Download What IS a population???

Microevolution
(small scale changes in allele
frequencies)
Reading
and
assignment:
Chp 18
The Evolution
18.1, 18.7,
of Populations
18.8
2004-2005
Part I. Chromosome Nomenclature
2004-2005
What is an allele?
slightly different forms of
a gene for a given trait
(found in sexually
reproducing
populations)
Caused by a difference in
an area on a chromosome
called a locus
 a locus (plural loci) is

the specific location of
a gene or DNA
sequence on a
chromosome.
A variant of the DNA
sequence at a given
locus is called an allele.
2004-2005
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How are alleles named?
Chromosomes have banding patterns that are
evident with staining; banding patterns correlate
with different section of the chromosome.
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Each of the bands is numbered,
beginning with 1 for the band nearest
the centromere
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Example:
The chromosomal locus of a gene might be written
"6p21.3".
6 - The chromosome
number.
p - The position is on the
chromosome's short
arm (p for petit in
French
21.3 - The numbers that
follow the letter
represent the position
on the arm: band 2,
section 1, sub-band 3.
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The ends of a chromosome are called
telomeres and labeled "ptel" and "qtel",
and so "2qtel" refers to the telomere of
the long arm of chromosome 2.
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Terms to know:
What IS a population???
 A group of individual of
the same species in a
specified area
What is a gene pool?
the genes of a given
population
What are morphological
characteristics?
Traits!
 Two types:
a. those with qualitative
differences (ex. Blue or
brown eyes
b. those with
quantitative differences
(ex. Individuals show a
range of incrementally
small variations in a
trait)
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You tell me!
Phenotype?
Genotype?
Dimorphism? (-ic)
(ex. X and Y chromosome)
Polymorphism? (-ic)
(ex. ABO blood group)
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 Natural selection acts on individuals
differential survival (“survival of the fittest”)
 differential reproductive success (who
bears more offspring?)
 True adaptation and
therefore “Evolution,”
acts on Populations

 genetic makeup of
population changes
over time

favorable traits
(greater fitness)
become more common
Birch trees like acid
soils 5-6.5 pH and adapt
well to coal banks
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Part II. How does one get to
be the „right‟ variety so that
survival is allowed…?
There are many factors that
influence
variations in populations most
importantly…
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1. Mutations – abrupt inheritable change
in a gene
 Must occur in reproductive cells or

gametes (not somatic cells) in order to be
passed on
Mutation creates variation

new mutations are constantly appearing
 Each human has about 100,000 genes with
an average of 2-7 NEW mutations not
present in parents
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Mutation changes DNA sequence
(BASES)
Changes amino acid sequence can:
 change protein
 change structure? (structural proteins)
 change function? (enzymes)

changes in protein may
change phenotype &
therefore change fitness
See handouts for Protein Synthesis and
Amino acid chart!!
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Ribosome
tRNA
mRNA
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2 types of
mutations:
A. Point mutations:
single base change
in DNA sequence
Result: change in :
• nucleotide
• mRNA
• Maybe even amino
acid (and therefore
protein formed)
• Examples: PKU,
sickle cell anemia,
Tay Sachs, MS, MD,
CF
2004-2005
B. Chromosomal
mutations
Change in chromosome
due to duplication, deletion,
inversion or translocation
Usually more drastic effect
on phenotype because so
many genes are involved!!
Ex. Downs Syndrome and
C21
Most of these mutations are
BAD, some LETHAL, but
some do survive!
Gene flips
sequence:
note color
change
Switch
of parts
2004-2005
Somatic cell mutations may also
occur, but are not inheritable (ex. Skin
cancer)
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2. Sex & Variation
 Sex is a source and promoter of Variation!
one ancestor can have many descendants
 sex causes recombination of genes
 offspring have new combinations
of traits = new phenotypes

 Sexual reproduction recombines alleles into
new arrangements in every offspring
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Events during sexual reproduction
that cause new combinations…
a. Independent
assortment
during meiosis
Video of Independent
Assortment
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Crossing over of Alleles in Homologous
Chromosomes (see Handout and video)
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c. Combination of two parental
genomes
 Occurs at

fertilization
Combines to
form unique
individual
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PART III. How Variation impacts natural
selection
Natural selection requires a source of
variation within the population
there have to be differences
 some individuals must be more fit than
others

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What actually changes in populations??
 Evolution of populations is really
measuring changes in allele frequency

all the genes & alleles in a population =
gene pool
 Factors that alter allele frequencies
in a population
a. natural selection
b. genetic drift
 founder effect
 bottleneck effect
c. gene flow
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a. Natural selection
 Natural selection adapts a population to
its environment
Ex. When an environment changes…
 climate change
 food source availability
 new predators or diseases

Result: combinations of alleles
that provide “fitness”
increase in the population
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b. Recall: Genetic drift
 Defined: random change in allele frequencies
over generations
 Magnitude of effect is greatest in small
populations
 chance events drive this!

