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Chapter 21 – PART 1
Population and Community Dynamics
Populations
Successful individuals of a species are able to
reproduce and adapt to their environments.
What role might their genetic makeup
play in meeting these two demands?
Populations
The population size of marine green turtles shows
little change over time, while the locust population in
the Canadian prairies varies widely from year to year.
•What might this suggest about the environments in
which they live?
•Which environment is most stable?
Populations
Population:
• A group of organisms of the same species that live in
the same habitat or ecosystem at the same time.
• Each individual has a different genotype.
– Differences in genotypes and environmental influences
account for differences among the phenotypes of individuals
of the same species.
• All of the genes that occur in a population are referred
to as the gene pool.
– Maintains continuity of traits from generation to generation.
Hardy- Weinberg Principle
• 1908 – G.H. Hardy & W. Weinberg,
independently derived the basic principle of
population genetics, the Hardy-Weinberg
principle
Hardy-Weinberg Terminology
Allele frequency:
• The proportion of allele copies in a population of a
given gene.
– Changes in populations can be measured in part by
looking for changes in allele frequencies.
– Note: not all genes exhibit variation
Fixed frequency:
• The frequency of an allele within a population when
only a single allele is present for a particular gene.
– The allele’s frequency is always 100% (only option)
The conditions under which no change
will occur are:
• 1) The populations must be closed (no
immigration/emigration)
• 2) Random mating takes place (no mating
preferences with respect to genotype)
• 3) No selection pressure. A single gene must
not affect the survival of the offspring
• 4) No mutations
• 5) Population must be large
Hardy-Weinberg Terminology
Genotypic ratio:
• The ratio of offspring with each possible allele
combination from a particular cross
Ex. 250 AA : 500 Aa : 250 aa
Phenotypic ratio:
• The ratio of offspring with a dominant trait to the
alternative, recessive trait.
Ex. 750 Squid : 250 not Squid
Hardy-Weinberg Principle
• The idea behind Hardy-Weinberg is that scientists
studying inheritance of traits in populations wanted
an equation to calculate the number of alleles in a
population.
• They did this by measuring the allele’s frequency.
http://www.khanacademy.org/science/biology/heredity-andgenetics/v/hardy-weinberg-principle?playlist=Biology
Solving a Hardy-Weinberg Problem
1.
2.
3.
4.
What am I being asked to find?
Create Dominance Hierarchy.
Identify what info. has been given to you.
If allele frequency is given, you already have either
p or q (easy to solve).
5. If no allele frequency is given, use info. you have
been given to find q2, then take √q2 to get q and
solve.
6. Be sure you are solving what the question is asking
(know your terminology)
Remember…
An allele is one of several forms of the same gene.
Ex. The gene for wing colour in moths has 2 alleles:
Brown and white
• Brown is dominant to white
Let’s say that A= brown and a= white
Write down the 3 possible genotypes:
Hardy-Weinberg Principle
In our example population there are 500 moths:
• 320 homozygous brown
• 160 heterozygous brown
• 20 white
• What are the genotype frequencies?
– Type of moth/total number of moths
• What are the allele frequencies?
Remember: each individual contributes 2 alleles to the gene pool
Hardy-Weinberg Principle
• 320 AA = 640 A (from AA genotype)
• 160 Aa = 160 A + 160 a (from Aa genotype)
• 20 aa = 40 a (from aa genotype)
• TOTAL: 1000  the number of alleles in the gene pool
Allele frequency:
• A=
• a=
Hardy-Weinberg Equation
For a gene with only 2 alleles
p+q=1
p= frequency of allele A (dominant)
q= frequency of allele a (recessive)
p2 + 2pq + q2 = 1
p2= frequency of homozygous dominant
2pq= frequency of heterozygous
q2= frequency of homozygous recessive
Example:
Suppose a certain allele A has a frequency of 0.6 in a population. The frequency of
allele a must be 0.4 because A + a must equal 1. (1 – 0.6 = 0.4). Let’s see what
happens during reproduction. We can arrange the alleles and their frequencies in
a Punnett square.
A (0.6)
a (0.4)
A (0.6)
AA (0.36)
Aa (0.24)
a (0.4)
Aa (0.24)
aa (0.16)
Genotype ratio: 36% AA; 48% Aa: 16% aa
Or use Hardy Weinberg:
Frequency of AA = p2 = (0.6)2 = 0.36
Frequency of Aa= 2pq = 2(0.6x0.4)= 0.48
Frequency aa = q2 =( 0.4)2 = 0.16
Unlike the genetic Punnett
square used to determine
individual traits, the eggs &
sperm of this Punnett
square represent the
genes for the entire
population.
