Download 16-1 16-2 lecture

Warm Up 2/22/16
Wk. 5
 Why don’t we look exactly like our
biological parents even though they
provided us with all of our genes?
Objective:
 Students will be able to explain the factors that
cause evolution and diversity
 Students will be able to recognize that new
mutations are constantly being generated in a
gene pool
 Students will be able to explain why natural
selection acts on phenotype rather than
genotype of an organism
16-1 & 16-2
GENES AND VARIATION
EVOLUTION AS GENETIC
CHANGE
Darwin in the Dark
 Darwin had two main
gaps in his theory of
evolution:
 Heritable traits??
 Yeah, I see
variation. How does
it appear? I don’t
know!
Darwin in the Dark
 By the 1930s, biologists understood
that genes control heritable traits
Changes in genes=heritable
variation=emergence of natural
selection
Ex: moth color
Darwin in the Dark
 Watson & Crick
Shed
light on mutations and
genetic variation
Remember?
 Genes have at
least two forms,
or alleles.
 Ex: Flower color
Variation and Gene Pools
 Population: A group of individuals of
the same species that interbreed
Ex: # of humans living in NY city
Variation and Gene Pools
 Members of a population interbreed
and so there is a common group of
genes they share
______: Consists of all genes
present in a population
Variation and Gene Pools
Variation and Gene Pool
 ________ of an allele is the # of
times a particular allele occurs in a
gene pool, compared with the number
of times other alleles for the same gene
occur.
Ex: Mice fur color (Next slide)
 Often expressed as a percentage
Variation and Gene Pool
Pause
WAIT
Variation and Gene Pools
 The relative frequency
of an allele has
NOTHING to do with
whether the allele is
dominant or recessive
 In the mouse fur
example, the recessive
allele occurs more
frequently than the
dominant allele
So How Are Genes and Evolution Related?
 Evolution is any change in the relative
frequency of alleles in a population
 Back to the mice fur coat example:
 If the relative frequency of the B allele
(black fur) in the mouse population
changed over time from 20% to 30%, the
population is evolving!
Sources of Genetic Variation
2 main sources of genetic
variation:
Mutations
Genetic Shuffling from sexual
reproduction
Pause
WAIT
Mutations
 Any change in a sequence of DNA
 Why do they occur?
Mistakes
in DNA replication
Radiation
Chemicals
Mutations
 Don’t always affect an
organism’s phenotype,
e.g., not all mutations
result in Marvel
superheroes!
 Many mutations do
produce changes in
phenotype
 May affect an
organism’s fitness or it
may have no affect on
fitness
Gene Shuffling
 Bringing together
new combinations
of genes
 How?
Meiosis (crossing
over, production
of gametes)
Gene Shuffling
 Sexual reproduction produces
many different phenotypes without
changing the relative frequency of
alleles in a population
 How?
Gene Shuffling
 Think of this like a deck of
playing cards
 Shuffling leads to many
different hands but does
not change the # of kings,
queens, aces, etc. in the
deck
 The probability of picking
a king from the deck will
always be 4/52 no matter
how many times you
shuffle the deck
Pause
WAIT
Single-Gene and Polygenic Traits
 The # of phenotypes produced for a
given trait depends on how many genes
control the trait
Single-Gene and Polygenic Traits
 Single trait gene:
Controlled by a single gene
that has two alleles (forms)
 Widow’s peak is a single
trait gene
 Allele for widow’s peak is
dominant over the allele for
hairline with no peak
 As a result, variation of this
gene leads to only two
distinct phenotypes.
Pause
WAIT
Single-Gene and Polygenic Traits
 Traits controlled by two or more genes
are called ____________.
 Each
gene has two or more alleles
 As a result one polygenic trait can have
many possible genotypes and phenotypes
Pause
WAIT
Single-Gene and Polygenic Traits
 An example of a polygenic trait is height
 Bell shaped curve
Evolution as Genetic Change
Why doesn’t natural selection
ever act directly on genes?
Evolution as Genetic Change
 It is the entire organism, not a single
gene, that either survives and
reproduces or dies without
reproducing.
 Natural selection can therefore only
affect which individuals survive and
reproduce and which do not.
Evolution as Genetic Change
 If an individual produces many
offspring its alleles stay in the gene pool
and may increase in frequency
 If individual dies without reproducing,
the individual does not contribute its
alleles to the population’s gene pool.
Celebrities who might not have a
chance to contribute their alleles to
the population gene pool
Natural Selection on Single-Gene Traits
 Natural selection on single gene traits
can lead to changes in allele frequencies
and thus to evolution
Natural Selection on Single-Gene Traits
 A population of normally brown lizards experiences
mutations that produce red and black forms.
 The allele frequencies may evolve to favor the allele
for black lizards since they can absorb more sunlight
and warm up faster on colder days.
Natural Selection on Single-Gene Traits
 The allele frequencies for red lizards will decrease
and not be common since they are easier to see by
their predators
Natural Selection on Polygenic Traits
 When traits are controlled by more
than one gene the effects of natural
selection are more complex
 Remember, multiple alleles on a trait
produces a range of phenotypes
Natural Selection on Polygenic Traits
 Polygenic traits produce a range of
phenotypes resembling a bell curve
Natural Selection on Polygenic Traits
 The fitness of individuals close to one
another on the curve will not be very
different
 The fitness can vary a great deal from one
end of such a curve to the other
 Where fitness varies, natural selection can
act
Natural Selection on Polygenic Traits
 Natural selection can affect the distributions
of phenotypes in any of 3 ways:
 Directional selection
 Stabilizing selection
 Disruptive selection
Directional Selection
 Directional selection: Occurs when individuals at
one end of the curve have higher fitness than
individuals in the middle or at the other end.
