Download Chap. 23 Evolution of Populations

Chapter 23
The Evolution of
Populations
Populations, Genes, and Evolution Are
Related
 Evolutionary changes occur from generation to generation,
causing descendants to differ from their ancestors
 Evolution is a property populations, not individuals
 A population is a group that includes all members of a species
living in a given area
 Genes and the environment interact to determine traits
Microevolution
 A change in allele frequencies in a population over
generations
 3 mechanisms cause allele frequency change:
 Natural selection
 Genetic drift
 Gene flow
 Only natural selection causes adaptive evolution
Genetic variation makes evolution possible
 Variation in heritable traits is a prerequisite for evolution
 Mendel’s work on pea plants provided evidence of discrete
heritable units (genes)
 Natural selection can only act on variation with a genetic
component
Genes and the environment interact to
determine traits
 In diploid individuals, each gene consists
of two alleles
 Homozygous - alleles are the same
 Heterozygous - alleles are different
 The specific alleles on an organism’s
chromosomes (its genotype) interact with
the environment to influence the
development of its physical and behavioral
traits (its phenotype)
Hamster example
 Coat color is determined by two alleles in hamsters
 The dominant allele encodes for an enzyme that catalyzes black pigment
 The recessive allele encodes for an enzyme that catalyzes brown pigment
 Hamsters with at least one dominant allele produce black
pigment (homozygous dominant or heterozygous)
 Hamsters with two recessive alleles produce brown pigment
(homozygous recessive)
Alleles, Genotype, and Phenotype
Coat-color allele B is
dominant, so heterozygous
hamsters have black coats
Each chromosome
has one allele of the
coat-color gene
phenotype
genotype
BB
B
Bb
B
B
bb
b
b
b
chromosomes
homozygous
heterozygous
homozygous
Gene Pool
 The gene pool is the sum of the genes in a population
 Population genetics deals with the frequency, distribution,
and inheritance of alleles in populations
 A gene pool consists of all the alleles of all the genes in all
individuals of a population
 For a given gene, the proportion of times a certain allele occurs
in a population relative to the occurrence of all the alleles for
that gene is called its allele frequency
More Hamster Examples
 Illustrate the idea of allele frequency
 A population of 25 hamsters contains 50 alleles of the coat
color gene (diploid)
 If 20 of those 50 alleles code for black coats, then the
frequency of the black allele is 0.40, or 40% (20/50 = 0.40)
A Gene Pool
Population: 25 individuals
Gene pool: 50 alleles
BBBBBBBB
BB
BB
BB
BB
BBBB b b b b
Bb
Bb
Bb
Bb
BBBB b b b b
Bb
Bb
Bb
Bb
BBBB b b b b
Bb
Bb
Bb
Bb
bbbbbbbb
bb
bb
bb
bb
bbbbbbbb
bb
bb
bb
bb
bb
bb
The gene pool for the
coat-color gene contains
20 copies of allele B and
30 copies for allele b
Gene Variation
 Can be measured as gene variability or nucleotide
variability
 Gene variability - average heterozygosity measures
the average percent of loci that are heterozygous in a
population
 Nucleotide variability - measured by comparing the
DNA sequences of pairs of individuals
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
 Either red, or white
 Quantitative characters vary along a continuum within a
population
 Skin color, height
Variation Between Populations
 Most species exhibit geographic variation,
differences between gene pools of separate populations
 Chromosomal variation among populations is due to drift, not
natural selection (more later)
 The island of Madeira is home to several isolated
populations of mice
 Some examples of geographic variation occur as a cline,
which is a graded change in a trait along a geographic axis
 Mummichog fish vary in a cold-adaptive allele along a
temperature gradient, due to natural selection
Figure 23.4
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
Figure 23.5
Ldh-Bb allele frequency
1.0
0.8
0.6
0.4
0.2
0
46
44
42
Maine
Cold (6°C)
40
38
36
Latitude (ºN)
34
32
Georgia
Warm (21ºC)
30
Formation of New Alleles
 A mutation is a change in nucleotide sequence of DNA
 Only mutations in gametes can be passed onto offspring
 A point mutation is a change in one base in a gene
The effects of point mutations can vary:
 Mutations in noncoding regions of DNA are harmless
 Mutations in a genes can be neutral due to redundancy in the
genetic code
 Mutations that result in a change in protein production are
often harmful
 Mutations that result in a change in protein production can
sometimes be beneficial
Sources of Genetic Variation
 New genes and alleles can arise by mutation or gene
duplication
Altering Gene Number or Position
 Chromosomal mutations that delete, disrupt, or rearrange many
