Transcript Slide 1

Lecture #2 – Evolution of
Populations
Image of a population of penguins
1
Key Concepts:
•
•
•
•
•
•
•
The Modern Synthesis
Populations and the Gene Pool
The Hardy-Weinberg Equilibrium
Micro-evolution
Sources of Genetic Variation
Natural Selection
Preservation of Genetic Variation
2
Review definitions
• Species – individual organisms capable of
mating and producing fertile offspring
• Population – a group of individuals of a
single species
• Community – a group of individuals of
different species
Images – species, population, community
3
The Modern Synthesis
integrates our knowledge about
evolution
•
•
•
•
Darwin’s natural selection
Mendel’s hereditary patterns
Particulate transfer (chromosomes)
Structure of the DNA molecule
All explain how the genetic structure
of populations changes over time
4
KEY POINT
Environmental factors act on the
individual to control the genetic future of
the population
Individuals don’t evolve…..populations do
* ** ** **
** ** * * * * * *** **
*
* *
* * *
* *
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Population = a +/- localized group
of individuals of one species
Image – population of iris
6
Critical Thinking
• How do we determine the boundaries of a
population???
7
Critical Thinking
• How do we determine the boundaries of a
population???
Boundaries are scale dependent
Some sub-populations overlap
Some are more isolated
We can look at populations at many different
scales – micro to meta
8
Recall basic genetic principles:
• In a diploid species (most are), every
individual has two copies of every gene
One copy came from each parent
• Most genes have different versions = alleles
• Diploid individuals are either heterozygous
or homozygous for each gene
Heterozygous = Aa
Homozygous = AA or aa
9
Recall basic genetic principles:
• The total number of alleles for any gene in
a population is the number of individuals in
the population x 2
If the population has 10 individuals, there are
20 copies of the A gene – some “A” alleles
and some “a” alleles
• All these alleles comprise the “gene pool”
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Hardy-Weinberg Theorem
• Gene pool = all alleles in a population
• All alleles have a frequency in the
population
There is a percentage of “A” and a
percentage of “a” that adds up to 100%
• Hardy-Weinberg Theorem demonstrates
that allele frequencies don’t change
through meiosis and fertilization alone
11
Hardy-Weinberg Theorem
• A simple, mathematical model
• Shows that repeated random meiosis and
fertilization events alone will not change the
distribution of alleles in a population
Even over many generations
p2 + 2pq + q2 = 1
we will not focus on the math – you’ll
work on this in lab
12
Hands On
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•
•
•
The equation: p2 + 2pq + q2 = 1 (Page 2)
p = the frequency of one allele
q = the frequency of the other allele
p + q MUST = 1 = 100% of the gene pool
13
Hands On
• If the allele frequencies are known, the
HW equilibrium can be demonstrated by a
Punnett square
Assume that we
know there are
½ T and ½ t
alleles
Paternal
Parent
Maternal
Parent
T
t
T
TT
Tt
t
Tt
tt
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Hands On
• What are the frequencies of each allele in
the F1 generation?
Assume that we
know there are
½ T and ½ t
alleles
Paternal
Parent
Maternal
Parent
T
t
T
TT
Tt
t
Tt
tt
15
Hands On – Results
• What are the frequencies of each allele in
the F1 generation?
Assume that we
know there are
½ T and ½ t
alleles
T
Paternal
Parent
t
Maternal
Parent
T
t
TT
Tt
.5x.5=.25
.5x.5=.25
Tt
tt
.5x.5=.25
.5x.5=.25
16
Hands On – Results
p2 + 2pq + q2 = 1
.52 + 2x.5x.5 + .52 = 1
25% TT : 50% Tt : 25% tt
50% T and 50% t = no change
Assume that we
know there are
½ T and ½ t
alleles
T
Paternal
Parent
t
Maternal
Parent
T
t
TT
Tt
.5x.5=.25
.5x.5=.25
Tt
tt
.5x.5=.25
.5x.5=.25
17
Hands On – Results
p2 + 2pq + q2 = 1
25% TT : 50% Tt : 25% tt
50% T and 50% t = no change
This goes on generation after generation
The phenotype remains 75% dominant and 25%
recessive
Assume that we
know there are
½ T and ½ t
alleles
T
Paternal
Parent
t
Maternal
Parent
T
t
TT
Tt
.5x.5=.25
.5x.5=.25
Tt
tt
.5x.5=.25
.5x.5=.25
18
Hands On
• How do you determine the allele
frequencies???
