the evolution of populations during their lifetimes...variation between populations •most species...
TRANSCRIPT
1/11/2012
1
LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
The Evolution of Populations
Chapter 23 Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve
during their lifetimes
• Natural selection acts on individuals, but only
populations evolve
• Consider, for example, a population of medium
ground finches on Daphne Major Island
– During a drought, large-beaked birds were
more likely to crack large seeds and survive
– The finch population evolved by natural
selection
© 2011 Pearson Education, Inc.
Figure 23.1 Figure 23.2
1976
(similar to the
prior 3 years)
1978
(after
drought)
Ave
rag
e b
ea
k d
ep
th (
mm
)
10
9
8
0
• Microevolution is a change in allele
frequencies in a population over generations
• Three mechanisms cause allele frequency
change:
– Natural selection
– Genetic drift
– Gene flow
• Only natural selection causes adaptive
evolution
© 2011 Pearson Education, Inc.
• Variation in heritable traits is a prerequisite for
evolution
• Mendel’s work on pea plants provided evidence
of discrete heritable units (genes)
Concept 23.1: Genetic variation makes
evolution possible
© 2011 Pearson Education, Inc.
1/11/2012
2
Genetic Variation
• Genetic variation among individuals is caused
by differences in genes or other DNA
segments
• Phenotype is the product of inherited genotype
and environmental influences
• Natural selection can only act on variation with
a genetic component
© 2011 Pearson Education, Inc.
Figure 23.3
(a) (b)
Figure 23.3a
(a)
Figure 23.3b
(b)
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
• Quantitative characters vary along a continuum
within a population
© 2011 Pearson Education, Inc.
• Genetic variation can be measured as gene
variability or nucleotide variability
• For gene variability, average heterozygosity
measures the average percent of loci that are
heterozygous in a population
• Nucleotide variability is measured by comparing
the DNA sequences of pairs of individuals
© 2011 Pearson Education, Inc.
1/11/2012
3
Variation Between Populations
• Most species exhibit geographic variation,
differences between gene pools of separate
populations
• For example, Madeira is home to several
isolated populations of mice
– Chromosomal variation among populations is
due to drift, not natural selection
© 2011 Pearson Education, Inc.
Figure 23.4
1 2.4
8.11 9.12 10.16
3.14
13.17
5.18
19
6
XX
7.15
1 2.19
9.10 11.12 13.17
3.8
15.18
4.16 5.14
XX
6.7
Figure 23.4a
1 2.4
8.11 9.12 10.16
3.14
13.17
5.18
19
6
XX
7.15
Figure 23.4b
1 2.19
9.10 11.12 13.17
3.8
15.18
4.16 5.14
XX
6.7
Figure 23.4c
• Some examples of geographic variation occur
as a cline, which is a graded change in a trait
along a geographic axis
• For example, mummichog fish vary in a cold-
adaptive allele along a temperature gradient
– This variation results from natural selection
© 2011 Pearson Education, Inc.
1/11/2012
4
Figure 23.5
1.0
0.8
0.6
0.4
0.2
0 46 44 42 40 38 36 34 32
Maine
Cold (6°C)
Latitude (ºN) Georgia
Warm (21ºC)
Ld
h-B
b a
llele
fre
qu
en
cy
30
Sources of Genetic Variation
• New genes and alleles can arise by mutation or
gene duplication
© 2011 Pearson Education, Inc.
Animation: Genetic Variation from Sexual Recombination
Formation of New Alleles
• A mutation is a change in nucleotide
sequence of DNA
• Only mutations in cells that produce gametes
can be passed to offspring
• A point mutation is a change in one base in a
gene
© 2011 Pearson Education, Inc.
• The effects of point mutations can vary:
– Mutations in noncoding regions of DNA are
often harmless
– Mutations in a genes can be neutral because
of redundancy in the genetic code
© 2011 Pearson Education, Inc.