Directional Selection
 Ex: A population of seed eating birds
experiences directional selection when a food
shortage causes the supply of small seeds to run
low (so more large seeds in circulation).
Stabilizing Selection
 Stabilizing selection: Individuals near the
center of the curve have higher fitness than
individuals at either end of the curve
 Keeps the center of the curve at its current
position, but it narrows the overall graph.
Stabilizing Selection
 Ex: Human babies born
at an average mass are
more likely to survive
than babies born either
much smaller (less
healthy) or much larger
(difficulty being born)
than average
Disruptive Selection
 Disruptive selection: Individuals at the upper and
lower ends of the curve have higher fitness than
individuals near the middle
 Selection acts most strongly against individuals of an
intermediate type
 If pressure of natural selection is strong enough and
lasts long enough, this situation can cause the single
curve to spilt into two

In other words, selection creates two distinct phenotypes
Disruptive Selection
 If pressure of natural
selection is strong
enough and lasts long
enough, this situation
can cause the single
curve to spilt into two
 In other words,
selection creates two
distinct phenotypes
Disruptive Selection
 Ex: Average sized seeds become
less common, and larger and
smaller seeds become more
common.
 As a result, the bird population
splits into two subgroups
specializing in eating different
sized seeds.
 Birds with unusually small or
large beaks would have higher
fitness
Question
 In disruptive selection, organisms
on which part of the curve have the
lowest fitness?
Question
How does the curve change in
stabilizing selection?
Hardy-Weinberg Principle
 States that allele and genotype frequencies
in a population will remain CONSTANT
from generation to generation.
 In absence of evolutionary change
Hardy-Weinberg Principle
 To achieve genetic equilibrium, five
conditions must be met:
1.) Large population
2.) Random mating
3.) No movement in or out of population
(no immigration or emigration)
4.) No mutations
5.) No natural selection
Hardy-Weinberg Equation
 First of all, WHYYY is it useful to
learn about this equation??
Can be applied to observations for
study of population futures.
Shows how prevalent alleles are in
a population.
Hardy-Weinberg Equation
 There are two important equations:
p + q=1
(Allele freq. equation)
2
p +
2pq +
2
q =1
(Genotype freq. equation)
Hardy-Weinberg Equation
p + q=1
 p= freq. of dominant allele (for example, A)
 q= freq. of recessive allele (for example, a)
 1= total freq. of all alleles in population
Hardy-Weinberg Equation
p2 + 2pq + q2=1
 p2= freq. of homozygous dominant genotype
(for example, AA)
 2pq= freq. of heterozygous genotype (for
example, Aa)
 q2= freq. of homozygous recessive genotype (for
example, aa)
Hardy-Weinberg Equation
 Get out a sheet of paper
 Label it “Hardy-Weinberg extended
practice”
 Do the following problem (few
minutes)
 We will discuss afterwards
Hardy-Weinberg Problems
The allele y occurs with a
frequency of 0.8 in a
population of clams. Give the
frequency of genotypes YY,
Yy, and yy. Show your work!
Hardy-Weinberg Problems
The allele y has a frequency q =
0.8.
You know that p + q = 1
Hardy-Weinberg Problems
 Using that equation:
p = 1 – 0.8 = 0.2.
 Now you can estimate the frequency of
each genotype using your p and q:
 YY
genotype frequency = p2 = 0.04
 Yy genotype frequency = 2pq = 0.32
 yy genotype frequency = q2 = 0.64.
Hardy-Weinberg Problems
 A population of aliens may be blue
(the dominant phenotype) or green
(the recessive phenotype). Blue aliens
have the genotype BB or Bb. Green
aliens have the genotype bb. The
frequency of the BB genotype is .50.
 What is the frequency of the B allele?
Hardy-Weinberg Problems
 Frequency of the BB genotype is .50
(given).
 Using the equation p2 + 2pq + q2=1 we
know that p2 is the homozygous dominant
genotype.
 So: p2=.50
Hardy-Weinberg Problems
To find the frequency of the B
allele (p) we need to take the
2
square root of p and .50 from
2
the equation p =.50
The result is then p=.707=freq.
of B allele!
Hardy-Weinberg Problems
 In the same problem, what is the frequency
of heterozygous aliens? (In other words
what is 2pq)
 We know:
 p2=.50 (given)
 p=.707 (solved on last slide)
Hardy-Weinberg Problems
 To answer the question, we first need to find q
(so we can solve for 2pq).
 Remember, p + q=1
 So, .707 + q=1
 q=.293
 Now that we know q, we can determine the
frequency of heterozygous aliens (2pq)
 2(p)(q)=2(.707)(.293)= .414=freq. of
heterozygous aliens
Overwhelmed?
 PowerPoint posted on website
 Hardy-Weinberg worksheet problems
1-3 with answers posted on website
 Use as study tool for Friday’s quiz!