1 chapter 13 chapter 13 population genetics outcome 2 pages 464 - 504
TRANSCRIPT
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CHAPTER 13 CHAPTER 13 Population Genetics
Outcome 2
Pages 464 - 504
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VARIATION Variation: refers to a condition in which one or
more members of a population differ in one or more traits.
A population is: a group of the same species living in the same region at a given time.
KINDS OF VARIATION in populations
The variation that is present in a population may be classified in several ways:
Structural: example: polydactyly; tail vs. no tail
Behavioural: example: territorial behaviour; mating
Biochemical: example: ABO Blood type; cat pigment
Developmental: example: ripening colours;
Physiological: example: Red / green colour blindness
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Geographical: evident when comparing locations (cline: gradual changes/ graded sequence within a population across a range)
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EXTENT OF VARIATIONin relation to a particular characteristic e.g skin colour
NO VARIATION
One Gene – One Trait Monomorphic: A population that shows no variation in
traits is monomorphic.
example: white plumage of wild short billed corellas
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VARIATION
One Gene – Two or more traits A population that shows two or more variations in a trait
is said to be polymorphic.
Discontinuous Variation: a few diverse and discrete traits (caused by monogenic inheritance)
example: flower colour of snapdragons (3 traits!)
Continuous Variation: ‘continuum’ of variation
example: adult male human height
Questions 1-3 p471
Chapter 14 - VariationChapter 14 - Variation 44
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CAUSES OF VARIATION
Variations seen in a population can have different
causes.
Some variations are due to:
Environmental Factors
Inherited Factors
A mixture of both (remember:
phenotype = genotype + environment
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ENVIRONMENTAL VARIATIONS
Are variations caused by factors of the organisms
external environment.
Casual Factors Include:
Soil Type Diet Up-bringing Temperature Substance Abuse
THE INTERNAL ENVIRONMENT
Variations caused by the internal environment
include:
Levels of hormones Types of hormones
Questions 4-6 p474
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INHERITED VARIATIONS
A number of inherited factors influence variation
within a species including:
Number of inherited chromosomes:
sources of variation: polyploidy/ annueploidy
Combination of chromosomes:
sources of variation: meiosis, crossing over
Alterations to genes
sources of variation: mutations, chromosome abberations
Number and type of alleles sources of variation: mutations,
Combination of allelessources of variation: multiple alleles, independent assortment
Monogenic inheritance sources of variation: 2 or more alleles
Polygenic inheritance sources of variation: 2 or more genes in combination
MONOGENIC INHERITANCE
Monogenic Inheritance: inherited variations (ie blood type) are due to the action of a single gene (with two or more alleles!).
These are called monogenic traits.
Table 13.4 p475 – Monogenic Traits
Question: Question: How much variation in ABO Blood types?
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POLYGENIC INHERITANCE
Polygenic Traits are those that cause variation in traits due to a number of different genes.
Examples of polygenic traits include:
Skin Pigmentation Fat content in Milk Seed Mass in Bean Plants
Chapter 14 - VariationChapter 14 - Variation 99
Polygenic VariationsPolygenic Variations
No of + No of + AllelesAlleles
Possible GenotypesPossible Genotypes
Gene 1 Gene 2Gene 1 Gene 2ColourColour
44 ++++ ++++ Very Very DarkDark
33 ++++
+-+-
+-+-
++++
DarkDark
22 ++++
+-+-
----
----
+-+-
++++
InterInter
DarkDark
11 +-+-
----
----
+-+-
MedMed
DarkDark
00 ---- ---- LightLight
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VARIATIONS- MUTATIONS
Gene mutation is the process which produces new alleles of genes in various species and so generates new genetic variation.
Questions 7-10 p479
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GENES IN POPULATIONS
The genetic information present in a population of organisms is known as a gene pool.
For example fleece colour in sheep (W=white & w=black)
Gene Pool – WW, Ww, ww
Gene pools are analysed and expressed in terms of frequencies of allele combinations.
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The Gene PoolThe Gene Pool
Activity Sheet – The Gene Pool p311
Activity Sheet – Factors Affecting Gene Pools p312
ALLELE FREQUENCY
When all genotypes in a population are known – we can directly count the number of each kind of allele and calculate its frequency.
