Download Natural Selection

Concept 23.3:
Natural selection, genetic drift, and gene flow
can alter a population’s genetic composition
Three major factors alter allele frequencies and
bring about most evolutionary change
– Natural selection
– Genetic drift
– Gene flow
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Genetic Drift
Unpredictable fluctuation in alleles frequency
from one generation to the next because of a
population finite size.
• Statistically, the smaller a sample
– The greater the chance of deviation from a
predicted result
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Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed to the
next generation in greater proportions
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• Genetic drift
– Describes how allele frequencies can fluctuate
unpredictably from one generation to the next
– Tends to reduce genetic variation
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
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Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CRCR
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CRCR
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CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
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Only 2 of
10 plants
leave
offspring
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
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 population’s gene pool
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.
Original
population
Figure 23.8 A
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Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect
– Can increase understanding of how human
activity affects other species
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.
Figure 23.8 B
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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
– Results from the movement of fertile
individuals or gametes
– Causes a population to gain or lose alleles
– 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
– Occurs in individuals in populations of all species
– Is not always heritable
(a) Map butterflies that
emerge in spring:
orange and brown
(b) Map butterflies that
emerge in late summer:
black and white
Figure 23.9 A, B
Seasonal differences in hormones
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Variation Within a Population
• Both discrete and quantitative characters
Contribute to variation within a population
• Discrete characters
– Can be classified on an either-or basis
– The different forms can be called morphs.
• 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
– Alleles of several loci affect the character
• height
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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
• Average heterozygosity: measures the
average percent of loci that are
heterozygous in a population
– At the molecular level:
nucleotide variability
Average heterozygosity tends to be greater than nucleotide
variability
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Variation Between Populations
• Most species exhibit geographic variation
– Differences between gene pools of separate
populations or population subgroups
Figure 23.10
1
2.4
3.14
5.18
8.11
9.12
10.16
13.17
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
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6
7.15
19
XX
5.14
6.7
XX
Some examples of geographic variation occur as a
cline: a graded change in a trait along a
geographic axis (parallels to the gradient in the environment)
Heights of yarrow plants grown in common garden
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 differences
at different elevations, researchers collected seeds
from various altitudes and planted them in a common
garden. They then measured the heights of the
resulting plants.
Mean height (cm)
EXPERIMENT
Atitude (m)
RESULTS The average plant sizes in the common
garden were inversely correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants’ natural
environments.
CONCLUSION The lesser but still measurable clinal variation
in yarrow plants grown at a common elevation demonstrates the
role of genetic as well as environmental differences.
Figure 23.11
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Sierra Nevada
Range
Great Basin
Plateau
Seed collection sites
A Closer Look at Natural Selection
• From the range of variations available in a
population, natural selection increases the
frequencies of certain genotypes, fitting
organisms to their environment over
generations
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Evolutionary Fitness
• The phrases “struggle for existence” and
“survival of the fittest”
– Are commonly used to describe natural
selection
– Can be misleading
• Reproductive success
– Is generally more subtle and depends on many
factors
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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
• In a more quantitative way:
Relative fitness
– 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 Stabilizing Selection
• Selection
– Favors certain genotypes by acting on the
phenotypes of certain organisms
• Three modes of selection are
– Directional
– Disruptive
– Stabilizing
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• Directional selection
– Favors individuals at one end of the
phenotypic range
• Disruptive selection
– Favors individuals at both extremes of the
phenotypic range
• Stabilizing selection
– Favors intermediate variants and acts against
extreme phenotypes
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The three modes of selection
Directional selection
Disruptive selection
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Stabilizing selection
Directional selection
shifts the overall
makeup of the
population by favoring
variants
at one extreme of the
distribution.
•
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In this case, darker mice are favored
because they live among dark rocks and a
darker fur color conceals them
from predators.
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.
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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.
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The Preservation of Genetic Variation
• Various mechanisms help to preserve genetic
variation in a population
– Diploidy (Eukaryotes are diploid)
• Maintains genetic variation in the form of
hidden recessive alleles
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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
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
Figure 23.13
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Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
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
On pecking a moth image
the blue jay receives a
food 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).
Parental population sample
Experimental group sample
Phenotypic diversity
0.06
0.05
0.04
Frequencyindependent control
0.03
0.02
0
Plain background
Patterned background
Figure 23.14
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20
60
40
80
Generation number
100
Neutral Variation
– Is genetic variation that appears to confer no
selective advantage
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Sexual Selection
– Is natural selection for mating success
– 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 male’s
appearance
Figure 23.15
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The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction
– Produces fewer reproductive offspring than asexual
reproduction, a so-called reproductive handicap
Sexual reproduction
Asexual reproduction
Generation 1
Female
Female
Generation 2
Male
Generation 3
Generation 4
Figure 23.16
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• If sexual reproduction is a handicap, why has it
persisted?
– It produces genetic variation that may aid in
disease resistance
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Why Natural Selection Cannot Fashion Perfect Organisms
1. Evolution is limited by historical constraints
– Each species has a legacy of descent with
modification. (birds 4 legs & wins?)
2. Adaptations are often compromises
– Seals swim well, walk not as well
3. Chance and natural selection interact
4. Selection can only edit existing variations
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