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces population to
small number & then
population recovers
& expands again
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Founder effect
 When a new population is started by
only a few individuals
some rare alleles may be at high
frequency; others may be missing
 skew the gene pool of
new population

 human populations that
started from small group
of colonists
 example: white people
colonizing New World
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Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance

 alleles lost from gene pool
 narrows the gene pool
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Effects of genetic drift show up in
inbred populations!
 Inbreeding (mating
between close
relatives)
increases the
frequency of
homozygous
individuals,
therefore lowering
the genetic
diversity
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Recall the Cheetahs
 All cheetahs share a small number of
alleles
less than 1% diversity
 as if all cheetahs are
identical twins

 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
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Conservation issues
 Bottlenecking is an
important concept in
conservation biology of
endangered species
loss of alleles from gene
pool
 reduces variation
 reduces ability to
adapt
 at risk populations

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c. Gene flow (pp 289):
defined as: physical movement of
alleles in and out of a population
by way of immigration (entering a
new place)and emigration (leaving
a place behind)
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 Population spread over large area
migrations = individuals move from one
area to another
 These sub-populations may have
different allele frequencies than larger
group

 Migrations cause genetic mixing across
regions = gene flow
new alleles are moving
into gene pool
 reduce differences
between populations

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Gene flow…
Counters (goes against) the evolutionary
effects of:
o mutations
o Natural selection
o Genetic drift
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Human evolution today
 Gene flow in human
populations is
increasing today

transferring alleles
between populations
Are we moving towards a blended world?
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Reading Assignment for Part I, II
and III of this PPT:
Reading assignment:
Chp 18
18.1, 18.7, 18.8
Complete Activity:
Biochemical Evidence for
Evolution NOW
2005-2006
Watch TED – Five Fingers of Evolution
Directions:
1. Watch this video and take notes – do
not do the „dig deeper‟ at this point.
2. Continue with PowerPoint when
finished
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Part IV. Microevolution
Measuring small changes originating in
.the genes or alleles and leading to
evolution at the population level.
Reading assignment:
18.2
pp.280-281
Hardy-Weinberg
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Population Genetics – the branch of
biology which studies and establishes
the genetic basis for evolutionary
change
 Combines Darwin's
Theory of Natural
Selection with….
 Mendel‟s
Principles of
Genetic
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Inheritance
Populations & gene pools
Concepts
1. A population is a localized group of
interbreeding individuals
Recall Mendel‟s Simple Cross:
P
Tt x Tt
T Tt t tt
F1
TT Tt Tt tt
TttT T
2. Gene pool is sum total of all genes or alleles in
the population
 Usually deals with one trait at a time
 remember difference between alleles & genes!
2004-2005
Allele: one of the alternate genes controlling a
particular trait
Example: T and t are the alleles which control the
trait of tallness. B and b are the alleles that
control the trait of eye color. T and B are not
alleles, but unrelated genes.
Allele frequency is how often an allele occurs
in a population
 refer to gene pool and count the number
present : express as % of total.
 how many A vs. a in whole population
2004-2005
Genotype – the genetic
makeup of an individual
(usually for one trait)
Phenotype – the expression of the
 Homozygous
genotype; the observable trait
dominant TT
(DNA Protein Observable Trait)
 Heterozygous Tt
 Homozygous
recessive tt
2004-2005
Hardy-Weinberg Principle (expressed
mathematically):
the frequencies of genes/alleles (and of
genotypes) will remain constant in a
population (genetic equilibrium) if
evolution is NOT occurring
G.H. Hardy
mathematician
W. Weinberg
physician
2004-2005
In order for this (genetic equilibrium) to
occur, the following must be true:
A non-evolving population will have:
1. very large population size (no genetic
drift)
2. no migration (movement in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (no selection)
Does this ever happen????
Think about it!
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Therefore…
This serves as a model ONLY!
For natural populations are rarely in H-W
equilibrium!!
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By using their equation we can …
… calculate the
frequencies of
genes and alleles
in a population
… use it as a base or
standard to
measure and
compare change in
the genetic makeup
of actual
populations
(Answers question: Is
evolution occurring
in this population?
And..if so, how
much? )
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They realized that there was a correlation between
genotypes and the gene or allele frequency
Hd
het
hr
Assume
that gene
.25
.50
.25
has two
AA
Aa
aa
alleles
A=.5
a = .5
(all the Dom genes + all the rec genes = 100% of
genes in the population)
2004-2005
Hardy-Weinberg theorem
Alleles
assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q

 frequencies must add to 100%, so:
p+q=1
BB
Bb
bb
2004-2005
Hardy-Weinberg theorem
 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 100%, so:
p2 + 2pq + q2 = 1
BB
Bb
bb
2004-2005
FYI : Math behind the equation
p + q = 1 (alleles = 100%)
(p + q)2 = 1 (we square this because each trait has
2 alleles)
p2 + (p+q) + (p+q) + q2 = 1
p2 + 2(pq) + q2 = 1
Hardy-Weinberg theorem
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Problem 1
IF the allele
frequency of D is
0.7, what is the
allele frequency of
d?
You must assume
HW population
(meaning at
equilibrium)
Answer:
Since: p(D) = 0.7
And: p + q = 1
Then: 1 – p = q
Therefore: q(d) = 0.3
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Problem 2
 If the allele freq of
g = 0.48, what is
the allele freq of
G?
Assume HW pop in
equilibrium
Answer:
Since: q(g) = 0.48
And: p + q = 1
Then: 1 – q = p
Therefore: p(G) = 0.52
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Answer (Quest 1) :
In a HW population, Since: p(A) = 0.8
And: p + q = 1
the freq of the
dominant allele, A, Then: 1 – q = p
Therefore: q(a) = 0.2
is 0.8.
Problem 3

