Population Genetics
The Hardy-Weinberg Law of Genetic
Equilibrium
In 1908 G. Hardy and W. Weinberg
independently proposed that the frequency of
alleles and genotypes in a population will
remain constant from generation to
generation if the population is stable and in
genetic equilibrium.
Five conditions are required in order for
a population to remain at Hardy-Weinberg
Equilibrium:
1. A large breeding population
2. Random mating
3. No change in allelic frequency due to
mutations
4. No immigration or emigration
5. No natural selection
If "A" and "a" are alleles for a particular gene
and each diploid individual in a population has
two such alleles, then
p can be designated as the frequency of the "A"
allele and
q as the frequency of the "a" allele.
Therefore, in a population of 100 individuals in
which 40% of the genes are "A",
p = 0.40.
The rest of the genes (60%) would be "a", so
q = 0.60.
This gives us Hardy-Weinberg's first
equation:
p+q=1
In this example, 0.40 + 0.60 = 1.00. These
values are referred to as allele
frequencies.
As long as certain conditions are met, the
frequency of the various combinations of these
alleles (AA, Aa, aa) can be determined by using
the second Hardy-Weinberg equation:
p2 + 2pq + q2 = 1
– p2 is the percentage of homozygous dominant
individuals in the population
– 2pq is the percentage of heterozygous individuals
in the population and
– q2 is the percentage of homozygous recessive
individuals in the population.
Determining the Genotype and
Allele Frequencies in a Population
Hardy-Weinberg Principle
Hardy-Weinberg Equation
Using a Punnett Square for Hardy
Weinberg Frequencies
Example:
The frequency of the T allele is 0.70. What is the frequency
of the t allele? What are the genotypic frequencies
expected in the next generation? (hint: start with the
male and female allelic frequencies)
A Punnett square can be used to determine the expected
genotype frequencies in the next generation. This Punnett square
has been scaled up to represent the genotype frequencies for the
gametes in an entire gene pool. In generic terms, p2 represents
the homozygous dominant offspring, 2pq represents the
heterozygous offspring, and q2 represents the homozygous
recessive offspring.
Hardy Weinberg Examples
Calculate the change in allelic frequencies
from the following phenotypic information.
• A free breeding moth population has 60%
white moths and 40% black moths. White
colour is dominant.
• In three years, the observed colour
percentages change to 65% white and
35% black.
How do we solve this?
• A free breeding moth population has 60% white moths and 40%
black moths. White colour is dominant.
• In three years, the observed colour percentages change to 65%
white and 35% black.
- ww = black, therefore ww genotypic frequency is
0.40
- Determine the frequency of the w allele:
- Use Hardy Weinberg q2 = 0.40
- q=
and p =
Populations
The five agents of evolutionary change are:
(A) mutation, a change in DNA; (B) gene flow, the migration of
alleles from one population to another; (C) non-random mating,
such as self-fertilization in flowers; (D) genetic drift, a change in
allele frequencies in a small population due to a chance event;
and (E) natural selection for favourable variations.
Genes flow between nearby
populations.
Genetic Drift
In every generation,
only some of the
plants in this
population
reproduce. When the
light pink and
heterozygous roses
in the second
generation did not
reproduce, the allele
for light pink petals
was quickly lost from
the gene pool.
The Bottleneck Effect
The parent population
contains roughly
equal numbers of
yellow and blue
alleles. A catastrophe
occurs and there are
only a few survivors.
Most of these
survivors have blue
alleles. Due to genetic
drift, the gene pool of
the next generation
will contain mostly
blue alleles.
• In text questions p. 563 #1-8
• Read and complete the Case Study on p.
567
Populations and Communities
POPULATION ECOLOGY
I. Terms
A. ecology: the study of the interactions of organisms
with one another and their physical and chemical
environment
B. habitat: the type of place where an individual or
population normally lives; physical features, chemical
features, other species present
C. population: a group of the same species occupying
a specific habitat at a specific time
D. community:
1. populations of all species that occupy a habitat
2. or groups of organisms with similar life-styles in a
habitat
E. ecosystem: community and its environment and
interrelated flow of energy
1. biotic: all living organisms
2. abiotic: all nonliving components: nutrients,
temperature, rainfall
F. biosphere: total of all places in which organisms live
in/on water, earth's crust, atmosphere
G. Niche: the population’s role in the
ecological community including all the
biotic and abiotic conditions needed for its
continued survival.