Allele Frequencies can have a value between 1 and 0.
If a particular allele has a frequency of 1 – then every organism will be homozygous with that particular allele.
See Figure Fig 13.24 p479
This population contains:
10 Sheep in total
2 black (homozygous - ww) 2 heterozygous white (Ww) 6 homozygous white (WW)
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The total number of alleles = 20
No of W alleles = 14
No of w alleles = 6
Freq of W alleles = (14/20) = 0.7
Freq of w alleles = (6/20) = 0.3
A convention commonly used is that the frequency value for the dominant trait is given the letter p, while the recessive trait is given the letter q.
Therefore p + q = 1 (0.7+0.3=1)
ALLELE FREQUENCY
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Hardy-Weinberg Equilibrium1908 G. H Hardy and W. Weinberg
recognised
a pattern in allele frequencies.
Hardy Weinberg Principle states:
• given the right conditions, allele frequencies remain constant, generation after generation
Conditions for a stable Allele Frequency: Must be a large population Members must mate at random (i.e. not selective
mating) All matings are equally fertile, producing equal numbers
of variable offspring The population is closed; no migration in or out
Hardy Weinberg Equilibrium: the allele frequencies
in a large closed population will remain constant from
Generation. (until an agent of change acts on the population)
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Hardy-Weinberg Equilibrium
This is said to be the Hardy-Weinberg Equilibrium and it is a formula used to calculate the proportions of genotypes in a population.
p2 + 2pq + q2 = 1
Where,
Proportion with WW= p2
Proportion with Ww
= 2pq
Proportion with ww
= q2
So, p2 + 2pq + q2 = 1
Page 481 - 482
If we know the p and/or q values for a population – we can calculate the proportions of heterozygous and homozygous genotypes.
We can always work out the number of q2
Q2 = number of homozygous recessives
Therefore,
√ Q2 = q
and, number of ps = 1 – q
Example:
Plant flower color with B = blue b = white
36 members of the population have white flowers!
(see Figure 13.28 – p482)
Biozone: Changes in Gene Pool, pages
Quick Check Page 483: 11 - 15
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Hardy-Weinberg Equilibrium
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CHANGE AGENTS IN POPULATIONS
Most gene pools are not ‘closed’, unaffected by chance or by selection factors (predators, disease
etc). Consequently, allele frequencies often change within populations. Lets examine some of these
agents of change.
Agents of Change in Populations
Selection: Natural Selection/ Artificial SelectionMigration: in and out of populationsChance: genetic drift due to chance events
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SELECTION AS AN AGENT OF CHANGE
Under natural conditions, members of the same
population compete with each other for: Access to living space Food Mating partners
In a polymorphic population, different phenotypes
may be selected for (selective advantage) and
have different survival and reproductive chances and
consequently have a higher fitness value.
The ‘agent of change’ that causes these differences
to occur between phenotypes is the selecting agent.
Note: a selective advantage is not static but is
Subject to change (consider: mega fauna existed in
Australia before human settlement!).
Genetic fitness varies in different environments.
(consider: tree kangaroos climb trees. Would they be
successful in a desert environment?)
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ARTIFICIAL SELECTION
When farmers, horticulturalists, horse breeders or geneticists deliberately select particular organisms from a population to be the parents of the next generation.
Under conditions of artificial selection – the parents that contribute to the next generation may not be the fittest (in a genetic sense).
LEVELS OF SELECTION
Complete Selection: against a phenotype occurs when any organism with a given phenotype cannot reproduce because of death before a reproductive age is reached or because of sterility.
Partial Selection: against a phenotype occurs when matings involving that phenotype produce on average fewer viable and fertile offspring relative to othermatings .
Chapter 14 - VariationChapter 14 - Variation 2121
Artificial Selection In DogsArtificial Selection In Dogs
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The Biggy!
NATURAL SELECTION
NATURAL SELECTION: The action of selecting agents on populations living in the wild is called natural selection.
Natural selection refers to any environmental agent that acts on a wild population and results in differential reproduction.
Differential reproduction occurs when one inherited variety (phenotypic trait!) in a population produced more viable offspring (live to reproductive age) than other varieites, and thus contribute disprooprtionally to the gene pool.
In wild populations more offspring are produced than will survive to maturity.
