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Biology 30
Population Genetics
The modern theory of evolution recognizes that the main source of
variation in a population lies in the differences in the _________ carried by the
chromosomes. Genes determine an organism’s appearance and mutations can
cause new variations to arise. These variations can be passed from one
generation to another.
Certain genotypes may be better equipped for survival than others. They may
be better at obtaining food and water, protecting themselves from predators or
have a higher reproductive potential. When these organisms reproduce, these
“successful” genes will be transmitted to the offspring. The offspring will be
better able to survive; therefore, subsequent generations would have an
increased frequency of these “successful” variant genes. Consequently, there
would be _________________, within the group, of individuals better adapted to
prevailing conditions.
Genes in Human Populations
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The principles of genetics, established by studies on plants and fruit flies,
can be applied to humans.
However, the study of human genetics presents some unique problems:
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Unlike garden peas and Drosophilia, humans produce few offspring,
which makes it difficult to determine the genotypes of both parents
& offspring for any particular trait.
Observing successive generations takes time. Drosophilia can
reproduce every 14 days.
Many human traits, including body size, weight and intelligence,
are affected by environment as well as genes.
_______________________ is one of the most common ways to study
human populations.
- A representative group of individuals within the population is
selected and the trends or frequencies displayed by the group are
used as indicators for the entire population.
- i.e. Trait for tongue rolling is sampled this way. Approximately
65% of the population carry this dominant gene.
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A population consists of all the members of a species that occupy a
particular area at the same time. i.e. Perch population of Lake Wabamun,
dandelion population of school field.
The members of a population are more likely to breed with one another
than with other populations of the same species.
Therefore, genes tend to stay in the population for generation after
generation.
The total of all the alleles for all of the genes in all the members of a
population at one time is called the population’s ________________.
Over time the size of a gene pool changes. The gene pool __________
when a mutation changes a gene and the mutation survives. The gene
pool ___________ when an allele dies out.
The more variety there is in a gene pool, the better the population can
survive in a ______________ environment.
_________________ is the change in the frequency of genes in a
population’s gene pool from one generation to the next.
Hardy-Weinberg Principle
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1908 – Godfrey Hardy & Wilhelm Weinberg, independently derived the
basic principle of population genetics, the Hardy-Weinberg principle.
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This law states that the frequencies of alleles in a population’s gene pool
will remain constant over generations as long as five factors stay constant
(genetic equilibrium):
1)
2)
3)
4)
5)
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Provides a model of an unchanging gene pool.
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If all of these conditions are met, the frequencies of two alleles, for
example A and a, will remain constant in a population for an indefinite
time (until conditions change).
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The Hardy-Weinberg principle allows us to study one trait at a time. We
will consider the simplest case, a trait that is controlled by a dominant
allele and a recessive allele. The mathematical expression for the HardyWeinberg equilibrium is as follows:
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Example #1:
Suppose a certain allele A has a frequency of 0.6 in a population. The
frequency of allele a must be _______ because A + a must equal 1.
We can arrange the alleles and their frequencies in a Punnett square.
Unlike the genetic
Punnett square used
to determine individual
traits, the eggs &
sperm of this Punnett
square represent the
genes for the entire
population.
 If p represents the frequency of the dominant allele (A), then p2
represents the frequency of the homozygous dominant offspring
______.
 If q represents the frequency of the dominant allele (A), then q2
represents the frequency of the homozygous recessive offspring
______.
 As shown in the Punnett square, there are two possible
recombinations that will result in the heterozygous genotype ____.
Therefore the frequency of the heterozygous genotype is
pq + pq = _______.
 Notice that the sum of all the genotypes is equal to 1.00 or 100
percent:
_________ + __________ + __________ = 1.00
_________ + __________ + __________ = 100 percent
_________ + __________ + __________ = 1.00
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The Hardy Weinberg equation can be used to calculate the proportion of a
population that carries recessive alleles for genetic conditions, i.e. sickle
cell anemia or cystic fibrosis. It can also be used to calculate the number
of individuals with a specific genotype.
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Using the example from before, calculate the number of individuals of
each genotype if the population was composed of 8000 individuals.
