Download Chapter 23: The Evolution of Populations

Chapter 23: The
Evolution of
Populations
S
Chapter 23 Assignment
1. Define the terms population, species, gene pool,
relative fitness, and neutral variation
2.
List the five conditions of Hardy-Weinberg
equilibrium.
3. Hardy – Weinberg Equilibrium Problems (to be
distributed later)
Overview: The Smallest Unit of
Evolution
S One misconception is that organisms evolve, in the
Darwinian sense, during their lifetimes
S Natural selection acts on individuals, but only
populations evolve
S Genetic variations in populations contribute to evolution
S Microevolution is a change in allele frequencies in a
population over generations
Concept 23.1: Mutation and sexual
reproduction produce the genetic
variation that makes evolution
possible
S
S Two processes, mutation and sexual
reproduction, produce the variation in gene pools
that contributes to differences among individuals
Genetic Variation
S Variation in individual genotype leads to
variation in individual phenotype
S Not all phenotypic variation is heritable
S Natural selection can only act on variation with a
genetic component
Fig. 23-2
(a)
(b)
Nonheritable variation: these caterpillars display phenotypes
based on their diets, not their genes.
Variation Within a Population
S Both discrete and quantitative characters
contribute to variation within a population
S Discrete characters can be classified on an either-or
basis
S Quantitative characters vary along a continuum
within a population
Average Heterozygosity
S Population geneticists measure
polymorphisms in a population by
determining the amount of heterozygosity
at the gene and molecular levels
S Average heterozygosity measures the
average percent of loci that are
heterozygous in a population
Variation Between Populations
S Geographic variation: differences between gene
pools of separate populations or population
subgroups
Fig. 23-3
1
2.4
3.14
5.18
6
7.15
8.11
9.12
10.16
13.17
19
XX
1
2.19
3.8
4.16
9.10 11.12 13.17 15.18
5.14
6.7
XX
• Some examples of geographic variation occur as
a cline, which is a graded change in a trait along
a geographic axis
• Examples: frequency of an allele present in the
population`
How do new alleles arise?
S Mutations:
S Gene (point mutations)
S Chromosomal
S Sexual reproduction
Mutation
S Mutations are changes in the nucleotide
sequence of DNA
S Mutations cause new genes and alleles to arise
S Only mutations in cells that produce gametes can
be passed to offspring
Point Mutations
S A point mutation is a change in one base in a gene
S Point mutations can vary in severity:
S Noncoding regions – point mutation has no effect on
gene expression
S Can be more severe – sickle-cell disease
S Rarely do mutations increase the organism’s fitness –
most mutations have a negative effect on the
organism
Mutations That Alter Gene
Number or Sequence
S Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
S Duplication of large chromosome segments is
usually harmful
S Duplication of small pieces of DNA is sometimes less
harmful and increases the genome size
S Duplicated genes can take on new functions by further
mutation
Mutation Rates
S Mutation rates are low in animals and plants
S Why?
S The average is about one mutation in every 100,000
genes per generation
S Mutations rates are often lower in prokaryotes and
higher in viruses
S Why are mutations higher in viruses?