• How do you find p and q???
• In this example, how do you know the
percentage of T and the percentage of t???
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Hands On – Results
• Remember that the recessive phenotype
is tt
• If you know the percentage of the
population that expresses the recessive
phenotype, then t (q) is the square root of
that number
p2 + 2pq + q2 = 1
25% TT : 50% Tt : 25% tt
75% express dominant; 25% express recessive
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√.25 = .5; determine p by subtraction
Hands On
• Clasp your hands
21
Hands On
• Count right thumbs up vs. left thumbs up
• Right thumb up is the recessive condition!
• Determine the distribution of T and t in our
class population
• Type up a summary of your results and
turn in tomorrow
22
Hardy-Weinberg Theorem
• Meiosis and fertilization randomly shuffle
alleles, but they don't change proportions
Like repeatedly shuffling a deck of cards
The laws of probability determine that the
proportion of alleles will not change from
generation to generation
• This stable distribution of alleles is the
Hardy-Weinberg equilibrium
Doesn’t happen in nature!!!
24
Conditions for H-W Equilibrium:
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No natural selection
Large population size
Isolated population
Random mating
No mutation
Doesn’t happen in nature!!!
The violation of each assumption acts as
an agent of microevolution
25
The value of H-W???
• It provides a null hypothesis to compare to
what actually happens in nature
• Allele frequencies DO change in nature
• BUT, they change only under the
conditions of microevolution
In nature, all the H-W assumptions are violated
• Result – populations DO evolve
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Critical Thinking
• What are the limitations of the HardyWeinberg theorem???
27
Critical Thinking
• What are the limitations of the HardyWeinberg theorem???
• The H-W model considers just one trait at a
time, and assumes that just one gene with 2
alleles (one completely dominant) controls
that trait
• Recall your basic genetics – is this
realistic???
28
Critical Thinking
• Reality is much more complex for most traits
in most organisms
Incomplete dominance or codominance
More than 2 alleles for many genes
Pleiotropy – one gene affects multiple traits
Polygenic traits – multiple genes affect one trait
Epistasis – one gene affects expression of
another gene
Environmental effects on phenotypic expression
• Reproductive success depends on the way
all genes and phenotypic traits interact 29
Individuals Do Not Evolve
• Individuals vary, but populations evolve
• Natural selection pressures make an
individual more or less likely to survive and
reproduce
• But, it is the cumulative effects of selection
on the genetic makeup of the whole
population that results in changes to the
species
The environment is a wall; natural selection is a gate
30
The environment is the wall;
natural selection is the gate
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** ** * * * * * *** **
*
* *
* * *
* *
?
*****
*****
31
Micro-evolution:
population-scale changes in allele
frequencies
•
•
•
•
•
Natural Selection
Genetic Drift
Gene Flow
Selective Mating
Mutation
Image – natural variation in
flower color; same image for
all these summary slides
32
Natural Selection – the essence of
Darwin’s theory
Cartoon – beaver with chainsaw paws 
“natural selection does not grant
organisms what they “need””
More on this later….
More on this later….
Differential reproductive success is the only
way to account for the accumulation of
favorable traits in a population
33
Micro-evolution:
population-scale changes in allele
frequencies
•
•
•
•
•
Natural Selection
Genetic Drift
Gene Flow
Selective Mating
Mutation
34
Genetic Drift – random changes in allele
frequency from generation to generation
• Reproductive events are samples of the
parent population
Larger samples are
more representative
than smaller samples
(probability theory)
Parent pop = 10% blue
1
2
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Larger pop = ~29% blue
Smaller pop = 100% blue
Genetic Drift – random changes in allele
frequency from generation to generation
• More pronounced in smaller and/or more
segregated populations
Bottleneck effect
Founder effect
Parent pop = 10% blue
1
2
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Segregated pop = ~29% blue
Segregated pop = 100% blue
Bottlenecking = extreme genetic drift
Diagram – bottlenecking
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Critical Thinking
• What events could cause a bottleneck???
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Critical Thinking
• What events could cause a bottleneck???