 Example #2:
Suppose a certain allele A has a frequency of 0.7 in a population (found in
70% of the genes). Calculate the expected frequencies of the three possible
genotypes.
If we were just given the distribution of genotypes, how could we predict the
frequency of the A and a alleles?
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Example #3: The frequency of a recessive trait in a population is 4%,
what are the allele frequencies?
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Hardy-Weinberg purposely ignored the external factors that influence
populations.
The Hardy-Weinberg principle points out that sexual reproduction
reshuffles genes but does not by itself cause ____________.
If the population does not demonstrate H-W equilibrium, (i.e. it’s allele
frequencies change over time), it is in evolutionary change!
Factors that bring about Evolutionary Change
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A population’s gene pool is very unstable. It is constantly influenced by
external factors – factors that were intentionally ignored by Hardy and
Weinberg.
These factors change a population’s genetic makeup, upset the tendency
toward genetic stability & lead to evolutionary change.
microevolution – the gradual change in allele frequencies (gene pool),
of a population over successive generations.
i.e. development of DDT resistance in some Anopheles mosquito
species is limiting the effectiveness of DDT over time.
1) Natural Selection
 The nonrandom survival & reproduction of certain genotypes from
one generation to the next.
 Certain traits may be selected for, while others may be selected
against.
 Individuals with greater fitness breed and pass on their favourable
characteristics to the next generation.
 Natural selection occurs when a _____________ produces a
phenotype that gives one individual a survival advantage over
another.
 Examples:
 In North America, individuals who are homozygous for
normal hemoglobin have a selective advantage over those
who are heterozygous or homozygous for the sickle cell
allele.
 Peppered moth in England – the frequency for the dark
wings in the peppered moth changed as levels of air
pollution changed.
 It is possible that human resistance to HIV, the Ebola virus
& the West Nile virus, and others will increase in frequency
as these viruses become more widespread in the
population.
2) Non-Random Mating
 Random mating means that there is no way to predict which males
will mate with which females.
 In natural populations, unrestricted random mating is probably
uncommon for two main reasons:
1)
2)
3) Mutations
 A mutation is any inheritable change in the DNA of an organism.
 Mutations occur in a cell as it undergoes ___________ to form an
egg or sperm.
 Two types:
1) Chromosome Mutation –
2) Gene Mutation –
 If the mutation gives selective advantage to individuals carrying it,
then it will increase in frequency and the population gene pool will
change over successive generations.
 While most mutations are neutral, some are harmful and a few are
even beneficial.
i.e. some people have a rare mutation in a gene that codes for a
protein receptor on the surface of white blood cells. In people
without the mutation, HIV can use the protein receptor to enter
white blood cells. Those that have the mutation, lack the receptor
and are therefore resistant to HIV infection.
 A mutation considered beneficial in one environment may be
detrimental in another environment.
i.e. Sickle Cell Anemia & Tay-Sachs Disease
- Are conditions expressed as homozygous recessive
- Carriers (heterozygotes) are usually symptom free.
- This is why disease still exists, or it would be selected out of the
population.
- Tay Sachs is a fatal genetic disease.
- Sickle cell anemia can be treated (not cured), and sometimes can
be fatal.
- African and south eastern Asian
- Eastern Jewish Populations
4) Genetic Drift
 Evolution can occur simply by chance.
 Random events may bring death or lack of parenthood to some
individuals. As a result, alleles may disappear from a population.
 I.e. Population of 10 guinea pigs. Only one member displays an
allele B, for black coat color. If this black coat individual does not
mate, the black allele will disappear from the population.
 A _________ population is more likely to lose alleles from its gene
pool than a large population is. (H-W Principle applies to large
populations that do not experience genetic drift since chance
events are unlikely to affect overall allele frequencies, i.e. predators
are unlikely to kill all members of a population that have a
recessive allele. There is, however, the rare chance that factors
like _______, ________________, or extensive habitat
fragmentation could cause genetic drift to occur in large
populations).
5) Gene Flow (Migration)
 Movement of members into ______________, or out of
_____________, a population alters its equilibrium.
 In immigration, new genes are _________ to the population.