Sexual Reproduction
S 3 mechanisms:
S Crossing over
S Independent assortment
S Fertilization
S Shuffle existing alleles into new combinations
S In organisms that reproduce sexually, recombination of alleles is
more important than mutation in producing the genetic
differences that make adaptation possible
Concept 23.2: The HardyWeinberg equation can be used to
test whether a population is
evolving
S
Gene Pools and Allele
Frequencies
• A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
S A gene pool consists of all the alleles for all loci
in a population
Fig. 23-5
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
Hardy-Weinberg Principle
S Determine if a population is evolving or not
S Calculate allele frequencies of hypothetical non-evolving
populations and compare to actual frequencies of sample
population
S Allele frequencies should remain constant from one generation
to the next if there is no evolution
S The frequency of an allele in a population can be
calculated
S For diploid organisms, the total number of alleles at a locus is the total
number of individuals x 2
S The total number of dominant alleles at a locus is 2 alleles for each
homozygous dominant individual plus 1 allele for each heterozygous
individual; the same logic applies for recessive alleles
S By convention, if there are 2 alleles at a locus, p
and q are used to represent their frequencies
S The frequency of all alleles in a population will
add up to 1
S For example, p + q = 1
Fig. 23-6
Alleles in the population
Frequencies of alleles
p = frequency of
CR allele
= 0.8
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
80%
chance
20%
chance
Each sperm:
80%
chance
20%
chance
S Hardy-Weinberg equilibrium describes the
constant frequency of alleles in such a gene pool
S If p and q represent the relative frequencies of the
only two possible alleles in a population at a
particular locus, then
S p2 + 2pq + q2 = 1
S where p2 and q2 represent the frequencies of the homozygous genotypes
and 2pq represents the frequency of the heterozygous genotype
Hardy-Weinberg Equilibrium
Equation
2
Sp
+ 2pq +
2
q
=1
S Where p2 and q2 represent the frequencies of
the homozygous genotypes and 2pq
represents the frequency of the
heterozygous genotype
Fig. 23-7-1
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
64% (p2)
CRCR
16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CW CW
Fig. 23-7-2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Fig. 23-7-3
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
Conditions for Hardy-Weinberg
Equilibrium
S The Hardy-Weinberg theorem describes a
hypothetical population
S In real populations, allele and genotype
frequencies do change over time
The five conditions for
nonevolving populations are
rarely met in nature:
S No mutations
S Random mating
S No natural selection
S Extremely large population size
S No gene flow
S Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
Hardy-Weinberg
Practice Problems and
Homework
S
Practice Problem 1
1. You have sampled a population in which you know
that the percentage of the homozygous recessive
genotype (aa) is 36%. Using that 36%, calculate the
following:
• The frequency of the "aa" genotype.
• The frequency of the "a" allele.
• The frequency of the "A" allele.
• The frequencies of the genotypes "AA" and "Aa.“
• The frequencies of the two possible phenotypes if
"A" is completely dominant over "a."
Concept 23.3: Natural selection,
genetic drift, and gene flow can alter
allele frequencies in a population
S
S Three major factors alter allele
frequencies and bring about most
evolutionary change:
S Natural selection
S Genetic drift
S Gene flow
Natural Selection
S Differential success in reproduction results in
certain alleles being passed to the next generation
in greater proportions
Genetic Drift
S The smaller a sample, the greater the chance of
deviation from a predicted result
S Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to the
next
S Genetic drift tends to reduce genetic variation
through losses of alleles
S 2 examples: Founder Effect, Bottleneck Effect
Fig. 23-8-1
CR CR
CR CR
CR CW
CR CR
CW CW
CR CW
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Fig. 23-8-2
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CW CW
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CR
CR CW
CR CW
Generation 2
p = 0.5
q = 0.5
Fig. 23-8-3
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CR
CR CR
CW CW
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
The Founder Effect
S The founder effect occurs when a few individuals
become isolated from a larger population
S Allele frequencies in the small founder
population can be different from those in the
larger parent population
The Bottleneck Effect
S The bottleneck effect is a sudden reduction in
population size due to a change in the
environment
S The resulting gene pool may no longer be
reflective of the original population’s gene pool
S If the population remains small, it may be further
affected by genetic drift
Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
Effects of Genetic Drift: A
Summary
1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to change
at random
3. Genetic drift can lead to a loss of genetic
variation within populations
4. Genetic drift can cause harmful alleles to
become fixed
Gene Flow
S Gene flow consists of the movement of alleles
among populations
