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Ch 16- Evolution of Populations
• Natural selection acts on individuals, but only
populations evolve.
• Genetic variations in populations contribute to
evolution.
• Population genetics is the study of how
populations change genetically over time and
integrates Mendelian genetics with the
Darwinian theory of evolution by natural
selection.
Gene Pools and Allele Frequencies
• A population is a
localized group of
individuals capable of
interbreeding and
producing fertile
offspring.
• The gene pool is all the
alleles in the population.
•Microevolution is change in the genetic makeup
of a population from generation to generation.
• Peppered Moths are an example of
microevolution. Before the Industrial Revolution
light moths were more common than dark moths.
Why?
The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem describes a
population that is not evolving.
• It states that frequencies of alleles and
genotypes in a population’s gene pool remain
constant from generation to generation,
provided that only Mendelian segregation and
recombination of alleles are at work.
• 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
– p+q=1
– And p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the
frequency of the heterozygous genotype
LE 23-4
Generation
1
X
CRCR
genotype
Generation
2
CWCW
genotype
Plants mate
All CRCW
(all pink flowers)
50% CW
gametes
50% CR
gametes
come together at random
Generation
3
25% CRCR
50% CRCW
50% CR
gametes
25% CWCW
50% CW
gametes
come together at random
Generation
4
25% CRCR
50% CRCW
25% CWCW
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Conditions for Hardy-Weinberg
Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population.
• In real populations, allele and genotype
frequencies do change over time.
• The five conditions for non-evolving populations
are rarely met in nature:
–
–
–
–
–
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection
LE 23-5
Gametes for each generation are
drawn at random from the gene pool
of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
CW
(20%)
p2
pq
64%
CRCR
16%
CRCW
(20%)
CR
(80%)
CW
Eggs
(80%)
qp
4%
CWCW
16%
CRCW
q2
Hardy-Weinberg theorem
• Counting Alleles
– assume 2 alleles = B, b
– frequency of dominant allele (B) = p
– frequency of recessive allele (b) = q
• frequencies must add to 1 (100%), so:
p+q=1
BB
Bb
bb
Hardy-Weinberg theorem
• Counting Individuals
– frequency of homozygous dominant: p x p = p2
– frequency of homozygous recessive: q x q = q2
– frequency of heterozygotes: (p x q) + (q x p) = 2pq
• frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
Bb
bb
H-W formulas
• Alleles:
p+q=1
B
• Individuals:
p2 + 2pq + q2 = 1
BB
BB
b
Bb
Bb
bb
bb
Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
What assume
Must
are the genotype
populationfrequencies?
is in H-W equilibrium!
Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.16
BB
Bb
bb
p2=.20
=.74
BB
2pq=.64
2pq=.10
Bb
q2=.16
bb
Null hypothesis
Sampled data
How do you explain
the data?
Population Genetics and Human
Health
• We can use the Hardy-Weinberg
equation to estimate the percentage of
the human population carrying the allele
for an inherited disease.
• Two processes, mutation and sexual
recombination, produce the variation in
gene pools that contributes to
differences among individuals
Mutation
• Mutations are changes in
the nucleotide sequence of
DNA.
• Mutations cause new genes
and alleles to arise.
• The dominant allele is not
always the most frequent
allele.
Point Mutations
• A point mutation is a change in one base in a
gene.
• It is usually harmless but may have significant
impact on phenotype.
• Sickle Cell Anemia
Application of H-W principle
• Sickle cell anemia
– inherit a mutation in gene coding for
hemoglobin
• oxygen-carrying blood protein
• recessive allele = HsHs
– normal allele = Hb
– low oxygen levels causes
RBC to sickle
• breakdown of RBC
• clogging small blood vessels
• damage to organs
– often lethal
Sickle cell frequency
• High frequency of heterozygotes
– 1 in 5 in Central Africans = HbHs
– unusual for allele with severe
detrimental effects in homozygotes
• 1 in 100 = HsHs
• usually die before reproductive age
Why is the Hs allele maintained at such high levels in
African populations?
