Transcript CHAPTER 23

CHAPTER 23
HOW POPULATIONS EVOLVE
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OVERVIEW
Natural selection acts on individuals, but only
populations evolve.
Genetic variations in populations contribute to
evolution.
Microevolution is defined as a change in allele
frequencies in a population over time and represents
evolutionary change on its smallest scale.
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I. Concept 23.1: Genetic Variation Makes Evolution
Possible
A. Two processes produce the genetic differences that are
the basis of evolution:
1. mutation
2. sexual reproduction
B. Genetic Variation
1. Variation in individual genotype leads to variation
in individual phenotype
2. Natural selection can only act on variation with a
genetic component.
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NONHERITABLE VARIATION
Difference in caterpillars of same species is due to diet
Caterpillars raised on oak
flowers resemble the flowers.
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Their siblings raised on oak
leaves resembled oak twigs.
C. Variation Within a Population
1. Both discrete and quantitative characters contribute
to variation within a population
2. Discrete characters can be classified on an either-or
basis (Ex: flower color)
3. Quantitative characters vary along a continuum
within a population (Ex: polygenic traits like skin
color)
4. Population geneticists measure genetic variation in a
population by determining the amount of
heterozygosity at the gene level and the molecular
level of DNA (nucleotide variability)
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5. Average heterozygosity measures the average
percent of loci that are heterozygous in a
population (gene variability)
6. Nucleotide variability is measured by comparing the
DNA sequences of base-pairs of individuals in a
population
7. Average heterozygosity tends to be greater than
nucleotide variability
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D. Variation Between Populations
1. Most species exhibit geographic variation which
results from differences in phenotypes or genotypes
between populations or between subgroups of a
single population that inhabit different areas
2. Some examples of geographic variation occur as a
cline, which is a graded change in a trait along a
geographic axis based on some environmental
variable like temperature
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CLINE
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E. Mutation
1. Defined as a change in the nucleotide sequence of
DNA
2. Mutations cause new genes and alleles to arise
3. Only mutations in cells that produce gametes can
be passed to offspring (germ mutation)
4. A point mutation is a change in one base in a gene
5. The effects of point mutations can vary:
 Mutations in noncoding regions of DNA are often
harmless
 Mutations in a gene might not affect protein
production because of redundancy in the genetic
code
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Mutations that result in a change in protein
production are often harmful
 Mutations that result in a change in protein
production can sometimes increase the fit
between organism and environment
F. Mutations That Alter Gene Number or Sequence
1. Chromosomal mutations that delete, disrupt, or
rearrange many gene loci are typically harmful
2. Duplication of large chromosome segments is
usually harmful
3. Gene duplication can be an important source of
new genetic variation.
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G. Mutation Rates
1. Mutation rates are low in animals and plants
2. The average is about one mutation in every 100,000
genes per generation
3. Mutations rates are often lower in prokaryotes and
higher in viruses
H. Sexual Reproduction
1. Sexual reproduction can shuffle existing alleles into
new combinations
2. In organisms that reproduce sexually, recombination
of alleles is more important than mutation in
producing the genetic differences that make
adaptation and evolution possible
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3. Three mechanisms that contribute to the unique
combinations of alleles are:
a. Crossing over
b. Independent assortment of chromosomes
c. Fertilization
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II. Concept 23.2: Hardy-Weinberg Equation
A. Gene Pools
1. A population is a group of individuals that belong to
the same species, live in the same area, and interbreed
to produce fertile offspring
 Individuals near the population’s center are, on
average, more closely related to one another than to
those individuals on the periphery
2. A gene pool consists of all the alleles for all loci in a
population (allele frequencies)
3. A locus is fixed if all individuals in a population are
homozygous for the same allele
4. Population Genetics—study of how populations
change genetically over time
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One Species (caribou); Two Populations
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B. Allele Frequencies
1. If there are 2 alleles at a locus, p and q are used to
represent their frequencies
2. The frequency of all alleles in a population will add
up to 1
 p+q=1
3. Example: (diploid population)
Imagine a population of 500 wildflower plants with
two alleles (CR and CW) at a locus that codes for flower
pigment (incomplete dominance)
20 plants are CW CW—white
320 plants are CR CR –red
160 plants are CR CW—pink
What are the allele frequencies for CW and CR?