• The effects of point mutations can vary:
– 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
1/11/2012
5
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Concept 23.2: The Hardy-Weinberg
equation can be used to test whether a
population is evolving
• The first step in testing whether evolution is
occurring in a population is to clarify what we
mean by a population
© 2011 Pearson Education, Inc.
Gene Pools and Allele Frequencies
• A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
• A gene pool consists of all the alleles for all loci
in a population
• A locus is fixed if all individuals in a population
are homozygous for the same allele
© 2011 Pearson Education, Inc.
Figure 23.6
Porcupine herd
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
AL
AS
KA
CA
NA
DA
MAP
AREA
Figure 23.6a
Porcupine herd
1/11/2012
6
Figure 23.6b
Fortymile herd
• The frequency of an allele in a population can
be calculated
– For diploid organisms, the total number of
alleles at a locus is the total number of
individuals times 2
– 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
© 2011 Pearson Education, Inc.
• By convention, 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
© 2011 Pearson Education, Inc.
• For example, 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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
1/11/2012
7
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
© 2011 Pearson Education, Inc.
Figure 23.7
Alleles in the population
Gametes produced
Each egg: Each sperm:
80%
chance
20%
chance 80%
chance
20%
chance
Frequencies of alleles
p = frequency of
q = frequency of
CW allele = 0.2
CR allele = 0.8
• Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
• Consider, for example, the same population of 500 wildflowers and 100 alleles where
– p freq CR 0.8
– q freq CW 0.2
© 2011 Pearson Education, Inc.
• 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)
(80%) (20%) Sperm
20% CW (q = 0.2)
CR CW
(80%)
(20%)
CR
CW
Eggs 64% (p2)
CRCR
16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CWCW
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
4% CW
(from CWCW plants)
16% CR
(from CRCW plants) +
+
Genotypes in the next generation:
16% CW
(from CRCW plants)
=
=
80% CR = 0.8 = p
20% CW = 0.2 = q
64% CRCR, 32% CRCW, and 4% CWCW plants
Figure 23.8a
80% CR (p = 0.8)
(80%) (20%)
20% CW (q = 0.2)
CR CW Sperm
(80%)
(20%)
CR
CW
Eggs 64% (p2)
CRCR 16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CWCW
1/11/2012
8
Figure 23.8b
(20%) CR Sperm
(80%)
(20%)
CR
CW
Eggs 64% (p2)
CRCR
16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CWCW
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
4% CW
(from CWCW plants)
16% CR
(from CRCW plants) +
+
Genotypes in the next generation:
16% CW
(from CRCW plants)
=
=
80% CR = 0.8 = p
20% CW = 0.2 = q
64% CRCR, 32% CRCW, and 4% CWCW plants
CW (80%)
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• The five conditions for nonevolving populations
are rarely met in nature:
© 2011 Pearson Education, Inc.
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
• Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
© 2011 Pearson Education, Inc.
Applying the Hardy-Weinberg Principle
• We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:
© 2011 Pearson Education, Inc.
1. The 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
1/11/2012
9
3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
4. The population is large
5. Migration has no effect as many other populations have similar allele frequencies
© 2011 Pearson Education, Inc.
• The occurrence of PKU is 1 per 10,000 births
– q2 0.0001
– q 0.01
• The frequency of normal alleles is
– p 1 – q 1 – 0.01 0.99
• The frequency of carriers is
– 2pq 2 0.99 0.01 0.0198
– or approximately 2% of the U.S. population
© 2011 Pearson Education, Inc.
• Three major factors alter allele frequencies and
bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Concept 23.3: Natural selection, genetic
drift, and gene flow can alter allele
frequencies in a population
© 2011 Pearson Education, Inc.
Natural Selection
• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
• For example, an allele that confers resistance
to DDT increased in frequency after DDT was
used widely in agriculture
© 2011 Pearson Education, Inc.