S Alleles can be transferred through the movement of
fertile individuals or gametes (for example, pollen)
S Gene flow tends to reduce differences between
populations over time
S Gene flow is more likely than mutation to alter allele
frequencies directly
Gene flow can decrease the
fitness of a population
S In bent grass, alleles for copper tolerance are
beneficial in populations near copper mines,
but harmful to populations in other soils
S Windblown pollen moves these alleles between
populations
S The movement of unfavorable alleles into a
population results in a decrease in fit between
organism and environment
Fig. 23-12
70
60
MINE
SOIL
NONMINE
SOIL
NONMINE
SOIL
50
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
80
Distance from mine edge (meters)
100
120
140
160
Gene flow can increase the
fitness of a population
S Insecticides have been used to target
mosquitoes that carry West Nile virus and
malaria
S Alleles have evolved in some populations that
confer insecticide resistance to these
mosquitoes
S The flow of insecticide resistance alleles into a
population can cause an increase in fitness
Concept 23.4: Natural selection is
the only mechanism that consistently
causes adaptive evolution
S
S Only natural selection consistently results in
adaptive evolution
A Closer Look at Natural
Selection
S Natural selection brings about adaptive evolution
by acting on an organism’s phenotype
Relative Fitness
S The phrases “struggle for existence” and
“survival of the fittest” are misleading as they
imply direct competition among individuals
S Reproductive success is generally more subtle and
depends on many factors
S Relative fitness is the contribution an individual
makes to the gene pool of the next generation,
relative to the contributions of other individuals
S Selection favors certain genotypes by acting on
the phenotypes of certain organisms
Directional, Disruptive, and
Stabilizing Selection
S Three modes of selection:
S Directional selection favors individuals at one end of the phenotypic
range
S Disruptive selection favors individuals at both extremes of the
phenotypic range
S Stabilizing selection favors intermediate variants and acts against
extreme phenotypes
Fig. 23-13a
Original population
Phenotypes (fur color)
Original population
Evolved population
(a) Directional selection
Fig. 23-13b
Original population
Phenotypes (fur color)
Evolved population
(b) Disruptive selection
Fig. 23-13c
Original population
Phenotypes (fur color)
Evolved population
(c) Stabilizing selection
The Key Role of Natural
Selection in Adaptive Evolution
S Natural selection increases the frequencies of
alleles that enhance survival and reproduction
S Adaptive evolution occurs as the match between
an organism and its environment increases
Fig. 23-14
(a) Color-changing ability in cuttlefish
Movable bones
(b) Movable jaw
bones in
snakes
S Because the environment can change, adaptive
evolution is a continuous process
S 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
Sexual Selection
S Sexual selection is natural selection for mating
success
S It can result in sexual dimorphism, marked
differences between the sexes in secondary sexual
characteristics
Fig. 23-15
S Intrasexual selection is competition among
individuals of one sex (often males) for mates of the
opposite sex
S Intersexual selection, often called mate choice,
occurs when individuals of one sex (usually females)
are choosy in selecting their mates
S Male showiness due to mate choice can increase a
male’s chances of attracting a female, while
decreasing his chances of survival
S How do female preferences evolve?
S The good genes hypothesis suggests that if a trait
is related to male health, both the male trait and
female preference for that trait should be selected
for
Fig. 23-16
EXPERIMEN
T
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm
Eggs
Offspring of
SC father
LC sperm
Offspring of
LC father
Fitness of these half-sibling offspring compared
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
Fig. 23-16a
EXPERIMEN
T
Female gray
tree frog
LC male gray
tree frog
SC male gray
tree frog
SC sperm
Eggs
Offspring of
SC father
LC sperm
Offspring of
LC father
Fitness of these half-sibling offspring compared
Fig. 23-16b
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
The Preservation of Genetic
Variation
S Various mechanisms help to preserve genetic
variation in a population
Diploidy
S Diploidy maintains genetic variation in the form
of hidden recessive alleles
Balancing Selection
S Balancing selection occurs when natural
selection maintains stable frequencies of two or
more phenotypic forms in a population
Heterozygote Advantage
• Heterozygote advantage occurs when
heterozygotes have a higher fitness than do
both homozygotes
• 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
Fig. 23-17
Frequencies of the
sickle-cell allele
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent Selection
• In 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
Fig. 23-18
“Right-mouthed”
1.0
“Left-mouthed”
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Neutral Variation
S Neutral variation is genetic variation
that appears to confer no selective
advantage or disadvantage
S For example,
S Variation in noncoding regions of DNA
S Variation in proteins that have little effect on protein
function or reproductive fitness
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