Download Genetic Variation

• Understanding the bottleneck effect can increase
understanding of how human activity affects other
species
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The Founder Effect
• The founder effect occurs when a few individuals
become isolated from a larger population
• It can affect allele frequencies in a population
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Gene Flow
• Gene flow consists of genetic additions or
subtractions from a population, resulting from
movement of fertile individuals or gametes
• Gene flow causes a population to gain or lose
alleles
• It 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
• It is not always heritable
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
Variation Within a Population
• Both discrete and quantitative characters
contribute to variation within a population
• Discrete characters can be classified on an eitheror 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 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 polymorphisms in
a population by determining the amount of
heterozygosity at the gene and molecular levels
• 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
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LE 23-10
1
2.4
3.14
8.11
9.12
10.16
5.18
6
13.17
19
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
5.14
7.15
XX
6.7
XX
• Some examples of geographic variation occur as
a cline, which is a graded change in a trait along a
geographic axis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-11
Heights of yarrow plants grown in common garden
Mean height (cm)
100
50
0
3,000
2,000
1,000
Sierra Nevada
Range
0
Seed collection sites
Great Basin
Plateau
A Closer Look at Natural Selection
• From the range of variations available in a
population, natural selection increases
frequencies of certain genotypes, fitting organisms
to their environment over generations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolutionary Fitness
• The phrases “struggle for existence” and “survival
of the fittest” are commonly used to describe
natural selection but can be misleading
• Reproductive success is generally more subtle
and depends on many factors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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, compared with contributions
of alternative genotypes for the same locus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Directional, Disruptive, and Stabilizing Selection
• Selection favors certain genotypes by acting on
the phenotypes of certain organisms
• Three modes of selection:
– Directional
– Disruptive
– Stabilizing
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
The Preservation of Genetic 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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Balancing Selection
• Balancing selection occurs when natural selection
maintains stable frequencies of two or more
phenotypic forms in a population
• Balancing selection 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
• It exemplifies the heterozygote advantage
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-13
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent Selection
• In frequency-dependent selection, the fitness of
any morph declines if it becomes too common in
the population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-14
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 a new set
of images (a new
feeding opportunity).
Parental population sample
0.6
Phenotypic
variation
Experimental group sample
0.5
0.4
Frequencyindependent control
0.3
0.2
0
Plain background
Patterned background
20
40
60
Generation number
80
100
Neutral Variation
• Neutral variation is genetic variation that appears
to confer no selective advantage
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Sexual Selection
• Sexual selection is natural selection for mating
success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Intrasexual selection is 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
• Selection may depend on the showiness of the
male’s appearance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction produces fewer reproductive
offspring than asexual reproduction, a so-called
“reproductive handicap”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-16
Asexual reproduction
Female
Sexual reproduction
Generation 1
Female
Generation 2
Male
Generation 3
Generation 4
• Sexual reproduction produces genetic variation
that may aid in disease resistance
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Why Natural Selection Cannot Fashion Perfect
Organisms
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance and natural selection interact
• Selection can only edit existing variations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings