POPULATION GENETICS

BIO 511 SMJ

 LESSON LEARNING OUTCOME
UPON COMPLETION OF THIS CHAPTER STUDENT SHOULD BE ABLE TO

• Understand the concept of a population and

polymorphism in populations.
• Use the Hardy-Weinberg equation to examine the
frequency of alleles and genotypes in a population.
• Identified factors that change allele and genotype
frequencies in populations

Brooker, CHAPTER 24,

Population Genetics,
Page 666
POPULATION GENETICS: an introduction

The central issue in population genetics is genetic variation

– Its extent within populations
– Why it exists?
– How it changes over the course of many generations

DEFINITION of population genetics

Studies the origin of variation, the transmission of variants from
parents to offspring generation after generation, and the
temporal changes that occur in a population because of
systematic and random evolutionary forces.
GENES IN POPULATION
• The focus is shifted away from
the individual and toward the
population of which the
individual is a member

• Conceptually, all of the alleles of

every gene in a population make
up the gene pool
– Only individuals that reproduce
contribute to the gene pool of the
next generation

• Population geneticists study the

genetic variation within the gene
pool and how it changes from
one generation to the next
 A large population is usually composed
What is a of smaller groups called local populations

Population? – These are also called demes

A population is a group – Members of a local population are far
of individuals of the likelier to breed with each other than
same species that with members of the general population
occupy the same region – Local populations are often separated
and can interbreed with from each other by moderate
each other geographic barriers

Figure 1.0 Two local populations of Douglas

fir separated by a wide river.
 Natural population
Populations typically are dynamic units that change commonly go through cycles
from one generation to the next. A population may of feast or famine during
change in which the populations will
1. Size shrinks and swells

2. Geographic location
Populations or Individuals
3. Genetic composition
MIGRATE to a new site
and establish a distinct
Population geneticists have developed mathematical population in this location
theories that predict how the gene pool will change in
response to fluctuations in the above
As populatio size and
POPULATION IS DYNAMIC UNIT location changes, their
genetic composition
generally changes as well
 In order to understand population
genetics we must first describe the
gene pool

DEFINITION of GENE POOL: all the

genes and their different alleles that are
present in a population of a particular
species of organism
Size of Genetic Pool

LARGE GENE POOL SMALL GENE POOL

• A large gene pool indicates high • A small gene pool indicates low
genetic diversity, increased genetic diversity, reduced
chances of biological fitness, and chances of acquiring biological
survival. fitness, and increased possibility
of extinction.
• Gene pool increases when
mutation occurs and survives. • Gene pool decreases when the
population size is significantly
reduced (e.g. famine, genetic
disease, etc.).
WHY IS IT IMPORTANT
TO DESCRIBE THE GENE
POOL?

• Gene pool gives an idea

of the number of genes,
the variety of genes and
the type of genes existing How is blood group O the most common if the
in a population. allele for O is recessive?

• It can be used to help

determine gene
frequencies or the ratio
between different types
of genes in a population. The proportion of any given phenotype is
not determined by the dominance or
recessiveness of the allele, but on the
frequency of the allele in the gene pool
Allele and Genotypic Frequencies DEFINE Gene Pool

• Two fundamental calculations are central to population genetics

Number of copies of an allele in a

population
Allele frequency =
Total number of all alleles for that
gene in a population

Number of individuals with a

particular genotype in a population
Genotype frequency =
Total number of all individuals in
a population
FREQUENCY OF ALLELES
• Consider a population of 100 pea plants
– 64 tall plants with the genotype TT
– 32 tall plants with the genotype Tt
– 4 dwarf plants with the genotype tt
Number of copies of allele t in
the population

Frequency of allele t =
Total number of alleles T and t in
Homozygotes the population
have two copies Heterozygotes have
of allele t only one

(2)(4) + 32
 Frequency of allele t =
(2)(64) + (2)(32) + (2)(4)

All individuals have two copies of each gene

 Frequency of allele t = 40 = 0.2, or 20%

200
FREQUENCY OF ALLELES

500 flowering plants

480 red flowers 20 white flowers

320 RR 160 Rr 20 rr

As there are 1000 copies of the genes for color,

the allele frequencies are (in both males and females):

Frequency of allele R:
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R

Frequency of allele r:
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r
FREQUENCY OF GENOTYPES
• Consider a population of 100 pea plants
– 64 tall plants with the genotype TT
– 32 tall plants with the genotype Tt
– 4 dwarf plants with the genotype tt
Number of individuals with
genotype tt in the population
 Frequency of genotype tt =
Total number of all individuals
in the population

