Chapter 20
Genes Within
Populations
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http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecEvolution/Evolution2PopGen.html
Genetic Variation and Evolution
• Genetic variation
– Differences in alleles of
genes found within
individuals in a population
– Produces phenotypic
variation in population
– Raw material for natural
selection
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http://en.wikipedia.org/wiki/File:Coquina_variation3.jpg
Genetic Variation and Evolution
• Evolution
– Change in a population over time
– How this change occurs
– Development of modern concept
traced to Charles Darwin
• “Descent with modification”
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• “Through time, species accumulate
differences; as a result, descendants
differ from their ancestors. In this way,
new species arise from existing ones.”
– Charles Darwin (On the Origin of
Species)
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http://images3.makefive.com/images/debate/history/all-time-greatest-visionaries/charles-darwin-7.jpg
• Darwin was not the first to propose a
theory of evolution
– Unlike his predecessors, however,
Darwin proposed natural selection
as the mechanism of evolution
• Rival theory of Jean-Baptiste Lamarck
was evolution by inheritance of
acquired characteristics
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http://www.macroevolution.net/images/jean-baptiste-lamarck-239-224-11.jpg
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Stretching
Proposed ancestor of
giraffes has characteristics
of modern-day okapi.
Stretching
The giraffe ancestor lengthened its
neck by stretching to reach tree
leaves, then passed the change to
offspring.
Reproduction
a. Lamarck’s theory: acquired variation is passed on to descendants.
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Some individuals born happen to have longer necks due to
genetic differences.
Reproduction
Individuals pass on their traits to next generation.
Natural
selection
Reproduction
Over many generations, longer-necked individuals are more
successful, perhaps because they can feed on taller trees, and
pass the long-neck trait on to their offspring.
b. Darwin’s theory: natural selection on genetically-based
variation leads to evolutionary change.
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• Population genetics
– Study of properties of genes
in a population
– Evolution results in a change
in the genetic composition of
a population
• Occurs at population level, not
individual level
– Genetic variation is the raw
material for selection
• In nature, genetic variation is
the rule
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http://fc01.deviantart.net/fs70/i/2011/077/d/8/genetic_variation__by_ravenousaddict-d3bwfna.jpg
• Polymorphic variation
– More than one allele at
frequencies greater than
mutation alone
• Single nucleotide
polymorphisms SNPs
– Used to assess patterns
in human and natural
populations
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Hardy–Weinberg principle
• Predicting changes in allele frequency
• Hardy–Weinberg equilibrium
– Proportions of genotypes do not change in
a population if…
1. No mutation takes place
2. No genes are transferred to or from other
sources (no immigration or emigration)
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
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• H-W principle can be written as a
binomial expansion for two alleles
– Used to calculate allele frequencies
– For 2 alleles, p and q
• p = B for black coat color
• Black cat is BB or Bb
• q = b for white coat color
• White cats are bb
p2 + 2pq + q2 = 1
BB + Bb + bb = 1
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Generation Two
B
p = 0.60
b
q = 0.40
B
p = 0.60
BB
p2 = 0.36
Bb
pq = 0.24
b
q = 0.40
Bb
pq = 0.24
bb
q2 = 0.16
Eggs
Sperm
p2 + 2 pq + q2 = 1
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• If all 5 assumptions for HardyWeinberg equilibrium are true, allele
and genotype frequencies do not
change from one generation to the
next
– In reality, most populations will not meet
all 5 assumptions
– To determine this, look for changes in
allele frequency
– Suggest hypotheses about what process
or processes are at work to cause
changes to the frequencies
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http://campus.murraystate.edu/academic/faculty/tderting/bio116/hardy_weinberg.jpg
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Five agents of evolutionary change
(violations of the 5 assumptions)
Mutation
• Mutation
– Rates generally low
– Other evolutionary
processes usually more
important in changing allele
frequency
– Ultimate source of genetic
variation
– Makes evolution possible
Mutagen
DNA
C
T
G
G
C
G
A
G
a. The ultimate source of
variation. Individual
mutations occur so
rarely that mutation
alone usually does
not change allele
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frequency much.
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• Gene flow
– Movement of alleles from
one population to another
– Several sources
• Animal physically
moves into new
population
• Drifting of gametes or
immature stages into
an area
• Mating of individuals
from adjacent
populations
Gene Flow
b. A very potent agent of
change. Individuals or
gametes move from one
population to another.
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• Nonrandom mating
– Assortative mating
• Phenotypically similar
individuals mate
• Increases proportion of
homozygous individuals
– Disassortative mating
• Phenotypically different
individuals mate
• Produces excess of
heterozygotes
Nonrandom Mating
Self-fertilization
c. Inbreeding is the most
common form. It does
not alter allele
frequency but reduces
the proportion of
heterozygotes.