Phenotypes with the highest survival rates and fitness values will make the greatest contribution to the gene pool of the next generation.
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NATURAL SELECTION
Chapter 14 - VariationChapter 14 - Variation 2424
Natural SelectionNatural Selection
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Natural Selection Tasks
Activity Sheet – Natural Selection p314
Activity Sheet – Darwin’s Finches p315
Activity Sheet – Artificial Selection p324
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MIGRATION AS AN AGENT OF CHANGE
Migration (in), also known as gene flow, can change the allele frequency of a population.
Unlike selection, which requires several generations to have an effect, changes due to migration can occur very quickly.
Emigration (out) can also change allele frequencies if the emigrant group is not a representative sample of the original population.
Human Migrations
Two great migrations occurred in human prehistory:
Homo Erectus from Africa to Europe and Asia (2 000 000 years ago).
Modern Homo Sapiens from Africa to Europe and Asia, and then onto Australia and the America’s. (130 000 years ago).
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CHANCE AS AN AGENT OF CHANGE
Chance effects can cause allele frequencies in a population to change over time.
When change effects operate, the direction of the change is unpredictable and can vary from one generation to the next.
The resulting pattern of change is known as random genetic drift.
The smaller the population, the greater the impact of chance events.
If the population is greatly reduced, the effect could include bottlenecking
Bottlenecking can also result in a population founded by a small under-represented sample of the original population. This is known as the Founder Effect.
Biozone: Genetic Drift p323
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Different examples of genetic drift can be seen on page 491.
These include:
The Bottleneck Effect The Founder Effect
Activity Sheet – The Founder Effect p321
Activity Sheet – Population Bottlenecks p322
CHANCE AS AN AGENT OF CHANGE
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EVOLUTION WITHIN A SPECIES
The allele frequencies in a gene pool of a population can change as a result of the action of change agents such as:
Migration Immigration Emigration Selection Genetic Drift
Members of one population may be separated into a number of populations spread over the range of the species.
Chapter 14 - VariationChapter 14 - Variation 3030
Through exposure to certain selection agents, these isolated populations may give rise to new races or sub-species.
EVOLUTION WITHIN A SPECIES
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STUDYING POPULATIONS USING mtDNA
Mitochondrial DNA (mtDNA) is an important tool for tracing the evolutionary history of a species.
In addition mtDNA can also be used to outline migrations of species through history.
mtDNA is structurally identical to nuclear DNA of the chromosomes, however there are several difference between the two that makes mtDNA useful for studying populations.
These differences include:
Descent via the maternal line – mtDNA is inherited from the mother only, so that all offspring receive just one kind of mtDNA exclusively from the mother.
Lack of Recombination – mtDNA passes unchanged from a female parent to all of her offspring.
High Copy Number – Each mitochondrion contains 2 to 10 mtDNA molecules, and each cell has up to several hundred mitochondria. So many copies of mtDNA are present in each cell.
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The coding region of mtDNA mutates very slowly, but the two non-coding regions in the D-loop of mtDNA have a higher mutation rate.
These are hypervariable regions or segments and are designated as HVR1 and HVR2.
STUDYING POPULATIONS USING mtDNA
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Each of us carries in our mtDNA a story about our maternal ancestry.
The mtDNA in each of your cells was inherited from your mother – she received her mtDNA from her mother, and so on, and so on through thousands of generations.
By identifying similarities between individuals mtDNA can suggest an ancestral link.
The distinctive mtDNA sequences found in different populations are known as haplogroups and each is designated by a capital letter.
STUDYING POPULATIONS USING mtDNA
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Studying Populations Studying Populations Using mtDNAUsing mtDNA
Chapter 14 - VariationChapter 14 - Variation 3535
Studying Populations Studying Populations Using mtDNAUsing mtDNA
Using mtDNA to Identify Using mtDNA to Identify Prehistoric Migration p497Prehistoric Migration p497
Quick Check Question 22 p498Quick Check Question 22 p498
Chapter 14 - VariationChapter 14 - Variation 3636
Chapter ReviewChapter Review
Biochallenge 1-2 p499Biochallenge 1-2 p499
Chapter Review QuestionsChapter Review Questions
Understand Key WordsUnderstand Key Words
Questions 1-6 p500-504Questions 1-6 p500-504