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Total number of alleles—1000
 CR—800 alleles
 The frequency of CR allele in the gene pool of this
population is 800/1000 = 0.8 or 80%.
 Therefore frequency of CW = ?
 0.2 or 20% WHY?????
 p = 0.8
 q = 0.2
4. Allele and genotype frequencies can be used to
test whether evolution is occurring in a population
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C. The Hardy Weinberg Principle
1. Describes the gene pool of a population that is not
evolving
2. If a population does not meet the criteria of the
Hardy-Weinberg principle, it can be concluded that
the population is evolving
3. The Hardy-Weinberg principle states that frequencies
of alleles and genotypes in a population remain
constant from generation to generation unless acted
upon by agents other than Mendelian segregation and
recombination of alleles
4. Such a gene pool is said to be in Hardy-Weinberg
equilibrium
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Selecting alleles at random from a gene pool
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5. If p and q represent the relative frequencies of the
only two possible alleles in a population at a
particular locus, then
p2 + 2pq + q2 = 1
AA Aa aa
where p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the
frequency of the heterozygous genotype
6. This equation can be used to determine the
frequencies of the possible genotypes if we know
the frequencies of alleles, or we can calculate the
frequencies of alleles in a gene pool if we know the
frequencies of genotypes.
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Hardy-Weinberg Principle
Gametes for each generation are
drawn at random from the gene
pool of the previous generation
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If the gametes come together at
random, the genotype frequencies
of this generation are in HardyWeinberg equilibrium.
7. The Hardy-Weinberg theorem describes a hypothetical
population
8. In real populations, allele and genotype frequencies do
change over time
9. The five conditions for nonevolving populations are
rarely met in nature:
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No mutations
Random mating
No natural selection
Extremely large population size
No gene flow (migration)
10. Natural populations can evolve at some loci, while
being in Hardy-Weinberg equilibrium at other loci
D. Applying the Hardy-Weinberg Principle
1. We can assume the locus that causes phenylketonuria
(PKU) is in Hardy-Weinberg equilibrium. (PKU is
recessive)
2. The occurrence of PKU is 1 per 10,000 births
 q2 = 0.0001
 q = 0.01
3. The frequency of normal allele is
 p = 1 – q = 1 – 0.01 = 0.99
4. The frequency of carriers is
 2pq = 2 x 0.99 x 0.01 = 0.0198
 or approximately 2% of the U.S. population
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III. Concept 23.3: Three Mechanisms That
Directly Alter Allele Frequencies
A. Any condition that is a deviation from the 5
criteria for the Hardy-Weinberg Theorem has the
potential to cause evolution.
B. Three major factors that alter allele frequencies and
bring about evolutionary change are:
1. Natural selection (adaptive)
2. Genetic drift (nonadaptive)
3. Gene flow (nonadaptive)
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C. Natural Selection
 Differential success in reproduction results in certain
alleles being passed to the next generation in
greater proportions
D. Genetic Drift
1. Defined as changes in the gene pool (allele
frequencies) of a small population due to chance
2. The smaller a sample, the greater the chance of
deviation from a predicted result
3. Genetic drift tends to reduce genetic variation
through losses of alleles
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GENETIC DRIFT
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GENETIC DRIFT
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GENETIC DRIFT
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4. May occur in small populations (less than 100) as
the result of two situations:
Bottleneck effect
Founder effect
5. Bottleneck effect
Defined as a sudden reduction in population size
due to a change in the environment (Ex: disaster)
Alleles may be lost
New gene pool may differ drastically from original
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BOTTLENECK EFFECT
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Case Study: Impact of Genetic Drift on the Greater
Prairie Chicken
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Loss of prairie habitat caused a severe reduction in
the population of greater prairie chickens in Illinois
The surviving birds had low levels of genetic
variation, and only 50% of their eggs hatched
Researchers used DNA from museum specimens to
compare genetic variation in the population before
and after the bottleneck
The results showed a loss of alleles at several loci
Researchers introduced greater prairie chickens from
population in other states and were successful in
introducing new alleles and increasing the egg hatch
rate to 90%
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6. Founder effect
 Occurs when a few individuals colonize a new
habitat or become isolated from a larger
population
 Allele frequencies in the small founder population
can be different from those in the larger parent
population