Bottlenecks occur when there is an extreme
and indiscriminate reduction in the
reproducing population
Disease
Herbivory
Malnutrition
Major disturbance (flood, fire)
Human intervention
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Conservation implications – cheetahs are a bottlenecked species
Image – cheetah
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Maps – historic and current range
of cheetahs
Extreme range
reduction due to
habitat destruction
and poaching
+
Cheetahs were
naturally bottlenecked
about 10,000 years
ago by the last major
ice age (kinked tail)
The species is at risk
of extinction 41
Australian Flame Robin, California Condor,
Mauritian Kestrel
…..and many more, all driven nearly to extinction…..
Images – bottlenecked and now endangered species
Some colorful results of a quick web search on “bottlenecked species”
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Founder Effect = extreme genetic drift
• Occurs when a single individual, or small
group of individuals, breaks off from a
larger population to colonize a new habitat
Islands
Other side of mountain
Other side of a river…
• This small group may not represent the
allele distribution of the parent population
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Founder Effect
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Long distance dispersal events can lead to
the founder effect
Image – a founding population of
seeds; possibly also the bird if it’s a
gravid female
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Critical Thinking
• What do you think follows long distance
dispersal to a new ecosystem???
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Critical Thinking
• What do you think follows long distance
dispersal to a new ecosystem???
• Adaptive radiation frequently leads to
many new, closely related species as the
organisms adapt to new habitat zones in
their new home
1
Founding
Population
2
3
4
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Hands On
• Genetic drift is random
• Some drift is expected with every generation
Genetic drift is not necessarily extreme
• Use the beads to explore this idea (Page 4)
Count out 50 beads each of 2 colors
Each bead represents an allele in the gene pool
Since B and b are in equal proportion, what is
the phenotypic makeup of the diploid
population???
50
p2 + 2pq + q2 = 1
Hands On – Results
• Genetic drift is random
• Some drift is expected with every generation
Genetic drift is not necessarily extreme
• Use the beads to explore this idea
Count out 50 beads each of 2 colors
Each bead represents an allele in the gene pool
Since B and b are in equal proportion, what is
the phenotypic makeup of the diploid
population???
75% express dominant; 25% express recessive
51
Hands On
• Now simulate random mating by shaking
up the beads and pulling out 2 beads at a
time, with your eyes closed
Be sure to return the beads to the “gene pool”
We are sampling with replacement – random
• Record each offspring allele structure
Be sure to assign a dominant and recessive
color
• Repeat 50 times
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Shake the pool each time to maintain random
Hands On
• Count and record the allele structure of
your second generation
• Make a new gene pool of 100 beads that
reflects this new allele structure
• Repeat the bead selection for a 3rd
generation
• Repeat for a total of 5 generations
53
Hands On
• Is your 5th generation allele structure the
same as your 1st generation???
• What is the phenotypic distribution of your
5th generation?
• What are your conclusions?
• Use your lab notebook to record
observations
54
Hands On
• Now simulate a bottleneck
• Shake up the beads and pull out 2 beads
at a time, with your eyes closed
Be sure to return the beads to the “gene pool”
We are sampling with replacement – random
• Record each offspring allele structure
Be sure to assign a dominant and recessive
color
• Repeat 5 times
55
Shake the pool each time to maintain random
Hands On
• Count and record the allele structure of
your second generation
• Make a new gene pool of 100 beads that
reflects this new allele structure
• Repeat the bead selection for a 3rd
generation
• Repeat for a total of 5 generations
56
Hands On
• Is your 5th generation allele structure the
same as your 1st generation???
• What is the phenotypic distribution of your
5th generation?
• What are your conclusions?
• Compare 50 vs. 5 reproductive “events”
• Use your lab notebook to record
observations, and type up a summary to
turn in tomorrow
57
Micro-evolution:
population-scale changes in allele
frequencies
•
•
•
•
•
Natural Selection
Genetic Drift
Gene Flow
Selective Mating
Mutation
58
Gene Flow
• Mixes alleles between populations
Immigration
Emigration
• Most populations are NOT completely
isolated
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Critical Thinking
• Will gene flow tend to increase or
decrease speciation???
60
Critical Thinking
• Will gene flow tend to increase or
decrease speciation???
• Gene flow tends to preserve species by
shuffling alleles between all subpopulations
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Gene Flow
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Hands On
• How could we demonstrate gene flow with
our beads???
63
Hands On – Results
• How could we demonstrate gene flow with
our beads???
Have bead pairs migrate between teams
• Would beads be more likely to migrate
within a bench or between benches???
What are the implications for speciation?