II. Characteristics of Populations
A. population size: number of individuals
making up a gene pool
1. dependent on births, immigration (into),
deaths, and emigration (exit)
2. zero population growth: near balance of
births and deaths
Change in population size = (births + immigration)–(deaths+ emigration)
ΔN =
(b + i) - (d + e)
3. exponential growth pattern (J-shaped
curve)
a. rate of increase (r) = net
reproduction/individual/unit time
b. growth rate formula: gr = ΔN/Δt
c. as longs as r is positive, population
will grow at ever- increasing rates,
measured by "doubling time"
d. the larger the reproductive base, the
greater the expansion size
4. biotic potential: maximum rate of increase
under ideal or nonlimiting conditions
a. age of beginning reproduction
b. how often reproduction occurs
c. how many offspring are born each
time
The per capita growth rate: change in population size
per individual over a given time frame;
expressed as: cgr = ΔN/N or cgr = Nfinal – N/N
where N = initial population size and ΔN is change
B. population density: number of individuals per
unit of area or volume at a given time
Dp = N/A or Dp = N/V
Population Density Calculations
DP = N/A
N = total numbers counted
A = area
There are 480 bison living in a 600 hectare region of Wood Buffalo National Park.
Calculate the population density.
C. population distribution: spatial pattern in which
members are dispersed through its habitat
1. clumped dispersion: live in clumps (most
common)
a. suitable physical, chemical and biological
conditions are patchy, not uniform (food,
water, shelter, shade, protection)
b. live in social groups
c. offspring not highly mobile, forced to "live
where they landed"
2. uniform dispersion: rare in nature, may be
due to competition for limited resources
3. random dispersion: when environmental
conditions are uniform and members are
neither attracting nor repelling each other
4. patterns may vary depending on seasons,
droughts, etc.
D. age structure: relative proportions of
individual of each age, especially with respect
to reproductive years
III. Limits on Growth of Populations
A. limiting factors: any resource in short supply
1. actual rate of increase influenced by
environmental conditions
2. environmental resistance
a. nutrient supply
b. predation
c. competition for space
d. pollution
e. metabolic wastes
B. carrying capacity: sustainable supply of
resources determines population in a given
environment
1. logistic growth: carrying capacity can vary
over time (S- shaped curve)
2. final population will depend on the
sustainable supply of resources
C. density-dependent controls: competition for
1. resources
2. predation
3. parasitism
4. pathogens
D. density-independent controls:
1. weather: may increase the death rate without
respect to numbers present; lightning, floods,
snowstorms
IV. Life History Patterns
A. demographics: study of age-specific patterns
of particular populations
B. survivorship curves and reproductive
patterns
Survivorship Curves
1. Type I: large mammals, few offspirng, low infant
mortality, extended life span (e.g. humans)
2. Type II: chances of survival or death are about
the same at any age (e.g. squirrels)
3. Type III: low survivorship or high mortality rates
early in life (e.g. oysters)
R-Selected:
Usually smaller
animals with short
life spans, high
fecundity and spend
little or no time
rearing their young.
E.g. insects
K-Selected:
Usually larger animals
with longer
life spans. low
fecundity and spend
considerable time
rearing their
young to assure their
survival. Eg:
bears, humans
Competition
Interspecific is
competition between two
different species.
Often occurs when two
species niches overlap.
Intraspecific competition
is between members of
the same population
o It is density dependent.
o Members compete over
food, space and breeding
rights.
V. Biotic Factors Influencing an
Ecosystem
A. Symbiotic Relationships: most relationships
between species in an ecosystem are brief:eat
or be eaten. However, some are long lasting and
are called symbiotic: “living together”
1. Mutualism: a relationship in which both parties
benefit.
– Eg: nitrogen fixing bacteria in root nodules of legumes
– Lichens – fungus and green algae
– Cleaning symbiosis: cleaner wrasse, Nile crocodile &
Egyptian plover
– Endosymbiosis: mitochondria and chloroplasts
2. Commensalism: means "at table together". It is used for
symbiotic relationships in which one organism consumes the
unused food of another. Some examples:
• the remora and the shark. The dorsal fin of the remora (a bony
fish) is modified into a sucker with which it forms a temporary
attachment to the shark. When the shark feeds, the remora
picks up scraps. The shark makes no attempt to prey on the
remora.