Genetic Drift
• The smaller a sample, the greater the chance of
deviation from a predicted result
• Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to the
next
• Genetic drift tends to reduce genetic variation
through losses of alleles
© 2011 Pearson Education, Inc.
Animation: Causes of Evolutionary Change
Figure 23.9-1
Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
CRCR CRCR
CRCW
CWCW CRCR
CRCW
CRCR CRCW
CRCR CRCW
1/11/2012
10
Figure 23.9-2
5 plants leave off-
spring
Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
CRCR CRCR
CRCW
CWCW CRCR
CRCW
CRCR CRCW
CRCR CRCW
CRCR CWCW
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Generation 2 p = 0.5 q = 0.5
Figure 23.9-3
5 plants leave off-
spring
Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
CRCR CRCR
CRCW
CWCW CRCR
CRCW
CRCR CRCW
CRCR CRCW
CRCR CWCW
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Generation 2 p = 0.5 q = 0.5
2 plants leave off-
spring
CRCR
CRCR CRCR
CRCR CRCR
CRCR CRCR
CRCR
CRCR CRCR
Generation 3 p = 1.0 q = 0.0
The Founder Effect
• The founder effect occurs when a few
individuals become isolated from a larger
population
• Allele frequencies in the small founder
population can be different from those in the
larger parent population
© 2011 Pearson Education, Inc.
The Bottleneck Effect
• The bottleneck effect is a sudden reduction in
population size due to a change in the
environment
• The resulting gene pool may no longer be
reflective of the original population’s gene pool
• If the population remains small, it may be further
affected by genetic drift
© 2011 Pearson Education, Inc.
Figure 23.10-1
Original
population
Figure 23.10-2
Original
population
Bottlenecking
event
1/11/2012
11
Figure 23.10-3
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect can
increase understanding of how human activity
affects other species
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Figure 23.11
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken (a)
Location Population
size
Number
of alleles
per locus
Percentage
of eggs
hatched
93
<50
5.2
3.7
5.8
5.8
99
96
1,000–25,000
<50
750,000
75,000–
200,000
Nebraska, 1998
(no bottleneck)
(b)
Kansas, 1998
(no bottleneck)
Illinois
1930–1960s
1993
Figure 23.11a
Pre-bottleneck
(Illinois, 1820) Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken (a)
Figure 23.11b
Location Population
size
Number
of alleles
per locus
Percentage
of eggs
hatched
93
<50
5.2
3.7
5.8
5.8
99
96
1,000–25,000
<50
750,000
75,000–
200,000
Nebraska, 1998
(no bottleneck)
(b)
Kansas, 1998
(no bottleneck)
Illinois
1930–1960s
1993
1/11/2012
12
Figure 23.11c
Greater prairie chicken
• 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%
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Gene Flow
• Gene flow consists of the movement of alleles
among populations
• Alleles can be transferred through the movement
of fertile individuals or gametes (for example,
pollen)
• Gene flow tends to reduce variation among
populations over time
© 2011 Pearson Education, Inc.
• Gene flow can decrease the fitness of a population
• Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland
– Mating causes gene flow between the central and eastern populations
– 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
© 2011 Pearson Education, Inc.
Figure 23.12
Population in which the
surviving females
eventually bred
Central
Eastern
Su
rviv
al
rate
(%
)
Females born
in central
population
Females born
in eastern
population
Parus major
60
50
40
30
20
10
0
Central
population
NORTH SEA Eastern
population Vlieland,
the Netherlands
2 km
1/11/2012
13
Figure 23.12a
Parus major
• Gene flow can increase the fitness of a population
• Consider, for example, the spread of alleles for
resistance to insecticides
– Insecticides have been used to target mosquitoes
that carry West Nile virus and malaria
– Alleles have evolved in some populations that
confer insecticide resistance to these mosquitoes
– The flow of insecticide resistance alleles into a
population can cause an increase in fitness
© 2011 Pearson Education, Inc.