4 % of dwarf plants
 Frequency of genotype tt =
64 + 32 + 4 in the population

4
 Frequency of genotype tt = = 0.04, or 4%
100
 In our pea plant example; POLYMORPHIC
Allele and Genotype GENES (2 Alleles  T and t)
Frequencies are always ≤ 1
• Frequency of T allele + frequency of t
For a given trait, the allele and allele = 1
genotype frequencies are always
less than or equal to 1 • Frequency of T allele = 1 – frequency of
t allele
i.e., less than or equal to
100%
Therefore,
For monomorphic gene Frequency of T allele = 1 – 0.2
The allele frequency for the = 0.8, or 80%
single allele will be equal or
be close to 1.0

For polymorphic genes

The frequencies of all alleles
should add up to 1.0
THE HARDY-WEINBERG EQUILIBRIUM
• The Hardy-Weinberg equation was formulated independently by Godfrey
Harold Hardy and Wilhelm Weinberg in 1908
– It is a simple mathematical expression that relates allele and genotype
frequencies in a population

• The HW equation is also called an equilibrium.

• IT CAN BE USED TO CALCULATE GENOTYPE FREQUENCIES BASED ON ALLELE
FREQUENCIES

The Hardy-Weinberg Equation:

p2 + 2pq + q2 = 1

Where:
p2 = frequency of AA genotype;
2pq = frequency of Aa plus aA genotype;
q2 = frequency of aa genotype
 The Hardy-Weinberg Equation in
Action!
• Consider a polymorphic gene that exists in two alleles,
A and a
– The frequency of allele A is denoted by the variable p
– The frequency of allele a is denoted by the variable q
•p+q=1
– For this gene, the Hardy-Weinberg equation states that
• (p + q)2 = 1
• p2 + 2pq + q2 = 1
Genotype Genotype
frequency of Genotype frequency of aa
AA frequency of Aa
• If p = 0.8 and q = 0.2, and if the population is in Hardy-Weinberg
equilibrium, then

1. frequency of AA = p2
= (0.8)2 = 0.64

2. frequency of Aa = 2pq
= 2(0.8)(0.2) = 0.32

3. frequency of aa = q2
= (0.2)2 = 0.04

• Figure 1.2 compares the Hardy-Weinberg equation with the

Punnett square approach
Figure 1.2 compares the
Hardy-Weinberg equation
with the Punnett square
approach
• To illustrate the
relationship between
allele frequencies and
genotypes, Figure 1.2
compares the Hardy-
Weinberg equation with
the way that gametes
combine randomly with
each other to produce
offspring.

• In a population, the
frequency of a gamete
carrying a particular .HETEROZYGOTES
HETEROZYGOTES CAN BE PRODUCED in TWO
allele is EQUAL to the DIFFERENT WAYS!!
allele frequency in that An offspring could inherit the A allele from its
population. father and a from its mother OR A from its
mother and a from its father  Frequency of
heterozygous is pq+pg = 2pq.
ASSUMPTIONS IN H-W THEOREM

THE ALLELE AND GENOTYPE FREQUENCIES DO NOT CHANGE

OVER THE COURSE OF MANY GENERATIONS!
• The Hardy-Weinberg equation predicts an equilibrium if certain conditions
exist in a population
1. No new mutations
2. No genetic drift. The population is so large
 Allele frequencies do not change due to random sampling errors
3. No migration
4. No natural selection
5. Random mating

The HW equation provides a quantitative relationship between the allele and

genotype frequencies  Refer to Figure 1.3
This genotype This genotype
predominates when predominates when the
the frequency of frequency of allele a is
allele a is low intermediate

This genotype
predominates when
the frequency of
allele a is high

Figure 1.3 The relationship between allele frequencies and genotype frequencies
according to the Hardy-Weinberg equilibrium
IN REALITY, NO POPULATION
SATISFIES THE HARDY-WEINBERG
EQUILIBRIUM COMPLETELY
BUT! WE KNOW THAT THERE ARE
CHANGES IN GENOTYPE AND ALLELE
FREQUENCIES in a gene pool from
generation to generation.

Evolution does occur within

populations.

Evolution within a
species/population =
MICROEVOLUTION

Microevolution refers to changes

in allele frequencies in a gene pool
from generation to generation.
Represents a gradual change in a
population.
 The alteration of the gene
GENETIC pool of a small population
DRIFT due to CHANCE

When the beetles reproduced, just by random

luck more brown genes than green genes
ended up in the offspring.

Two factors may cause genetic

drift:
• Bottleneck effect may lead to • Founder effect may lead to reduced
reduced genetic variability variability when a few individuals
following some large disturbance from a large population colonize an
that removes a large portion of isolated habitat.
the population.