Suggests some selective advantage of being
heterozygous…
Fig. 16.16
Malaria
Single-celled eukaryote parasite (Plasmodium)
spends part of its life cycle in red blood cells
1
2
3
Mutations That Alter Gene Number or
Sequence
• Chromosomal mutations that delete, disrupt,
or rearrange many loci are typically harmful.
• Gene duplication is nearly always harmful.
• Down’s Syndrome
Mutation Rates
• Mutation rates are low in animals and plants.
• The average is about one mutation in every
100,000 genes per generation.
• Mutations are more rapid in microorganisms.
Sexual Recombination
• Sexual recombination is far more important
than mutation in producing the genetic
differences that make adaptation possible.
• Three major factors alter allele frequencies
and bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Natural Selection
• Differential success in reproduction results in certain
alleles being passed to the next generation in greater
proportions.
Genetic Drift
• The smaller a sample, the greater the chance of
deviation from a predicted result.
• Genetic drift is changes of allele frequencies in
the gene pool due to chance rather than
selection.
• Genetic drift tends to reduce genetic variation
through losses of alleles.
– Bottleneck Effect
LE 23-7
CWCW
CR CR
CR CR
CR CW
Only 5 of
10 plants
leave
offspring
CR CR
CWCW
CR CR
CR CW
CWCW
CR CR
CR CW
CR CW
CR CR
CWCW
CR CW
CR C R
CR CR
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CR CR
CR C R
CR CR
CR CR
CR CR
CR CR
CR CR
C R CR
C R CW
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
• The bottleneck effect is a sudden change in
the environment that may drastically reduce
the size of a population.
• The resulting gene pool may no longer be
reflective of the original population’s gene
pool.
LE 23-8
Original
population
Bottlenecking
event
Surviving
population
The Founder Effect
• The founder effect occurs when a few
individuals become isolated from a larger
population. (new island, moved to an isolated
area)
• It can affect allele frequencies in a population.
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 make gene pools similar and reduces
possibilities of allele frequency differences.
Fig. 16.6
Genetic Variation
• Genetic variation occurs in individuals in
populations of all species.
• It is not always heritable.
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
Directional, Disruptive, and Stabilizing
Selection
• Selection favors certain genotypes by acting on
the phenotypes of certain organisms.
• Three modes of selection:
– Directional
– Disruptive
– Stabilizing
• Directional Selection
– One extreme form of a trait is most successful in
the environment; the moderate and other extreme
forms are less successful.
• Stabalizing Selection
– The moderate form of a trait is most successful in
the environment; BOTH extreme forms are less
successful.
• Disruptive Selection
– Both extreme forms of a trait are most successful
in the environment; the moderate form is less
successful.
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
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.
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.
Heterozygote Advantage
• In tropical Africa, where malaria is common:
– homozygous dominant (normal)
• die or reduced reproduction from malaria: HbHb
– homozygous recessive
• die or reduced reproduction from sickle cell anemia: HsHs
– heterozygote carriers are relatively free of both: HbHs
• survive & reproduce more, more common in population
Hypothesis:
In malaria-infected cells,
the O2 level is lowered
enough to cause sickling
which kills the cell &
destroys the parasite.
Frequency of sickle cell allele &
distribution of malaria
• The sickle-cell allele causes mutations in
hemoglobin but also confers malaria
resistance.
• It exemplifies the heterozygote advantage.
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.
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
Experimental group sample
Phenotypic
variation
0.5
0.4
Frequencyindependent control
0.3
0.2
0
Plain background
Patterned background
20
40
60
Generation number
80
100
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.
• Intrasexual selection is competition among
individuals of one sex for mates of the
opposite sex.
• Cat Fight
• 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.
Mating Dance
The Evolutionary Enigma of Sexual
Reproduction
• Sexual reproduction produces fewer
reproductive offspring than asexual
reproduction, a so-called “reproductive
handicap”.