• Gene flow is an important agent of evolutionary change in human populations
© 2011 Pearson Education, Inc.
• 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
Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
© 2011 Pearson Education, Inc.
A Closer Look at Natural Selection
• Natural selection brings about adaptive
evolution by acting on an organism’s
phenotype
© 2011 Pearson Education, Inc.
Relative Fitness
• The phrases “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
© 2011 Pearson Education, Inc.
1/11/2012
14
• 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
© 2011 Pearson Education, Inc.
Directional, Disruptive, and Stabilizing
Selection
• 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
© 2011 Pearson Education, Inc.
Figure 23.13
Original population
Phenotypes (fur color)
Fre
qu
en
cy
of
in
div
idu
als
Original
population
Evolved
population
(a) Directional selection (b) Disruptive selection (c) Stabilizing selection
Figure 23.13a
Original
population
Evolved
population
(a) Directional selection
Figure 23.13b
Original
population
Evolved
population
(b) Disruptive selection
Figure 23.13c
Original
population
Evolved
population
(c) Stabilizing selection
1/11/2012
15
The Key Role of Natural Selection in
Adaptive Evolution
• Striking adaptation have arisen by natural
selection
– For example, cuttlefish can change color
rapidly for camouflage
– For example, the jaws of snakes allow them
to swallow prey larger than their heads
© 2011 Pearson Education, Inc.
Figure 23.14
Bones shown in
green are movable.
Ligament
Figure 23.14a
• Natural selection increases the frequencies of
alleles that enhance survival and reproduction
• Adaptive evolution occurs as the match between
an organism and its environment increases
• Because the environment can change, adaptive
evolution is a continuous process
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
1/11/2012
16
Figure 23.15
• Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
• Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
• Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
Figure 23.16 EXPERIMENT
Recording of SC
male’s call
Recording of LC
male’s call
LC male gray tree frog
SC male gray tree frog
Female gray tree frog
SC sperm Eggs LC sperm
Offspring of Offspring of SC father LC father
Survival and growth of these half-sibling offspring compared
RESULTS
Time to metamorphosis
Larval survival
Larval growth
NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.
Offspring Performance 1995 1996
LC better NSD
NSD
LC better (shorter)
LC better (shorter)
LC better
Figure 23.16a
EXPERIMENT
Recording of SC
male’s call
Recording of LC
male’s call
LC male gray tree frog
SC male gray tree frog
Female gray tree frog
SC sperm Eggs LC sperm
Offspring of Offspring of SC father LC father
Survival and growth of these half-sibling offspring compared
Figure 23.16b
RESULTS
Time to metamorphosis
Larval survival
Larval growth
NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.
Offspring Performance 1995 1996
LC better NSD
NSD
LC better (shorter)
LC better (shorter)
LC better
1/11/2012
17
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• 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
Heterozygote Advantage
© 2011 Pearson Education, Inc.
Figure 23.17
Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote)
Key
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%
• 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
Frequency-Dependent Selection
© 2011 Pearson Education, Inc.
1/11/2012
18
“Left-mouthed”
P. microlepis
“Right-mouthed”
P. microlepis
1.0
0.5
0 1981
Sample year
’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Fre
qu
en
cy o
f
“le
ft-m
ou
the
d” in
div
idu
als
Figure 23.18
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
© 2011 Pearson Education, Inc.
Figure 23.19 Figure 23.UN01
CRCR
CWCW
CRCW
Figure 23.UN02
Original
population
Evolved
population
Directional selection
Disruptive selection
Stabilizing selection
Figure 23.UN03
Sampling sites
(1–8 represent
pairs of sites)
Allele
frequencies
lap94 alleles Other lap alleles
Salinity increases toward the open ocean
Long Island
Sound
Atlantic
Ocean
Data from R. K. Koehn and T. J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).
1
2
2 3 4 5 6 7 8 9 10 11
1
2 3 4 5 6
7 8
9
10
11
1/11/2012
19
Figure 23.UN04