• The surviving population often

does not represent the allele
frequency in the original
population.
Bottleneck effect GENETIC
DRIFT
• In nature, a population
can be reduced
dramatically in size by a
natural disaster for
example

• Such a disaster
randomly eliminates
individuals regardless of
their genotype

• The period of the

bottleneck, when the
population size is very
small, may be
influenced by genetic
drift
Founder effect GENETIC
DRIFT
 NATURAL GENE
FLOW
SELECTION
• Gene flow is the exchange of
genes between populations,
• The process in nature by which which are usually of the same
only the organisms best adapted species.
to their environment tend to
survive and transmit their genetic
characteristics in increasing • Examples of gene flow within a
numbers to succeeding species include the migration
generations while those less and then breeding of organisms,
adapted tend to be eliminated or the exchange of pollen.

• The only agent that results in • Gene transfer between species

adaptation to environment. includes the formation of hybrid
organisms.
 Mutation is a change in an organism’s
DNA and is represented by changing
MUTATION alleles.

Mutation: Some “green genes” randomly mutated to “brown genes” (although since
any particular mutation is rare, this process alone cannot account for a big change in
allele frequency over one generation).

 Mutations can be transmitted in gametes to offspring, and immediately

affect the composition of the gene pool.
 Mutation can result in several different types of change in DNA sequences;
these can either have no effect, alter the product of a gene, or prevent the
gene from functioning.
 Due to the damaging effects that mutations can have on cells, organisms
have evolved mechanisms such as DNA repair to remove mutations.
POPULATION GENETICS

VARIATIONS WITHIN POPULATIONS

Some genes are Monomorphic, but most are Polymorphic

POLYMORPHISM MONOMORPHIC
• The term polymorphism refers to the • A monomorphic gene exists
observation that many traits display predominantly as a single allele
variation within a population – By convention, when a single
allele is found in at least 99% of
• At the DNA level, polymorphism is all cases, the gene is considered
due to two or more alleles that monomorphic
influence the phenotype
– In other words, it is due to genetic • In most natural populations, genes
are polymorphic.
variation
• However in small poplulation that
• Polymorphic is also used to describe a are near extinction, genetic
gene that commonly exists as 2 or variation is expected to be low
more alleles in a population because of the gene pool is derived
from a small number of individuals.
 EXAMPE 1 of
MONOmorphism
MONOMORPHISM in
AFRICAN CHEETAH
• When population’s size drops,
there are fewer animals to pair
off as mates.
• As a result, mating can occur
between more closely related
pairs. This is inbreeding, and it is
known to occur amongst
fragmented cheetah populations.
• Inbreeding serves to make
populations more homogenous.
Closely related animals tend to
share the same genes.
• Their offspring will therefore be
less genetically diverse than
outbred animals.
• As inbreeding proceeds,
homozygosity in an individual,
and monomorphism in the
population increases.
EXAMPE 1 of
polymorphism

Fig 1.1 POLYMORPHISM IN THE

HAWAIIAN HAPPY-FACE SPIDER
All individuals are from the same species, Theridion grallator
But they differ in alleles that affect color and pattern
POPULATION GENETICS

VARIATIONS BETWEEN POPULATIONS

 VARIATION BETWEEN POPULATIONS
GEOGRAPHIC VARIATIONS CLINAL VARIATIONS

• Refers to variation over geography • Clinal variation refers to a gradual

-- that is, populations in different change in some feature across
geography.
areas look different from each
other. • For example, if you go from south
to north in the northern
hemisphere you frequently find that
• There may not be any obvious populations of mammals are
pattern to such variation but smaller in the south and gradually
sometimes there is. as you go north you find that within
a species the individuals are larger
and larger.
• One common pattern of geographic
variation is clinal variation. • Such a pattern of gradual change is
called a cline so this form of
variation is called clinal variation.
GEOGRAPHIC VARIATIONS
1. Geographic variation in phenotypic
traits of monarch butterflies from
different populations in North America
and Hawaii.

2. Monarchs in these populations are

exposed to different climates, host
plant species, and natural enemies, and
experience different levels of
geographic isolation and migratory
strategies.

3. Divergent selective forces may have

affected traits associated with monarch
flight ability, including wing size and
shape, and monarch responses to
different host plant species.
CLINAL
VARIATIONS
Cline, a type of geographic variation, is a graded variation in individuals that
correspond to gradual changes in the environment.

Example: Height
variation in yarrow
plant along an
altitudinal gradient.
Can you think of a
reason for the plants
to evolve
differently?
Clinal Variation of the Burchell's Zebra's Coat

Burchell's zebra (Equus burchelli) is distributed in the southern and eastern

regions of Africa. Comparing the populations from the north to the south
there is a progressive reduction of the thickness of the black stripes and the
percentage of the coat that they cover. This phenomenon of a species’
characteristics changing gradually with geographic distribution is called clinal
variation.
END