Download GENETICS & EVOLUTION: population genetics

GENETICS & EVOLUTION:
POPULATION GENETICS
Chapter 3.3
Overview





Microevolution and mutation
Hardy-Weinberg Principle and Equilibrium
Natural Selection
Genetic drift
Gene flow
Overview: The Smallest Unit of Evolution
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One misconception is that organisms evolve, in the
Darwinian sense, during their lifetimes
Natural selection acts on individuals, but only
populations evolve
Genetic variations in populations contribute to
evolution
Microevolution is a change in allele frequencies in a
population over generations
Fig. 23-1
Mutation and sexual reproduction produce the
genetic variation that makes evolution possible

Two processes, mutation and sexual reproduction,
produce the variation in gene pools that contributes
to differences among individuals
Gene Pools and Allele Frequencies
•
•
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The first step in testing whether evolution is
occurring in a population is to clarify what we
mean by a population
A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
A gene pool consists of all the alleles for all loci in a
population
A locus is fixed if all individuals in a population are
homozygous for the same allele
Fig. 23-5
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
Fig. 23-5a
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range

The frequency of an allele in a population can be
calculated
 For
diploid organisms, the total number of alleles at a locus
is the total number of individuals x 2
 The total number of dominant alleles at a locus is 2 alleles
for each homozygous dominant individual plus 1 allele for
each heterozygous individual; the same logic applies for
recessive alleles
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
By convention, if there are 2 alleles at a locus, p and q
are used to represent their frequencies
The frequency of all alleles in a population will add up
to 1
 For
example, p + q = 1
The Hardy-Weinberg Principle
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
The Hardy-Weinberg principle describes a
population that is not evolving
If a population does not meet the criteria of the
Hardy-Weinberg principle, it can be concluded that
the population is evolving
Hardy-Weinberg Equilibrium
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
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The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a population
remain constant from generation to generation
In a given population where gametes contribute to
the next generation randomly, allele frequencies will
not change
Mendelian inheritance preserves genetic variation in
a population
Fig. 23-6
Alleles in the population
Frequencies of alleles
p = frequency of
CR allele
= 0.8
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
80%
chance
20%
chance
Each sperm:
80%
chance
20%
chance
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
Hardy-Weinberg equilibrium describes the constant
frequency of alleles in such a gene pool
If p and q represent the relative frequencies of the
only two possible alleles in a population at a
particular locus, then
 p2
+ 2pq + q2 = 1
 where p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the frequency
of the heterozygous genotype
Fig. 23-7-1
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
64% (p2)
CRCR
16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CW CW
Fig. 23-7-2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Fig. 23-7-3
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
Fig. 23-7-4
20% CW (q = 0.2)
80% CR ( p = 0.8)
Sperm
CR
(80%)
CW
(20%)
64% ( p2)
CR CR
16% ( pq)
CR CW
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW
= 20% CW = 0.2 = q
+ 16% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
Conditions for Hardy-Weinberg
Equilibrium
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The Hardy-Weinberg theorem describes a
hypothetical population
In real populations, allele and genotype frequencies
do change over time
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The five conditions for nonevolving populations are
rarely met in nature:
 No
mutations
 Random mating
 No natural selection
 Extremely large population size
 No gene flow

Natural populations can evolve at some loci, while being
in Hardy-Weinberg equilibrium at other loci
Applying the Hardy-Weinberg Principle

We can assume the locus that causes phenylketonuria
(PKU) is in Hardy-Weinberg equilibrium given that:
 The
PKU gene mutation rate is low
 Mate selection is random with respect to whether or not an
individual is a carrier for the PKU allele
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Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions
The population is large
Migration has no effect as many other populations
have similar allele frequencies
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The occurrence of PKU is 1 per 10,000 births
 q2
q

= 0.01
The frequency of normal alleles is
p

= 0.0001
= 1 – q = 1 – 0.01 = 0.99
The frequency of carriers is
 2pq
 or
= 2 x 0.99 x 0.01 = 0.0198
approximately 2% of the U.S. population

Required conditions are rarely (if ever) met
Changes in gene pool frequencies are likely
 When gene pool frequencies change, microevolution has
occurred

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Deviations from a Hardy-Weinberg equilibrium indicate
that evolution has taken place
Hence, microevolution (via genetic mutations)
 The
raw material for evolutionary change
 Provides new combinations of alleles
 Some might be more adaptive than others
Natural selection, genetic drift, and gene flow
can alter allele frequencies in a population

Three major factors alter allele frequencies and
bring about most evolutionary change:
 Natural
selection
 Genetic drift
 Gene flow
You should now be able to:
1.
2.
3.
4.
Explain why the majority of point mutations are
harmless
Explain how sexual recombination generates
genetic variability
Define the terms population, species, gene pool,
relative fitness, and neutral variation
List the five conditions of Hardy-Weinberg
equilibrium
5.
6.
7.
Apply the Hardy-Weinberg equation to a
population genetics problem
Explain why natural selection is the only mechanism
that consistently produces adaptive change
Explain the role of population size in genetic drift
Allele Frequencies
Red short-horned cattle are homozygous for the
red allele, white cattle are homozygous for the
white allele, and roan cattle are heterozygotes.
Population A consists of 36% red, 16% white,
and 48% roan cattle. What are the allele
frequencies?
a.
b.
c.
d.
red = 0.36, white = 0.16
red = 0.6, white = 0.4
red = 0.5, white = 0.5
Allele frequencies cannot be determined unless the
population is in equilibrium.
Copyright © 2008 Pearson
Education, Inc., publishing as
Pearson Benjamin Cummings.
Hardy-Weinberg Equilibrium
Determination
Which of these populations are in HardyWeinberg equilibrium?
a.
b.
c.
d.
A
B
both A and B
neither A nor B
Copyright © 2008 Pearson
Education, Inc., publishing as
Pearson Benjamin Cummings.
Cystic Fibrosis 1
The frequency of cystic fibrosis, a recessive genetic
disease, is 1 per 2,500 births among Northern
Europeans. Assuming random mating, what is the
frequency of carriers?
a.
b.
c.
d.
1/2,500
1/50
1/25
The frequency cannot be calculated because selection
violates Hardy-Weinberg assumptions.
Copyright © 2008 Pearson
Education, Inc., publishing as
Pearson Benjamin Cummings.
Cystic Fibrosis 2
Until the 1950s, infants born with cystic fibrosis did not
survive longer than a few months. If the frequency of
carriers was 4% in the year 1900, what proportion of CF
alleles was eliminated in one generation?
a.
b.
c.
d.
e.
100%
50%
4%
2%
<0.1%
Copyright © 2008 Pearson
Education, Inc., publishing as
Pearson Benjamin Cummings.