 In emigration, genes are _________ from the population.
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Speciation refers to the formation of a new _______________.
There is an enormous diversification between species that evolution alone
cannot explain.
A group of similar organisms that can interbreed and produce
fertile offspring in the natural environment.
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It is important to note that speciation and evolution are NOT necessarily
the same. Natural selection does not always cause speciation! (i.e. The
evolution of the peppered moth did not lead to a new species).
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There are two general pathways that can lead to the development of a
new species:
1) Gradual Speciation (Transformation)
 Gradually, and naturally over time as a result of mutation and
adaptation to changing environmental conditions.
 i.e. evolution of mammoths
ancestral mammoth
steppe mammoth
wooly mammoth
2) Divergence
 One or more species arise from a parent species.
 i.e. The small hoofed Hyracotherium is thought to have been the
common ancestor of modern horses, tapirs & rhinoceroses.
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Speciation can also be instantaneous – due to events such as nondisjunction.
Populations and Communities
 a population refers to all of the individuals of the same species living
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in the same place at a certain time
a community is made up of the populations of all organisms that
occupy an area
the study of a community involves only the organisms, whereas the
study of an ecosystem involves that abiotic and biotic components of
an area
a habitat is the physical area where a species lives
within a habitat, every population occupies an ecological niche – this
is referred to as the populations ecological role in the community,
including the biotic and abiotic factors under which a species can
successfully survive and reproduce
Distribution of Populations
 population patterns can be divided into three patterns:
1) Clumped distribution
- occurs when individuals are grouped in patches or aggregations
- organisms are distributed according to certain environmental
factors
- eg.) in river valleys, trees often grow only on the south slopes
and grasses dominate the north slope – plant distribution is
found in “clumps”
2) Random distribution
- occurs when there is neither attraction nor repulsion among
members of the population
- arbitrary and not very common
3) Uniform distribution
- occurs when there is competition among individuals for factors
such as moisture, nutrients, light and space
Size and Density of Populations
 population size: the number of organisms of the same species
sharing the same habitat at a certain time
 these numbers may arise from an exact count or an estimate of the
total population size using sampling methods
 eg.)In 1981, there were 27642 northern pike in Sylvan Lake, Alberta
 population density: the number of organisms per unit space
 the density (D) of any population is calculated by dividing the total
numbers counted (N) by the area (A) occupied by the population:
D=N
A
i.e. If 200 lemmings were living in a 25ha (hectare) area of tundra near
Churchill, Manitoba in 1980, their population density would be:
 we can compare population densities by determining if there have
been changes within the same population over a certain time period
(we call this the rate of change)
 Rate of density change can be expressed as follows:
rate of density change = change in density
change in time
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or
R = ∆D
T
D must be calculated showing the most recent dates minus the
density at the earliest date – this will show whether there has
been an increase or decrease in the population
 Four factors determine population size:
1) _________: the number of offspring of a species born in one year
2) _________: the number of individuals of a species that die in one year
3) _____________: the number of individuals of a species moving into an
existing population
4) ______________: the number of individuals of a species moving out of
an existing population
 population growth can be determined by the following formula:
PG
or
ΔN = (births+immigration) – (deaths + emigration) =
(b+i)-(d+e)
(factors that ______ the pop’n) – (factors that ______ the pop’n)
i.e. The Alberta greater sage grouse population was about 4375 in 1970.
By 2002, the population was estimated to be only about 350 birds.
Assuming that the population decreased at a constant rate, calculate the
growth rate (gr): a change in the number of individuals in a population
over a specific time frame, for the population from 1970 to 2002.
 Note: the calculation of growth rate does not factor in the initial size of
the population. The amount of increase in a large population will
always be larger than that of a small population. Why?
 To compare populations of the same species that are different sizes or
that live in different habitats, we use the rate of change per individual.