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7. Effects of Genetic Drift Summary
a. Genetic drift is significant in small populations
b. Genetic drift causes allele frequencies to
change at random
c. Genetic drift can lead to a loss of genetic
variation within populations
d. Genetic drift can cause harmful alleles to
become fixed
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E. Gene Flow
1. Defined as the transfer of alleles among
populations due to the migration of fertile
individuals or gametes (pollen)
2. Tends to reduce differences between
populations over time
3. More likely than mutation to alter allele
frequencies directly
4. Can increase or decrease the fit between
between organism and environment
5. Can introduce new alleles into a population
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IV. Concept 23.4: Natural Selection
Only natural selection leads to the adaptation of an
organism to its environment (adaptive evolution)
 Natural selection brings about adaptive evolution by
acting on an organism’s phenotype
A. Relative Fitness
1. Defined as the contribution an individual makes
to the gene pool of the next generation, relative
to the contributions of other individuals
2. Natural selection acts on the genotype indirectly
by how the genotype affects the phenotype
3. How an organism benefits from a particular allele
depends on the genetic and environmental
contexts in which it is expressed
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B. Three Modes of Selection:
Depends
on which
phenotypes in a population
are favored.
1. Directional Selection
Favors phenotype of
one extreme
Most common when
species migrate to new
and different habitats or
during major
environmental change
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2. Disruptive Selection
Favors individuals at
both extremes of
the phenotypic
range
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3. Stabilizing Selection
Favors intermediate
variants and acts
against extreme
phenotypes
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THREE MODES OF NATURAL SELECTION
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C. Key Role of Natural Selection in Adaptive Evolution
1. Natural selection increases the frequencies of alleles
that enhance survival and reproduction
2. Adaptive evolution occurs as the match between an
organism and its environment increases
3. Because the environment can change, adaptive
evolution is a continuous process
4. Genetic drift and gene flow do not consistently lead
to adaptive evolution as they can increase or
decrease the match between an organism and its
environment
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D. Sexual Selection
1. Defined as natural selection for mating success
2. Can result in sexual dimorphism (marked
differences between the sexes in secondary sexual
characteristics not directly associated with
reproduction)
3. Intrasexual selection is competition among
individuals of one sex (often males) for mates of
the opposite sex
4. Intersexual selection, often called mate choice,
occurs when individuals of one sex (usually
females) are choosy in selecting their mates
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SEXUAL SELECTION
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E. Mechanisms for Preserving Genetic Variation in a
Population
1. Diploidy
Maintains genetic variation in the form of hidden
recessive alleles
2. Balancing selection
 Occurs when natural selection maintains stable
frequencies of two or more phenotypic forms in a
population (polymorphism)
Two mechanisms that help maintain balanced
polymorphism
a. Heterozygote advantage
--Occurs when heterozygotes have a higher fitness
than do both homozygotes
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--Natural selection will tend to maintain two or
more alleles at that locus
--The sickle-cell allele causes mutations in
hemoglobin but also confers malaria resistance
--Defined in terms of the genotype, not the
phenotype
b. Frequency-dependent selection
--the fitness of a phenotype declines if it
becomes too common in the population
--Selection can favor whichever phenotype is less
common in a population
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c. Neutral Variation
--genetic variation that appears to confer no
selective advantage or disadvantage
F. Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
3. Adaptations are often compromises
4. Chance, natural selection, and the environment
interact
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You should now be able to:
1. Explain why the majority of point mutations are
harmless
2. Explain how sexual recombination generates
genetic variability
3. Define the terms population, species, gene pool,
relative fitness, and neutral variation
4. List the five conditions of Hardy-Weinberg
equilibrium
5. Apply the Hardy-Weinberg equation to a population
genetics problem
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6. Explain why natural selection is the only mechanism
that consistently produces adaptive change
7. Explain the role of population size in genetic drift
8. Distinguish among the following sets of terms:
directional, disruptive, and stabilizing selection;
intrasexual and intersexual selection
9. List four reasons why natural selection cannot
produce perfect organisms
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