NATURAL SELECTION

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NATURAL SELECTION

• Review for Lecture 221

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Reviewing Chapter 20

• Darwins theory to explain how evolution happens
• Individuals have variations
• Variations are genetic
• only some offspring survive and reproduce
• Natural Selection survival and reproduction of
  the fittest.

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Reviewing Chapter 20 concepts

• Homology similarities shared by species with a
  common ancestor
• Analogy similarities without a close ancestor.
  
• Phylogeny family tree

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Figure 21.2a
Structural homology
Humerus
Radius and ulna
Carpals
Metacarpals
Phalanges
Turtle
Human
Horse
Bird
Bat
Seal
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Figure 21.2c
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Figure 21.2b
Developmental homology
Both the chick and the human have gill pouches
and tails
Gill pouch
Tail
Chick
Human
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Figure 21.4, left
Human coccyx
Capuchin monkey tail (used for balance,
locomotion)
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Figure 21.4, right
Erect hair on chimp (insulation, emotional
display)
Human goosebumps
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Reviewing Darwin's concept

• Natural Selection survival and reproduction of
  the fittest.

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Did Mycobacterium tuberculosis become resistant
to rifampin by natural selection?

•      
• how to answer
• compare with Darwins postulates

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Darwin's four postulates

• Individuals have variations. What was the TB
  variation in this case?
• Variations are genetic. What experimental
  results could show that this variation is
  genetic?
• only some offspring survive and reproduce. What
  experimental results could show that this TB
  variation affects survival or reproduction of TB
  germs?
• Natural Selection survival and reproduction of
  the fittest.

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On to chapter 22.

•       most important concept
• When a particular allele increases the
  survivorship or fecundity or immigration of
  individuals, the frequency of that allele
  increases in the population's next generation.

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most important concept

• When a particular allele increases the
  survivorship or fecundity or immigration of
  individuals, the frequency of that allele
  increases in the population's next generation.
  a modern view of Natural Selection
• Darwin's concept of Natural Selection is a theory
  to explain evolution.
• DNA -gt more DNA --gt RNA -gt protein (adaptation)
  is still important in understanding the
  relationship between Natural Selection and
  inheritance.

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To measure genetic variation (and
even just to think about genetic variation)
biologists use frequency distributions.

• Example fromchapter 1

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To measure genetic variation (and
even just to think about genetic variation)
biologists use frequency distributions.

• Example fromchapter 10

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Figure 22.7b
For example, directional selection caused
overallbody size to increase in a cliff swallow
population
40
35
30
25
20
Nonsurvivors N 1853
15
10
5
Difference in average
0
Percentage of birds
40
Survivors N 1027
35
30
25
20
15
10
5
0
6
1
2
3
4
5
7
8
9
10
11
12
Body size class
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Figure 22.7a
Directional selection changes the average value
of a trait.
Normal distribution
Before selection
During selection
Number of individuals
After selection
Value of a trait
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Figure 22.9b
For example, only juvenile blackbellied
seedcrackers with very longor very short beaks
survived long enough to breed.
30
20
Number of individuals
10
0
11
6
7
10
8
9
Beak length (mm)
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Figure 22.9a
Disruptive selection increases the amount of
variation in a trait.
Normal distribution
Before selection
Low fitness
Number of individuals
During selection
After selection
Value of a trait
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Figure 22.8b
For example, very small and very large babies are
most likely to die, leaving a narrower
distribution of birthweights.
100
20
70
50
Mortality
15
30
20
Percentage of mortality
Percentage of Population
Heavy mortality on extremes
10
10
7
5
5
3
2
0
1
2
3
4
5
6
7
8
9
10
11
Birthweight (pounds)
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Figure 22.8a
Stabilizing selection reduces the amount of
variation in a trait.
Normal distribution
Before selection
High fitness
During selection
Number of individuals
After selection
Value of a trait
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Figure 21.7 a,b
In tundra habitats above timberline, the alpine
skypilot is pollinated primarily by bumblebees.
28 24 20 16 12 8 4 0
Number of individuals
10 12 14 16 18 20 22
Tundra flower big and sweet-smelling
Flower size (mm)
In forested habitats below timberline, the alpine
skypilot is pollinated primarily by flies.
10
8
6
Number of individuals
4
2
0
10 12 14 16 18 20 22
Below-timberline flower small and
skunky-smelling
Flower size (mm)
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Figure 21.8
110
100
80
60
Bee visits received
40
20
0
3
2
1
0
1
2
4
Size score
Large flowers
Small flowers
Short stems
Tall stems
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Figure 21.9
1.0
0.8
0.6
Relative fitness (fecundity)
0.4
0.2
0
40
0
20
60
80
100
110
Bee visits received
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Figure 21.11
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Applying Darwin's ideas