This is called per capita growth rate (cgr).
i.e. If a colony of 200 cranes had 40 births & 55 deaths, with no
migration, what is the per capita growth rate of the crane population?
 in mature ecosystems, populations tend to remain relatively stable
over the long term – this is called dynamic equilibrium or steady
state
 dynamic equilibrium is similar to homeostasis; populations will adjust
to changes in the environment to maintain equilibrium
 populations can either be classified as “open” or “closed”
- in open populations, all four factors (natality, mortality,
immigration and emigration) are functioning
- in closed populations, immigration and emigration do not occur,
so changes in natality and mortality will be the only factors that
influence population size
Growth Curves
 Three types of growth curves:
1) J-shaped population curves (ideal environment)
 If a few relatively active individuals are placed in an ideal
environment: unlimited space, food, water, without disease and
predation, the population can be expected to reproduce at its
maximum physiological rate (this is called its biotic potential
which is defined on the next page).
 The only limiting factors would be the rate of gamete formation,
mating and survival of offspring.
 Defined by a brief lag phase, followed by a steep increase in
numbers, known as exponential growth. This gives us the
characteristic “J” shaped curve.
 This rapid growth period is usually followed by a sharp decline
in the population.
**In real life situations, limiting factors curtail growth and the curves tend to
level off!
2) Growth curves for open populations
 typically form “s-shaped” curves
 a characteristic growth phase, followed by a stationary phase
 where the curve levels off, the maximum number of individuals
that the environment can support has been reached - this
number is now the new carrying capacity (K)
 The highest possible per capita growth rate for a population is
called its _________ ____________ (r).
 biotic potential determines the carrying capacity of that
environment
 there are six factors that regulate biotic potential:
1. Offspring (fecundity): the number of offspring per reproductive
cycle
2. Capacity for survival: the number of offspring that survive to
reach reproductive age
3. Maturity: the age of reproductive maturity
4. Procreation: the number of times in a life span the organism
reproduces
5. Life span
6. Gender ratio – the more females, the greater the biotic
potential.
7. Mate availability
 Limiting factors are factors that limit a habitat’s carrying
capacity & therefore limit population growth. (aka
environmental resistance)
 there are two general categories of limiting factors in an
environment:
- density independent:
- abiotic
- affect members of a population regardless of population
density
-
density dependent:
- biotic
- factors that arise from population density that affect
members of that population. These have a bigger impact on
large populations.
3) Growth curves for closed populations (limited resources)
 four definite phases can be identified in this curve
I) LAG PHASE
- the delay that occurs before the population enters a phase of
active reproduction
II) GROWTH PHASE
- the population increases at its fastest rate during this phase
- the rate of natality is greater than the rate of mortality
- cell cultures and yeasts can grow exponentially (2, 4, 8, 16…)
- the expected population increase in a given time (I) can be
calculated from the following formula:
I = growth rate (R) x current population (N)
- i.e.) If the growth rate of paramecia in a closed population was
7.5% per day, and the initial population was 200, there should
be an increase of 15 paramecia on the first day (after that it is
compounded)
III) STATIONARY PHASE
- the point where the population size no longer increases
- a lack of space, a shortage of nutrients and an accumulation of
toxic metabolic wastes cause a reduction in the rate of increase
- the rates of natality are equal to the rates of mortality
IV) DEATH PHASE
- the mortality rate exceeds the natality rate
- nutrients run out and wastes accumulate
r and K Population Reproductive Strategies
 K selected strategies:
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These populations are found in stable environments who live
close to the carrying capacity (K) of the environment.
- populations become crowded, causing intraspecific competition
- members are usually large in size and produce young that are
slow-growing and require parental care
- low reproduction rate
- i.e.) elk, bear, humans
- Summary of strategy:
 r selected strategies:
- undergo many unpredictable changes
- usually populations that are small in size
- have short life spans and an early reproductive age
- high reproductive rate
- the offspring grow rapidly and little parental care is needed
- a sudden environmental change can result is a large number of
deaths. i.e. Alberta – these organisms experience growth in the
summer, but die in large numbers as simmer ends.