loci are typically harmful
 Duplication of small pieces of DNA increases genome size and is
usually less harmful
 Duplicated genes can take on new functions by further mutation
 An ancestral odor-detecting gene has been duplicated many times:
humans have 1,000 copies of the gene, mice have 1,300
Rapid Reproduction
 Mutation rates are low in animals and plants
 The average is about one mutation in every 100,000
genes per generation
 Mutations rates are often lower in prokaryotes and
higher in viruses
Sexual Reproduction
 Sexual reproduction can shuffle existing alleles into new
combinations
 In organisms that reproduce sexually, recombination of
alleles is more important than mutation in producing the
genetic differences that make adaptation possible
Evolution
 Evolution is the change of allele frequencies within a population
 A casual observer might define evolution on the basis of changes in the
outward appearance or behaviors of the members of a population
 A population geneticist, however, defines evolution as the changes in allele
frequencies that occur in a gene pool over time
 If allele frequencies change from one generation to the next, the
population is evolving
 If allele frequencies do not change from generation to generation, the
population is not evolving
Hardy-Weinberg Principle
 The equilibrium population is a hypothetical population in which
evolution does not occur, an idealized population in which allele
frequencies do not change from generation to generation
 The Hardy-Weinberg principle, mathematical model, 1908
 English mathematician Godfrey H. Hardy
 German physician, Wilhelm Weinberg
 The Hardy-Weinberg principle demonstrates that, under
certain conditions, the frequencies of alleles and genotypes in a
sexually reproducing population remain constant from one
generation to the next
Frequency of each allele can be
measured
 If there are 2 alleles at a locus, p and q are used to
represent their frequencies
 The frequency of all alleles in a population will add up to 1
 For example, p + q = 1
 Consider a population of wildflowers that is incompletely
dominant for color:
 320 red flowers (CRCR)
 160 pink flowers (CRCW)
 20 white flowers (CWCW)
 Calculate the number of copies of each allele:
 CR  (320  2)  160  800
 CW  (20  2)  160  200
 To calculate the frequency of each allele:
 p  freq CR  800 / (800  200)  0.8
 q  freq CW  200 / (800  200)  0.2
 The sum of alleles is always 1
 0.8  0.2  1
The Hardy-Weinberg Principle
 The Hardy-Weinberg principle describes a population
that is not evolving
 If a population does not meet the criteria of the Hardy-
Weinberg principle, it can be concluded that the
population is evolving
Hardy-Weinberg Equilibrium
 The Hardy-Weinberg principle states that frequencies of
alleles and genotypes in a population remain constant from
generation to generation
 In a given population where gametes contribute to the next
generation randomly, allele frequencies will not change
 Mendelian inheritance preserves genetic variation in a population
Figure 23.7
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
 Hardy-Weinberg equilibrium describes the constant
frequency of alleles in such a gene pool
 Consider, for example, the same population of 500
wildflowers and 1000 alleles where
 p  freq CR  0.8
 q  freq CW  0.2
 The frequency of genotypes can be calculated
 CRCR  p2  (0.8)2  0.64
 CRCW  2pq  2(0.8)(0.2)  0.32
 CWCW  q2  (0.2)2  0.04
 The frequency of genotypes can be confirmed using a
Punnett square
© 2011 Pearson Education, Inc.
Figure 23.8
80% CR (p = 0.8)
20% CW (q = 0.2)
CR (80%)
Sperm
CW (20%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRC W
16% (qp)
CRCW
(20%)
4% (q2)
CW CW
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
 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
 where p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the
frequency of the heterozygous genotype
Conditions for Hardy-Weinberg
Equilibrium
 The Hardy-Weinberg theorem describes a hypothetical
population that is not evolving
 In real populations, allele and genotype frequencies do
change over time
5 conditions for non-evolving
populations are rarely met in nature:
1.
2.
3.
4.
5.
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
Natural populations can evolve at some loci & be
in equilibrium at other loci
Applying Hardy-Weinberg Principle
 We can assume locus that causes phenylketonuria (PKU) is
in Hardy-Weinberg equilibrium given that:
1. PKU gene mutation rate is low
2. Mate selection is random with respect to whether
or not an individual is a carrier for the PKU allele
3. Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions
4. Population is large
5. Migration has no effect as other populations have
similar allele frequencies
 The Biology Place Lab bench activity
What Causes Evolution?