• Epiphytes are plants that live perched on sturdier plants. They
do not take any nourishment from their host and simply benefit
from being better exposed to sunlight. Orchids and bromeliads
are examples.
3. Parasitism: A parasite is an
organism that
– lives on or in the body of
another organism (the host)
– from whose tissues it gets its
nourishment, and
– to whom it does some damage
• Animals are parasitized by viruses,
bacteria, fungi,
protozoans,flatworms (tapeworms
and flukes), nematodes, insects
(fleas,lice), and arachnids (mites).
• Plants are parasitized by viruses,
bacteria, fungi, nematodes, and a
few other plants.
Parasites damage their host in two major ways:
– consuming its tissues, e.g., hookworms
– liberating toxins, for example,
o Tetanus bacilli secrete tetanus toxin which interferes with synaptic
transmission.
The relationship between parasite and host varies
along a spectrum that extends from
– "hit and run" parasites that live in their host for a brief
period and then move on to another with or without killing
the first
– parasites that establish chronic infections. Both parasite
and host must evolve to ensure the survival of both
because if the parasite kills its host before it can move on,
it destroys its own meal ticket.
Symbiotic Relationships and
Defense Mechanisms
•
•
•
•
•
•
•
•
Mutualism- Gobi and shrimp
Mutualism- Ant and Caterpillar
Mimicry- Viceroy Butterfly
Parasitism- Parasitic Wasp and Aphids
Commensalism- Clown Fish and Anenome
Parasitism- Malaria
Parasitism- Tapeworm
Defense Mechanisms
B. Predator-Prey Relationships: eg: a wasp (Heterospilus prosopidus)
that feeds on a beetle (Callosobruchus chinensis)
• - wasp population follows beetle population – a couple of generations
behind
• - population fluctuation due to availability of food source
Regular population fluctuations
• eg: Lemmings – irrespective of food
supply
– as lemmings consume vegetation –
overgrazing results in lack of vegetation
– lemmings emigrate en masse – death march
to sea
– population is drastically reduced
– vegetation returns
– cycle repeats
– However, if food is plentiful, lemmings migrate
anyway!!
C. Ecological Succession
• Available Niche- if a niche is available a
population will occupy it.
• Competitive Exclusion Principle: no two
populations can occupy the same niche
-the more qualified pop. will take over
• Ecological Succession: the sequence of
changes that occurs in a community
- in a mature community the final occupants of the
available niches have eliminated all competition
- called a climax community
• Succession is Predictable
- succession results from the way communities change
their physical environment
Primary succession occurs in areas where no
plants have lived before.
- Pioneer plants must create detritus (litter) and
soil before other plants can succeed them.
- first stage :formation of soil
- soon bacteria, fungi and small plants begin to
take nourishment from soil
- bare sand is first colonized by grassescottonwoods are first trees
- 50 - 100 years from start, pine trees appear
-100-150 years from start, black oak trees appear
= this is the climax community
Secondary succession occurs after a natural
community has been disturbed.
• Our knowledge of succession patterns enables
us to predict the organisms that will appear in
each stage of succession and the time required
for each stage.
- often occurs where man has bared the land
- colonizing seeds from many plants arrive
- those which can survive open conditions (wind,
direct sunlight) will continue to thrive
There are different forms of succession occurring
at different times in different places but there are
some general truths:
1. The composition of species changes during succession,
but the changes are more rapid at earlier stages.
2. The total number of species increases from the start then
becomes stabilized in the older stages. Especially true of
heterotrophs whose variety is greater in the later stages
of succession.
3. Total biomass and non-living organic matter increases
during succession
4. Food webs become more complex, relationships
become more specialized
5. Percent of organic matter utilized increases at each
trophic level
VI. Human Population Growth
A. world population 5.6 billion in 1994 – now?
B. side-stepping natural controls
1. expanded to new habitats and climate
zones
2. agriculture increased the carrying
capacity of land
3. medicine and sanitation removed many
limiting factors
VII. Population Growth and Economic Development
A. demographic transition model: changes in
population growth linked to 4 stages of economic
development
1. preindustrial stage: living conditions harsh, birth
and death rates high, little increase in population
size
2. transitional stage: living conditions improve,
death rate drops, birth rate remains high
3. industrial stage: growth slows
4. postindustrial stage: zero population growth
reached, birth rate falls below death rate
Population Histograms
Breaking down the population by age groups to make predictions of population trends.