64
Micro-evolution:
population-scale changes in allele
frequencies
•
•
•
•
•
Natural Selection
Genetic Drift
Gene Flow
Selective Mating
Mutation
65
Selective Breeding
Image – male peacock with mating display
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Critical Thinking
• Animal behaviors are obvious examples
• Can you think of others???
67
Critical Thinking
• Animal behaviors are obvious examples
• Can you think of others???
• Proximity is important even in species that
do not have mating behaviors
Many plants and fungi are randomly fertilized
or pollinated…..but generally the exchange is
between closer neighbors
Image –
fungi spores
68
Micro-evolution:
population-scale changes in allele
frequencies
•
•
•
•
•
Natural Selection
Genetic Drift
Gene Flow
Selective Mating
Mutation
69
Mutations
• Random, rare, but
regular events
• The only source of
completely new traits
Diagram – mutations in
DNA strand
just for fun…..
Cartoon - jackalope
70
Evolution =
random events
x
“the gate”
* ** ** **
** ** * * * * * *** **
*
* *
* * *
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Review: Micro-evolution:
population-scale changes in allele
frequencies
•
•
•
•
•
Natural Selection
Genetic Drift
Gene Flow
Selective Mating
Mutation
72
Sources of Genetic Variation
• Natural selection acts on natural variation
• Where does this variation come from???
Meiosis
Mutation
• Additional mechanisms help preserve
variation (later)
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Meiosis = key source of variation
Diagram – meiosis I
74
Diagram – meiosis II
75
Random, Independent Assortment
of Homologous Chromosomes
n=2
Diagram – results of meiosis with n=2
76
Probability theory reveals that for
random, independent events:
• If each event has 2 possible outcomes
In this case, one side of the plate or the other
• The possible number of distribution
combinations = 2n, where n = the number of
events
In this case, the distribution event is the
distribution of chromosomes to the gametes
n = the haploid number of chromosomes
• If n is 2, then combinations are 22 = 4
77
Random, Independent Assortment
of Homologous Chromosomes
n=2
Diagram – results of meiosis with n=2
Four
possible
distributions
78
Probability theory reveals that for
random, independent events:
• If each event has 2 possible outcomes
In this case, one side of the plate or the other
• The possible number of distribution
combinations = 2n, where n = the number of
events
In this case, distribution refers to the distribution
of chromosomes to the gametes
n = the haploid number of chromosomes
• If n is 23, then combinations are 223 = 8.4
million!
79
Probability is Multiplicative:
8.4 million x 8.4 million > 70 trillion!!!
That is the number of possible combinations
of maternal and paternal chromosomes in
the offspring of a randomly mating pair of
humans
80
Recombination
increases the
potential
variation to
infinity
Diagram – recombination
81
Critical Thinking
• Can meiosis produce totally new traits???
82
Critical Thinking
• Can meiosis produce totally new traits???
• No – remember, normal meiosis just
shuffles the alleles
• Only mutation can make entirely new
alleles
83
Natural Selection as a Mechanism
of Evolutionary Adaptation
• Natural selection acts on the variation
produced by meiosis and mutation
• Selection increases the “fitness” of a
population in a given environment
• Fitness = ???
84
Natural Selection as a Mechanism
of Evolutionary Adaptation
• Natural selection acts on the variation
produced by meiosis and mutation
• Selection increases the “fitness” of a
population in a given environment
• Fitness = reproductive success
NOT big, NOT smart, NOT strong
The production of successful offspring is the
key
85
Natural selection has limits
• Individuals vary in fitness
Natural selection promotes the most fit
• Selection acts on the phenotype – the
whole, complex organism
Results from the combination of many different
genes for any organism
These genes are expressed in the whole,
complex environment
• Selection is always constrained by the
whole, complex evolutionary history of the
species
86
Critical Thinking
• Can evolution respond to “needs”???
Beaver cartoon again
87
Critical Thinking
• Can evolution respond to “needs”???
• NO!!!
• Evolution is a combination of random
events + successful reproduction in a
given environment
• The environment is the wall; natural
selection is the gate!!!!