 From the conditions that disturb a Hardy-Weinberg
equilibrium, we predict five causes of evolutionary change
 Mutation
 Gene flow
 Small population size
 Non-random mating
 Natural selection
Mutation
 The original source of genetic variability
 Mutations arise spontaneously, not as a result of or in
anticipation of, environmental necessity
 Can be beneficial, harmful, or neutral
 Little or no immediate effect
 Can be passed to offspring only if they occur in cells that give
rise to gametes
 Source of new alleles
Mutations Occur Spontaneously
1 Start with bacterial
colonies that have never
been exposed to antibiotics
2 Use velvet to transfer
colonies to identical positions
in three dishes containing the
antibiotic streptomycin
3 Incubate the dishes
4 Only streptomycinresistant colonies grow;
the few colonies are in
the exact same positions
in each dish
Gene Flow
 Gene flow - movement of alleles from one population to another
 Gene flow between populations changes allele frequencies
 Movement of individuals between populations commonly causes
gene flow
 Alleles can move between populations even if organisms do not
 Pollen (sperm) and seeds from flowering plants move and
distribute alleles
Pollen Can Be an Agent of Gene Flow
Great tit (Parus major) on the island of
Vlieland
 The Mating causes gene flow between the central and
eastern populations of the island
 Immigration from the mainland introduces alleles that
decrease fitness
 Natural selection selects for alleles that increase fitness
 Birds in the central region with high immigration have
a lower fitness
 Birds in the east with low immigration have a higher
fitness
Figure 23.12
60
Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
population
Central
Eastern
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
Evolutionary effect of gene flow
 To increase the genetic similarity of different populations
of a species
 Mixing alleles prevents the development of large
differences in genetic compositions of populations
 If gene flow between populations of a species is blocked,
the resulting genetic differences may grow so large that
one of the populations becomes a new species
Genetic Drift
 Genetic drift describes how allele frequencies fluctuate
unpredictably from one generation to the next
 The smaller a sample, the greater the chance of deviation from a
predicted result
 Genetic drift tends to reduce genetic variation through loss of
alleles
 In small populations, drift can result in the complete loss of an
allele in a few generations
Figure 23.9-1
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Figure 23.9-2
CRCR
CRCR
CRCW
CWCW
CRCR
5
plants
leave
offspring
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.9-3
CRCR
CRCR
CRCW
CWCW
CRCR
5
plants
leave
offspring
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Two Causes of Genetic Drift
1.
Population bottleneck
 A drastic reduction in population size brought about by natural
catastrophe or overhunting
 Can rapidly change allele frequencies and reduce genetic variation
 A bottleneck has been documented in the northern elephant seal
 Hunted almost to extinction in the 1800s, the elephant seals were
reduced to 20 individuals by 1890s
 A hunting ban allowed the population to increase to 30,000
 Biochemical analysis shows that present-day northern elephant
seals are almost genetically identical
 Despite their numbers, their lack of genetic variation leaves them
little flexibility to evolve if their environmental circumstances
change
Population Bottlenecks Reduce Variation
2. Founder Effect
 The founder effect - occurs when a small number of
individuals leave a large population and establish a new isolated
population
 By chance, the allele frequencies of founders may differ from
those of the original population
 Over time, the new population may exhibit allele frequencies
that differ from the original population
A Human Example of the Founder Effect in
Amish populations
Case Study: Impact of Genetic Drift
on the Greater Prairie Chicken
 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
Figure 23.11
Post-bottleneck
(Illinois, 1993)
Pre-bottleneck
(Illinois, 1820)
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
Location
Illinois
1930–1960s
1993
Percentage
of eggs
hatched
Population
size
Number
of alleles
per locus
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
 Researchers compared 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
populations in other states and were successful in
introducing new alleles, increasing the egg hatch rate to
90%
Mating within a Population
 Organisms within a population rarely mate randomly
 Nonrandom mating will not change the overall frequency of alleles in a
population
 Nonrandom mating will change the distribution of genotypes and of
phenotypes in a population
 Many organisms have limited mobility and remain near their place of
birth, hatching, or germination, increasing the likelihood of inbreeding
 In animals, nonrandom mating can arise if individuals have preferences
that influence their choice of mates
 Nonrandom mating may lead to inbreeding
 Increased chance of producing homozygous offspring
Nonrandom Mating Among Snow Geese
 Life on an Island
Natural Selection
 All genotypes are not equally beneficial
 Natural selection favors certain alleles at the expense of others
 Those individuals with the selective advantage have higher reproductive
success, so their alleles are passed on to the next generation
 The evolution of penicillin-resistant bacteria illustrates the
relationship between natural selection and evolution
Natural Selection Is The Only Mechanism That
Consistently Causes Adaptive Evolution
 Evolution by natural selection involves both change and “sorting”
 New genetic variations arise by chance
 Beneficial alleles are “sorted” and favored by natural selection
 Only natural selection consistently results in adaptive evolution
 Natural selection brings about adaptive evolution by acting on an
organism’s phenotype
 What do we mean by “adaptive
evolution?