1. What is the freq of
2.
the recessive
allele? (a)
What are the
genotypic freq of
AA, Aa, aa?
Answer (Quest 2) :
„genotypic freq‟ refers to genotype
Genotypes have TWO alleles
Therefore we use:
p2 + 2PQ + q2 = 1
(.8)2 + 2(.8)(.2) + (.2)2 = 1
.64 + .32
+ .04 = 1
hd
het
hr
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Problem 4
 If 16% of HW
population display
the recessive aa
trait, then what is
the freq of the
recessive allele, a?
If aa (q2)= 0.16 (16%, use decimals!)
Then freq of just q („a‟ allele) is 0.4
(square root of 0.16)
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Problem 5
 49% of the HW
population display the
recessive (bb) trait.
 The rest of the
population express the
dominant phenotype.
1. What is the freq of the
recessive allele, b?
2. What are the genotypic
freq of BB and Bb?




Since q2 (bb) = .49,
then q = .7 (Quest 1)
Since q = .7 Recall p + q = 1
Then p = .3
(Quest 2)
p2 + 2PQ + q2 = 1
(.3)2 + 2(.3)(.7) + (.7)2 = 1
.09 + .42
+ .49 = 1
hd
het
hr
2005-2006
THINK:
If 149 are tasters (TT or Tt)
Among 212 college
which are the (p2 = 2pq),
students, 149 are tasters then 63 of the 212 are nonof the chemical, PTC
tasters.
(phenylthiocarbamide). 63/212 = .3 = q2, then q = .55
The ability to taste PTC
(take sq rt of q2)
Problem 6

is dominant to the
If q = .55, then p = .45
(Quest 1)
inability to taste PTC.
(Quest 2)
1. What are the freq of the 2
2 = 1
p
+
2PQ
+
q
alleles of T and t?
(.45)2 + 2(.45)(.55) + (.55)2 = 1
2. What are the genotypic
.20 + .50
+ .30 = 1
freq within the
Hd
het
hr
population?
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Class Problem: Who can taste PTC?
 Find the % of non-tasters in the room = q2
 Solve for q and p
 Substitute in the p2 + 2PQ + q2 = 1 and

determine genotypic frequencies!
Compare our results with those of the N.Amer
population which is:
 55% or .55 are tasters (p2 + 2pq)
 45% or .45 are non-tasters (q2)
 Then q = .67 and p = .33
 Hint: (always figure q first because we don‟t know if
pop is HD or HET!)
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Application to determine evolution
 So…the allele freq (and genotypic freq) can be



measured across generations using this
approach.
If p and q do not change freq from one
generation to the next then the population is in
equilibrium – neither of the alleles is being
selected for or against
A change in gene freq is an indicator of natural
selection at work!
A change in gene freq can be observed if it
shows up over generations – remember that
populations evolve; individuals don‟t. 2005-2006
How do allele frequencies change?
2004-2005
Modern Synthesis
 Evolution since Darwin


comprehensive theory of evolution
took form in early 1940s
synthesis of natural selection &
Mendelian inheritance (genetics)
 also called Neo-Darwinism
 Theodosius Dobzhansky (geneticist)
 Ernst Mayr (evolutionary biologist)
Dobzhansky
 George Gaylord Simpson
(paleontologist
Mayr
2004-2005
Real world application of H-W
 Frequency of allele in human population
Example:
What % of human population carries
allele for PKU (phenylketonuria )
 ~ 1 in 10,000 babies born in the US is
born with PKU, which results in mental
retardation & other problems if untreated
 disease is caused by a recessive allele

 PKU = homozygous recessive (aa)
2004-2005
H-W & PKU disease
 frequency of homozygous recessive individuals




q2 (aa) = 1 in 10,000 = 0.0001
frequency of recessive allele (q):
q = √0.0001 = 0.01
frequency of dominant allele (p):
p (A) = 1 – 0.01 = 0.99
frequency of carriers, heterozygotes:
2pq = 2 x (0.99 x 0.01) = 0.0198 = ~2%
~2% of the US population carries the PKU allele
300,000,000 x .02 = 6,000,000
2004-2005
Instant Replay! Watch TED – Five
Fingers of Evolution
Directions:
1. Watch this video again, now with a
little more perspective!
2. Go to the „flip‟ at this site and ”Dig
Deeper” and play the simulation game!
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Essential Questions
 How do populations change over time?
 What factors can cause changes in

populations over time?
How did modern understandings of
genetics impact evolutionary thought?
2004-2005
Any Questions??
2005-2006