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• Genetic drift
– In small populations, allele
frequency may change by chance
alone
– Magnitude of genetic drift is
negatively related to population
size
– Change in alleles is random
– Founder effect
– Bottleneck effect
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http://evolution.berkeley.edu/evosite/evo101/images/beetles_mech3.gif
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Founder effect
• Alters allele frequencies
in small populations
– Large “parent” population
– Small number of
individuals “drift” from
population
– Can lead to the random
loss of alleles in isolated
populations  loss of
genetic diversity
Genetic Drift
d. Statistical accidents.
The random fluctuation
in allele frequencies
increases as population
size decreases.
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Bottleneck effect
• If organisms do not move from place to
place their populations may be drastically
reduced (reduction of numbers “in place”)
• Survivors may constitute a random
genetic sample of the original population
• Results in loss of genetic variability
• We see this happen with habitat
destruction causing species to become
isolated
21
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UNITED
STATES
population in 1890,
reduced to inhabiting
Guadalupe only
current population
Guadalupe
MEXICO
• Northern Elephant Seal
– Bottleneck case study
– Nearly hunted to extinction in 19th century
– As a result, species has lost almost all of its
genetic variation
– Population has rebounded, now numbers in tens
of thousands, but still has high homozygosity
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http://evolution.berkeley.edu/evolibrary/images/interviews/naturalselection1.gif
• Selection (5th agent of evolutionary
change)
– Some individuals leave behind more
progeny than others, and the rate at
which they do so is affected by
phenotype and behavior
– Artificial selection (human influenced)
– Natural selection
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http://www.historyrv.com/blog/wp-content/uploads/2011/12/natural_selection.png
• Three conditions for natural
selection to occur and to result in
evolutionary change
1. Genetic variation must exist
among individuals in a population
2. Differential fitness – variation
among individuals must result in
differences in the number of
offspring surviving in the next
generation
3. Heritability – variation must be
genetically inherited
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• Natural selection and evolution are not
the same
– Natural selection is a process (mechanism)
• Only one of several processes that can result in
evolution
– Evolution is the historical record, or
outcome, of change through time
– Result of evolution driven by natural
selection is that populations become better
adapted to their environment
27
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Three agents of selection
• Selection to avoid predators
– Pocket mice come in different colors
• Dark-colored mice living on rocks are favored
• Light-colored mice living on sand are favored
Light coat color favored by
natural selection because
it matches sand color
Light coat color pocket mouse
is vulnerable on lava rock
Dark coat color favored by
natural selection because 28
it matches black lava color
http://www.nicerweb.com/bio3400/Locked/media/ch26/geographic_variation.html
Three agents of selection
• Selection to match climate conditions
– Alleles coding for enzymes that differ in their optimal
temperature vary over latitude in many species
– In mummichogs, the frequency of lactate
dehydrogenase-B allele changes with temperature &
latitude in the Chesapeake Bay
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http://snhs-plin.barry.edu/Research/MUMMICHOG_FUNDULUS_HETEROCLITUS_files/Mummichog.jpg
Three agents of selection
• Selection for pesticide
and microbial resistance
– Antibiotic resistance is
common among many
pathogenic bacteria
• MRSA (Methicillin-resistant
Staphylococcus aureus)
• MDR-TB (Multi-drug-resistant
tuberculosis)
http://www.niaid.nih.gov/topics/tuberculosis/understanding/
whatistb/visualtour/pages/xdr-tb.aspx
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Pesticide molecule
Target site
– Houseflies have
evolved pesticide
resistant alleles at…
• pen gene decreases
insecticide uptake
• kdr and dld-r genes
decrease target sites
for insecticide
Resistant target site
Insect cell membrane
a. Insect cells with resistance allele at pen gene:
decreased uptake of the pesticide.
Target site
b. Insect cells with resistance allele at kdr gene:
decreased number of target sites for the pesticide.
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Fitness and its measurement
• Fitness
– Individuals with one phenotype
leave more surviving offspring in
the next generation than
individuals with alternative
phenotypes
– Differential fitness is a “relative”
concept
• the most fit phenotype is simply the
one that produces, on average, the
greatest number of offspring
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Image by B. Tyler
• Fitness has many components
– Survival
– Sexual selection – some individuals
more successful at attracting mates
– Number of offspring per mating
– Traits favored for one component
may be a disadvantage for others
• Selection favors phenotypes
with the greatest fitness
– Phenotype with greater fitness
usually increases in frequency
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Image by B. Tyler
Interactions
• Mutations and genetic drift may counter
selection
– In nature, mutation rates are rarely high
enough to counter selection
– Selection is nonrandom but genetic drift is
random
• Drift may decrease an allele favored by
selection
• Selection usually overwhelms drift except in
small populations
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• Gene flow can be
– Constructive
• Spread beneficial mutation
to other populations
– Constraining
Endangered Florida panther may
exemplify both of these
processes…Texas panthers were
brought in to increase genetic
diversity to counter male infertility,
but since Texas panthers are
adapted to a different environment,
there is concern of “diluting”
adaptive genes in the Florida
population.