The Aging Population in Canada
Canada's population
will undergo
considerable aging in
the 21st century:
· Fertility rate below replacement rate
· Increased life expectancy due to
improvements in public health
Population over Age 65:
· 12.7 % in 2001
· 14.4 % in 2011
· 17.9 % in 2021
Source: Calculation from Statistics
Canada's "Population Projections for
Canada, Provinces and Territories 20002026". Cat. No. 91-520
2001
2011
2021
Chaos Theory
• Read text
• It is applied to population growth
• Biologix Populations on LearnAlberta
19.2 The Causes of Gene Pool Change
• In this section, you will:
• outline the conditions that are required in order to
maintain genetic equilibrium in a population
• identify and compare the effects of mutations, gene
flow, non-random mating, and genetic drift on gene pool
diversity
• apply, quantitatively, the Hardy-Weinberg principle to
published data, and infer the significance of your results
• distinguish between the founder effect and the
bottleneck effect on gene pools
• explain how the process of natural selection is related to
the microevolution of a population
• explain the cause of heterozygote advantage and how it
affects a gene pool
• describe strategies that are used in captive breeding
and population management
• explain that genetic engineering may have both
intended and unintended effects on gene pools
Chapter 19 Review
• Explain the difference between phenotype and
genotype.
• Are all inheritable mutations disadvantageous?
Explain why or why not.
• Which factors can cause changes in the gene
pool of a population?
• What could cause the frequency of homozygous
genotypes to increase in a gene pool?
• How do human activities contribute to gene flow
and genetic drift in natural populations?
Concept Organizer
Chapter 19 Summary
• A gene pool contains all the alleles for all the
genes in a population that can be passed on to
the next generation. Population geneticists study
gene pools. The Hardy-Weinberg principle is a
mathematical model that population geneticists
use to determine allele frequencies and
genotype frequencies in a population. According
to the principle, allele frequencies in a
population will remain constant in succeeding
generations unless acted upon by outside
forces.
Chapter 19 Summary
• The total of the allele frequencies, p and q, for one gene
always equals 1.00, or 100 percent of the alleles. A
change in the allele frequencies over time indicates that
a population is undergoing microevolution.
Hardy-Weinberg equation: p2 + 2pq + q2 = 1.00
• The letters p and q represent the frequencies of the
dominant and recessive alleles, respectively. The
frequency of the homozygous dominant genotype is
represented by p2, the frequency of the heterozygous
genotype is represented by 2pq, and the frequency of
the homozygous recessive genotype is represented by
q2. If the population size (N) is known, the number of
individuals with a particular genotype can be calculated
using the equation:
p2(N) + 2pq(N) + q2(N) = N
• The more diverse the gene pool of a population, the
better is the population’s chance of survival should the
environment change.
Chapter 19 Summary
• Inheritable mutations can be neutral,
beneficial, or detrimental, depending on
the environment. Mutations that provide a
selective advantage will increase in
frequency due to natural selection.
• Gene flow due to emigration and
immigration of individuals increases the
genetic diversity of a population that
receives new members, but decreases the
genetic diversity among populations.
Chapter 19 Summary
• Non-random mating due to mate selection
based on phenotypic differences leads to sexual
selection. Inbreeding, another form of nonrandom mating, increases the frequency of
homozygous genotypes in a gene pool.
• Genetic drift can result in the loss of alleles from
small populations due to chance events, as well
as an increase in the frequency of previously
rare alleles. The formation of an isolated
population from a small founding population or
population bottleneck may lead to inbreeding
and a loss of genetic diversity in the population.
Chapter 19 Summary
• The process of natural selection selects
for favourable variations and directly leads
to the adaptation of species to their
environments. Harmful recessive alleles
may be maintained in a population by
heterozygous carriers, particularly if the
carrier state has greater fitness (called
heterozygote advantage) compared with
homozygous individuals, under certain
environmental conditions.
• Human activities can affect the amount of
gene flow between and genetic drift within
natural populations.