
Male
Generation 1
Generation 2
Generation 3
Generation 4
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If sexual reproduction is a handicap, why has it
persisted?
It produces genetic variation that may aid in
disease resistance
Why Natural Selection Cannot Fashion Perfect Organisms
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Why Natural Selection Cannot Fashion Perfect Organisms
Evolution is limited by historical constraints
Adaptations are often compromises
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Chance and natural selection interact
Selection can only edit existing variations