Relative Fitness
 “struggle for existence” and “survival of the fittest” are misleading
as they imply direct competition among individuals
 Reproductive success is generally more subtle and depends on
many factors
 Relative fitness is the contribution an individual makes to the
gene pool of the next generation, relative to the contributions of
other individuals
 Selection favors certain genotypes by acting on the phenotypes of
certain organisms
Three modes of selection:
 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
Frequency of
individuals
Figure 23.13
Original
population
Evolved
population
(a) Directional selection
Original population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
The Key Role of Natural Selection
in Adaptive Evolution
 Adaptations have arisen
by natural selection
 Cuttlefish can change
color rapidly for
camouflage
 Jaws of snakes
 Cuttlefish changing colors
All Genotypes are not Equally Beneficial
 Evolutionary changes are not “good” or “progressive” in any absolute
sense
 Penicillin-resistant bacteria were favored only because of the presence
of penicillin
 What happens in an environment free of penicillin?
 Evolution is a compromise between opposing pressures
 The long neck of male giraffes was favored only because it confers a
distinct advantage in combat to establish dominance
 The long neck of a giraffe is a compromise between the advantage of being
able to win contests with other males and a disadvantage of vulnerability
while drinking water
A Compromise Between Opposing
Environmental Pressures
How Does Natural Selection Work?
 Natural selection stems from unequal reproduction
 Natural selection is often associated with the phrase “survival
of the fittest”
 The fittest individuals are those that not only survive, but are able to
leave the most offspring behind
 It is reproductive success that determines the future of an individual’s alleles
 Because the environment can change, adaptive evolution is a
continuous process
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
Sexual Selection
 Intrasexual selection is competition among individuals of one
sex for mates (usually males) of the opposite sex
 Intersexual selection, mate choice, occurs when individuals
of one sex (usually females) are choosy in selecting mates
 Male showiness due to mate choice can increase a male’s chances
of attracting a female, while decreasing his chances of survival
 How do female preferences evolve?
 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 increase in frequency
Figure 23.16
EXPERIMENT
Recording of LC
male’s call
Recording of SC
male’s call
Female gray
tree frog
LC male gray
tree frog
SC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
SC father
Offspring of
LC father
Survival and growth of these half-sibling offspring compared
RESULTS
Offspring Performance
1995
1996
Larval survival
LC better
NSD
Larval growth
NSD
LC better
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
 Neutral variation is genetic variation that does not
confer a selective advantage or disadvantage
 Various mechanisms help to preserve genetic variation in
a population
Diploidy
 Diploidy maintains genetic variation in the form of
hidden recessive alleles
 Heterozygotes can carry recessive alleles that are hidden
from the effects of selection
Balancing Selection
 Balancing selection occurs when natural selection
maintains stable frequencies of two or more phenotypic
forms in a population
 Balancing selection includes
 Heterozygote advantage
 Frequency-dependent selection
Heterozygote Advantage
 Occurs when heterozygotes
have a higher fitness than do
both homozygotes
 Natural selection tends to
maintain two or more alleles
at that locus
 The sickle-cell allele causes
mutations in hemoglobin but
also confers malaria
resistance
Figure 23.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
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
 For example, frequency-dependent selection selects for
approximately equal numbers of “right-mouthed” and
“left-mouthed” scale-eating fish
Figure 23.18
“Left-mouthed”
P. microlepis
1.0
Frequency of
“left-mouthed” individuals
“Right-mouthed”
P. microlepis
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Natural Selection Cannot Fashion
Perfect Organisms
 Selection can act only on existing variations
 Evolution is limited by historical constraints
 Adaptations are often compromises
 Chance, natural selection, and the environment interact
Figure 23.19