• Did some skyrockets inherit adaptations which
  helped them survive and reproduce better than
  others in a specific habitat?

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Figure 22.9a
Disruptive selection increases the amount of
variation in a trait.
Normal distribution
Before selection
Low fitness
Number of individuals
During selection
After selection
Value of a trait
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natural selection EXPERIMENTS

• Other experiments over centuries
• fossils
• mummies
• ice men and other frozen specimens
• cave coprology etc.
• museum pelts

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EVIDENCE WE CAN OBSERVE

• Extinctions
• fossils
• structural homologies
• developmental homologies
• genetic homologies
• vestigial traits
• changes in adaptations
• repeated patterns in all of the above

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In Hardy-Weinberg EquilibriumFrequency
distributions DO NOT CHANGE

• The Hardy-Weinberg Law is a mathematical proof
  that frequency distribution of alleles stays in
  equilibrium in other words, genetic frequency
  cannot change from one generation to another
  unless something "selects" one allele over its
  alternatives these "somethings" which do change
  the equilibrium are important.

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Hardy-Weinberg equilibrium no evolution

• conditions
• no mutation
• no migration ( no gene flow)
• large population with no genetic drift
• random mating
• no selection no genetic advantage in survival
  or reproduction

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No net Mutation

• A can mutate to a only if
• a is equally likely to mutate to A

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No Migration No Gene Flow

• When gene flow occurs,
• New individuals join a gene pool
• certain phenotypes could be more likely to
  migrate, changing the allele frequencies

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No Genetic Drift

• change in the allele frequencies in a population
  due to random chance.
• small populations are very likely to lose genetic
  diversity by luck and by inbreeding.

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No Natural Selection

• Because obviously some genotypes produce
  phenotypes which are more likely to survive and
  breed.

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All Mating Must be RANDOM

• Because a genetic difference in mating success
  will obviously lead to a higher frequency of
  offspring with the sexy genes.
• So no sexual selection allowed
• and no small populations with inbreeding allowed
  either.

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Hardy-Weinberg equilibrium no evolution

• The factors which can change the Hardy-Weinberg
  equilibrium give biologists
• a method of measuring the rate of evolution and
• they guide biologists to focus on the possible
  sources of changes in frequency distribution

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Hardy-Weinberg equilibrium no evolution

• conditions
• no mutation
• no migration ( no gene flow)
• large population with no genetic drift
• random mating
• no selection no genetic advantage in survival
  or reproduction

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Opposite of Hardy-Weinberg evolution change
in allele frequency

• Mechanisms that change allele frequencies in
  populations
• Natural selection
• Mutation
• Gene flow
• Genetic drift
• BUT natural selection is the only mechanism that
  results in adaptation and leads to increased
  fitness.

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Non-random mating

• In nature, matings between individuals are
  seldom, if ever, random.
• Individuals may choose mates. (Unattractive
  individuals may be excluded from the gene pool.)
• Individuals may compete for mates.
• In small populations, matings between relatives
  are common. This is known as inbreeding.