If the phenotype “works”, the genotype
passes through the gate
88
Hands On
• Calculate the allele distribution to the F1
with the dominant phenotype resulting in
a 20% decline in the reproductive success
rate (Page 3, with a twist)
• The twist – start with a 50/50 distribution of
dominant and recessive alleles in the gene
pool
89
Hands On – Results
•
•
•
•
•
•
p2 + 2pq + q2 = 1
50% T; 50% t
25% TT : 50% Tt : 25% tt
75% dominate phenotype; 25% recessive
F1 would include .75 x .80 = 60% TT or Tt
By subtraction = 40% tt
The frequency of t is thus .63 (the √.4)
• By subtraction, the frequency of T is now .37
90
Hands On
• Calculate the allele distribution to the F1
with the recessive phenotype resulting in
100% mortality (Page 3, with a twist)
• The twist – start with a 50/50 distribution of
dominant and recessive alleles in the gene
pool
91
Hands On – Results
•
•
•
•
•
p2 + 2pq + q2 = 1
50% T; 50% t
25% TT : 50% Tt : 25% tt
75% dominate phenotype; 25% recessive
F1 would include zero tt
The t allele only present in heterozygotes
The frequency of t is thus (½ of 2pq) / (p2 + 2pq)
• The frequency of t = .5(.5) / (.25 + .5) = .33
92
• By subtraction, the frequency of T is now .67
Hands On
• In either case would either the T or t allele
become extinct?
• Why or why not?
93
Hands On – Results
• In either case would either the T or t allele
become extinct?
• Why or why not?
I think the T allele can become extinct since
the phenotype is deleterious (I only did the
math out one more generation)
I think the t allele will always remain hidden in
heterozygous individuals
• Homework – carry both questions out to 5
generations, or extinction (print clearly!) 94
Patterns of Change by Natural
Selection
• Directional Selection
• Diversifying Selection (AKA disruptive)
• Stabilizing Selection
Diagram –
patterns of
natural
selection
95
Remember, all populations exhibit a
range of natural variation
Diagram – patterns of natural selection
96
Directional Selection
• Phenotypes at one extreme of the range
are most successful
Color
Pattern
Form
Metabolic processes
• The population shifts to favor
the successful phenotype
Diagram –
directional
selection
97
Diversifying Selection
• Multiple, but not all, phenotypes are
successful
Patchy environments
Sub-populations migrate to new habitats
• The population begins to fragment and
new species begin to diverge
Diagram –
diversifying
selection
98
Stabilizing Selection
• The intermediate phenotypes are most
successful
Homogenous environments
Stable conditions
• The range of variation within the
population is reduced
Diagram –
stabilizing
selection
99
Critical Thinking
• Which selection mode will most quickly
lead to the development of diversity???
100
Critical Thinking
• Which selection mode will most quickly
lead to the development of diversity???
• Diversifying selection tends to produce
multiple species, and the parent species
may also persist
101
Critical Thinking
• Can you think of a real-life example of an
adaptive phenotype???
102
Critical Thinking
• Can you think of a real-life example of an
adaptive phenotype???
• Everything!
Variation is random
Selection is adaptive
103
Preservation of Natural Variation
• Diploidy
• Balanced Polymorphism
• Neutral Variation
Images – natural variation in flower color
104
Diploidy – 2 alleles for every gene
• Recessive alleles retained in heterozygotes
Not expressed
Not eliminated, even if the recessive trait is
aa may be eliminated, while Aa is preserved in
the population
• Recessive alleles function as latent
variation that may prove helpful if
environment changes
105
Balanced Polymorphism
• Heterozygote advantage
• Frequency dependent selection
• Phenotypic variation
106
Balanced Polymorphism – heterozygote
advantage
a mutation in the gene that codes for hemoglobin causes a single amino acid
substitution in the protein, RBC shape changes from round to sickle shape
Sickle-cell Anemia
Map – global distribution of sickle cell allele
Images – normal and sickled red blood cells
107
Balanced Polymorphisms – Frequency
Dependent Selection
rare clone is less infected
Graph – frequency dependent selection results
108
Balanced Polymorphisms – Phenotypic Variation
multiple morphotypes are favored by heterogeneous
(patchy) environment
Images – balanced polymorphisms in asters and snakes
109
Neutral Variation
• Genetic variation that has no apparent
effect on fitness
• Not affected by natural selection
• May provide an important base for future
selection, if environmental conditions
change
110
Key Concepts: QUESTIONS???
•
•
•
•
•
•
•
The Modern Synthesis
Populations and the Gene Pool
The Hardy-Weinberg Equilibrium
Micro-evolution
Sources of Genetic Variation
Natural Selection
Preservation of Genetic Variation
111