• Can impede adaptation by
continual flow of inferior
alleles from other
populations
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http://www.floridapanther.com/articles/FLORIDA%20PANTHER.htm
• Oscillating selection
– Selection favors one phenotype at one
time and another phenotype at another
time
– Effect will be to maintain genetic variation
in the population
– Medium ground finch of Galápagos Islands
• Birds with big bills favored during drought
• Birds with smaller bills favored in wet conditions
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• Heterozygote advantage
– Heterozygotes are favored over
homozygotes
– Works to maintain both alleles in the
population
– Sickle cell anemia
• Hereditary disease affecting hemoglobin
• Causes severe anemia
• Homozygotes for sickle cell allele usually die
before reproducing (without medical treatment)
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• Why is the sickle
cell allele not
eliminated?
• Leading cause of
death in central
Africa is malaria
• Heterozygotes for
sickle cell allele
do not suffer
anemia and are
much less
susceptible to
malaria
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Normal red
blood cells
Sickled red
blood cells
Sickle cell
allele in Africa
1–5%
5–10%
10–20%
Geographic
distribution of
P. falciparum
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Selection
• Many traits affected by more than one gene
• Selection operates on all the genes for the
trait
• Changes the population depending on which
genotypes are favored
• Types of selection
– Disruptive
– Directional
– Stabilizing
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– Available seeds fall
into 2 categories
– Favors bill sizes for
one or the other
0
25
50
75
100
125
Body Size (g)
Selection for small and large individuals
Number of Individuals
• Disruptive selection
• Acts to eliminate
intermediate types
• Different beak sizes
of African blackbellied seedcracker
finch
Number of Individuals
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Two peaks
form
0
25
50
75
100
125
Body Size (g)
a. Disruptive selection
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• Birds with intermediate-sized beaks are
at a disadvantage with both seed types
– they are unable to open large seeds
and too clumsy to efficiently process
small seeds
42
– Now fewer have that
behavior
Number of Individuals
0
25
50
75
100
125
Body Size (g)
Selection for larger individuals
Number of Individuals
• Directional selection
• Acts to eliminate
one extreme
• Often occurs in
nature when the
environment
changes
• In Drosophila,
artificially selected
flies that moved
toward the light
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Peak shifts
0
25
50
75
100
125
Body Size (g)
b. Directional selection
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0
25
50
75
100
125
Body Size (g)
Selection for mid-size individuals
Number of Individuals
• Stabilizing selection
• Acts to eliminate
both extremes
• Makes intermediate
more common by
eliminating extremes
• In humans, infants
with intermediate
weight at birth have
the highest survival
rate
Number of Individuals
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Distribution
gets narrower
0
25
50
75
100
125
Body Size (g)
c. Stabilizing selection
44
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births in population
infant mortality
100
70
15
50
30
20
10
10
7
5
Percent Infant Mortality
Percent of Births in Population
20
5
3
2
2
3
4
5
6
7
8
9 10
Birth Weight in Pounds
Stabilizing selection for birth weight in
humans
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Experimental studies
• To study evolution, biologists have
traditionally investigated what has
happened in the past
– Fossils or DNA evidence
• Laboratory studies on fruit flies common
for more than 50 years
• Only recently started with lab and field
experiments
46
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• Pike cichlid
(predator) rarely
found above waterfall
Killifish
(Rivulus hartii)
Guppy
(Poecilia reticulata)
– The killifish rarely eats
guppies
– Guppy males larger
and gaudier (more
colorful)
• Predator common
below waterfall
Pikecichlid
(Crenicichla alta)
Guppy
(Poecilia reticulata)
– Individuals more drab
(less colorful) and
reproduce earlier
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Limits of selection
• Multiple phenotypic effects of alleles
– Larger clutch size leads to thinner shelled eggs
• Lack of genetic variation
– Gene pool of thoroughbreds limited and
performance times have not improved for more
than 50 years
– Phenotypic variation may not have genetic basis
• Interactions between genes – epistasis
– Selective advantage of an allele at one gene may
vary from one genotype to another
48
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Kentucky Derby Winning Time
(seconds)
130
125
120
115
110
1900
1920
1940
1960
1980
2000
Year
Selection for increased speed in racehorses
is no longer effective
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