H.W. Equation Contd.....
p= frequency of A = 0.8 or 80%
q= frequency of a = 0.2 or 20%
2
p
AA genotype =
Aa genotype =
Aa genotype =
+ 2pq +
2
q
=1
Practice
• Pg. 720 #1-3
• Pg. 722 #5-8
• Hardy-Weinberg Worksheets
– Discuss Tomorrow
Chapter 21 – PART 2
Changes in Gene Pools
Review of Hardy-Weinberg…
Conditions for H.W. Equilibrium include:
1.
2.
3.
4.
5.
In a H.W. Equation:
p=
p2=
q=
q2=
2pq=
When is a Gene Pool predicted to change?
• When a population is small, chance fluctuations in
numbers will cause changes in allele frequencies
• When individuals migrate
• Mutations: new alleles will arise or existing ones will
change
• Natural Selection occurs
• When mating is not random
• In other words.....gene pools are always changing
because the environment is in a constant state of
change. Hardy-Weinberg only exists as a model, not in
real life.
5 Agents of Evolutionary Change
Mutation
Gene Flow
Genetic Drift
Non-random mating
Selection
Genetic Drift
Is a change in the genetic makeup of a
population resulting from chance (random
events).
Ex. Small populations - can lead to fixation of alleles.
• Increases the % of homozygous individuals within a
population and reduces its genetic diversity
Watch to 3:50
https://www.youtube.com/watch?v=mjQ_yN5znyk&feature=related
Genetic Drift
Amplified in low populations
Founder Effect
Genetic drift that results when a small number of
individuals separate from their original population
and find a new population.
• Allele frequencies likely to not be the same as those of
original population
Bottleneck Effect
A dramatic, often temporary, reduction in population
size, usually resulting in significant genetic drift.
• Frequency of alleles in the survivors is very different
from that in the original population.
– Narrows gene pool
Ex. Elephant seals (Fig 4 pg. 724)
https://www.youtube.com/watch?v=Q6JEA2olNts
Bottleneck Effect
Gene Flow (Migration)
The movement of alleles from one
population to another through the
movement of individuals or gametes.
• Alters both populations
– Occurs in wild populations
Ex #1 Seed & pollen distribution by wind & insects
Ex #2 Migration of animals
– sub-populations may have different allele frequencies
– causes genetic mixing across regions
– reduce differences between populations
Mutations
Can be beneficial or
harmful
• Mutation creates
variation
– new mutations are
constantly appearing
• Mutation changes the DNA sequence
– changes in protein may change phenotype &
therefore change fitness
Natural Selection
Differential survival & reproduction due to changing
environmental conditions
– climate change
– food source availability
– predators, parasites, diseases
– toxins
• Combinations of alleles that provide “fitness”
increase in the population
– adaptive evolutionary change
Ex. The Peppered Moth & Malaria/Sickle Cell Anemia
Natural Selection
Non-Random Mating
Sexual Selection
• Differential reproductive success that results from
variation in the ability to obtain mates
• When combined with evolutionary pressures can
create Sexual Dimorphism within a species:
– Striking differences in the physical appearance of males
and females not usually applied to behavioral
differences between sexes.
https://www.youtube.com/watch?v=9GgAbyYDFeg&list=PLPTIy3JA29L7xnWCaic2A4t8XTLNu64
OZ&index=1&feature=plpp_video
Antibiotic-Resistant Bacteria
• Case Study on page 727-728
Speciation
• Speciation refers to the formation of a new
species.
• 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.
It is important to note that speciation and
evolution are NOT necessarily the same.
Natural selection does not always cause
speciation! (Ex. The evolution of the peppered
moth did not lead to a new species).
How does speciation occur???
a. Instantaneous Speciation
• Occurring in one generation because of major
changes to the chromosomes
• Usually a result of nondisjunction
• Polyploids can mate with each other, but not
with members of parent generations, because
of different chromosome numbers.
b. Gradual Speciation
• Most species arose slowly and gradually
evolved differences through time.
• i.e. Galapagos finches
c. Geographic Speciation
• Speciation occurs if a population is divided
into 2 or more smaller populations, that are
physically separated from one another.
• i.e. mountains or bodies of water from floods
establish physical barriers. Over time the
species cannot reproduce within the original
group
d. Punctuated Equilibrium
• Periodic rapid evolution (within 100-1000
generations ) followed by little change over a
long time
• Suggests that population remain stable and
unchanging for very long periods of time
e. Phyletic Gradualism
• Evolution occurs at a constant rate over time
Practice
• For each type of change in a gene pool that
we have discussed, write down how each
affects the alleles in a population.
– Genetic Drift
– Gene Flow
– Mutations
– Natural Selection
– Non-Random Mating
• Pg. 730 #2-12