- i.e.) insects, bacteria, annual plants & algae
Life History Patterns
 population cycles that include growth and decline can occur in many
populations
 the snowshoe hare and lynx have cycles that are about 11 years in
length
Human Population Growth & Age Pyramids
 Growth curves show how populations change over time – not the age
distribution of the members
 With age pyramids, we are able to predict whether a population will
grow, stabilize, or decline
 an age pyramid with a wide base is characteristic of a rapidly growing
population – it indicates a high number of young offspring, but also
shows the number of animals capable of reproduction
 population histograms with a narrower base are often approaching
zero population growth, and those with a more narrow base than
middle section are showing declining population growth
 factors that affect population growth:
- industrial revolution/technology
- advances in medicine
- weather
Intraspecific and Interspecific Competition
 “If two populations of organisms occupy the same ecological niche,
one of the populations will be eliminated” – this is known as Gause’s
Principle and is due to interspecific competition
 interspecific competition occurs between similar species for a
limited resource (food, water…)
 intraspecific competition occurs within an ecological niche of
members within the same species
Predation
 two main ways for animals to avoid predation are:
1) camouflage – an adaptation in form, shape or behavior that better
ables an organism to avoid a predator
eg.) toxins produced by alder, birch or poplar trees deter animals and
insects
eg.) butterflies taste bitter to birds
eg.) During the winter, rabbits change their coloring to blend with the
environment
2) mimicry – involves developing a similar color pattern, shape or
behavior that has provided another organism with some survival
advantage
eg.) eyespots on butterflies
 coevolution can occurs between two species – this is when the
selection pressure
Symbiotic Relationships
 a symbiosis is a relationship in which two different organisms live in a
close association
 there are three main types of symbiotic relationships:
1) PARASITISM
- parasites obtain nourishment from their hosts, but do not
usually kill their hosts but often will affect the host in a
detrimental way
- eg.) Dutch elm disease – a parasitic fungus uses the tree for
food
2) COMMENSALISM
- commensalism is an association between two organisms in
which one benefits and the other is unaffected
- eg.) the fox and caribou in the arctic – fox will often follow
migrating caribou because the caribou kick the snow out of the
way so the foxes will have a path to travel on
3) MUTUALISM
- a relationship in which two different organisms live together and
both benefit from a relationship
- eg.) nitrogen fixing bacteria and legume plants (biology 20!) –
the plant feed the bacteria sugar and the bacteria make nitrates
for the plant
- eg.) pollination
Chaos Theory
 Scientists are interested in studying very complex phenomena which
seem to defy long term prediction. Eg. Biological communities &
populations, weather.
 A new way of examining why some features of nature are so
unpredictable is known as chaos theory. This theory assumes that
randomness is a basic feature of many complex systems, long-term
predictions may be extremely difficult.
 Even though features of nature are so unpredictable, they often share
similar characteristics:
1) Outcomes of processes in a complex system are extremely sensitive to
small differences in the conditions that were present when the process
began.
2) Once a process is underway, the relationships among the interacting
parts of the phenomena can change as a result of the interactions
themselves.
3) Two systems that appear similar at the start may end up being very
different, but how the two will differ is unpredictable.
 Chaos is a normal feature in biological systems.
 The inability to predict the precise makeup of a community does not
mean that communities are entirely unpredictable: communities tend
to undergo predictable changes over time called succession.
Succession
 succession is the slow, orderly progressive replacement of the
community by another during an areas development
 succession ends by reaching a climax community
 there are two possible types of succession:
1) Primary Succession
- occurs in an area which no community previously existed
- eg.) invasion of plant life of a newly formed volcanic island
2) Secondary Succession
- occurs following the complete or partial destruction of a
community
- eg.) regrowth after a forest fire
- the first plants and animals to appear are called the pioneer
community
- lichen, mosses and insects are often considered pioneer species
- pioneer communities develop into seral communities which
have plants and animals with longer life cycles than pioneer
species
- in the end, a climax community is formed where there is a
high rate of survival of all species
 some generalizations about succession:
- species composition changes more rapidly during the earlier
stages of succession
- the total number of species increases dramatically during the
early stages of succession, begins to level off during
intermediary stages, and usually declines and the climax
community becomes established
- food webs become more complex and the relationships more
clearly defined as succession proceeds
- both the total biomass and nonliving organic matter increase
during succession and begin to level off during the
establishment of the climax community