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Figure 22.5
SURVEYING ALLELIC DIVERSITY IN POPULATIONS
1. Take blood samples from many individuals and
isolate proteins.
2. Load protein samples from different
individuals into wells in gel.
3. Put gel into an electric field. Proteins
separate according to charge and mass.
4. Treat gel with a solution that stains a
specific enzyme. One band implies that the
individual is homozygous at the locus for the
enzyme. Two bands imply that the individual is
heterozygous at this locus.
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Chapter 22
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Table 22.3
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Inbreeding

• Inbreeding increases the proportion of
  homozygotes and reduces the proportion of
  heterozygotes in any populationin which it
  occurs. (Fig 22.6)
• Inbreeding depression is the loss of fitness that
  takes place when homozygosity is increased.
  (Table 22.3)

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Sexual Selection

• Another type of non-random mating
• a continuing controversy.
• Darwin invented this term to explain cases in
  which bright colors and fancy equipment, like a
  peacock's tail, seem to evolve simply to attract
  mates despite their probable disadvantages in
  survival.

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Figure 22.10 a,b,c
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Figure 22.11a
Males compete for the opportunity to mate with
females.
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Darwin's four postulates

• Individuals have variations
• Variations are genetic
• only some offspring survive and reproduce
• Natural Selection survival and reproduction of
  the fittest.

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Mechanisms of Evolutionary Change

• Evolution is defined as a change in allele
  frequencies over time.
• Natural selection acts on individuals, but
  evolutionary change occurs in populations.

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Analyzing Allele Frequency Change The
Hardy-Weinberg Model

• If no evolution is occurring, then allele
  frequencies will be the same in a parental and
  offspring generation.

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Figure 22.3
DERIVING THE HARDY-WEINBERG PRINCIPLE-A NUMERICAL
EXAMPLE
P1 frequency of allele A1 0.7
1. Suppose that the allele frequencies in the
parental generation were 0.7 and 0.3.
P2 frequency of allele A2 0.3
Gametesfrom parent generation
2. 70 of the gametes in the gene pool carry
allele A1 and 30 carry allele A2 .
3. Pick two gametes at random from the gene pool
to form offspring. Three genotypes are possible.
A2
A1
A1
A2
A1
A1
A2
A2
.07 x 0.30.21
.03 x 0.70.21
0.7 x 0.7 0.49
0.3 x 0.3 0.09
0.21 0.21 0.42
Homozygous
Heterozygous
Homozygous
4. Calculate the frequencies of these three
combinations of alleles.
Gametesfrom offspring generation
5. When the offspring breed, imagine that their
gametes go into a gene pool.
6. Calculate the frequencies of the two alleles
in this gene pool.
42 of the gametes are from A1A2 parents. Half of
these carry A1and half carry A2
49 of the gametes are from A1A1 parents. All of
these carry A1
9 of the gametes are from A2A2 parents. All of
these carry A2
BEHOLD! The allele frequencies of A1and A2 have
not changed from parent generation to offspring
generation. Evolution has not occurred.
P1 frequency of allele A1 (0.49 1/2(0.42))
(0.49 0.21) 0.7
P2 frequency of allele A2 (1/2(0.42) 0.09)
(0.21 0.09) 0.3
Genotype frequencies will be given by p12
2p1p2 p22 as long as all Hardy-Weinberg
assumptions are met
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Figure 22.4
1. Start long-term experiment by placing 10 mL of
identical growth medium and a genetically
identical E. coli cell to many replicate tubes.
2. Incubate overnight. Average population in each
tube is now 5 x 108 cells.
3. Remove 0.1 mL from each tube and move to 10
mL of fresh medium. Freeze remaining cells for
later analysis.
4. Take cells from generation 1 and add a genetic
marker so that they can be identified.
5. Put an equal number of cells from generation 1
and a later generation in fresh growth medium.
6. Incubate overnight and count the cells. Which
are more numerous?
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most important concept

•      
• Darwins theory NATURAL SELECTION

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most important concept

•      
• DNA -gtmore DNA --gt RNA -gt adaptation
• This "central dogma" of today's molecular biology
  has applications
• in natural selection.

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MORE ABOUT

• Darwin http//www.queens.edu/faculty/jannr/darwin.
  htm
• Creationism
• http//www.queens.edu/faculty/jannr/creationism.ht
  m
• Evolution
• http//www.queens.edu/faculty/jannr/evolution.htm