Transcript AOS2_ch13_population genetics_2012_student

AOS 2
On completion of this unit the student should be able to analyse and evaluate evidence for
evolutionary change and evolutionary relationships, and describe mechanisms for change
including the effect of human intervention on evolutionary processes.
Key knowledge
• change in populations: gene pool, allele frequencies; selection pressures; genetic drift;
• natural selection as a mechanism of evolution;
• geological time: scale; relative and actual dating techniques;
• evidence of evolution: fossil record, biogeography, comparative anatomy; molecular evidence;
• patterns of evolution: divergent, convergent; allopatric speciation, extinction;
• the development of evolutionary theory;
• evolutionary relationships: conservation of genes; genome phylogeny; mitochondrial evolution;
• hominid evolution: patterns, origin;
• interrelationships between biological, cultural and technological evolution;
• human intervention in evolutionary processes
– selective breeding
– application of gene technologies: cloning of organisms; transformation; stem cell
differentiation; genetic screening, gene therapy.
1. Population Genetics
EL:
What is a population?
• A group of organisms
of the same species
living within a
particular
geographical area at
a given time.
Orangutan distribution in
Borneo, Malaysia in 1999.
What is variation?
• Members of a
population differ in
one or more traits
Variation in a Population
Variation exists in
members of a
population and may be:
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Structural
Biochemical
Physiological
Developmental
Behavioural
Geographic
For the next 15 minutes, use
pages 466-470 to summarise
what each of these terms
means – use words or pictures
Monomorphic Variation
• Members of a
population may
show no variation
for a particular
trait.
Corellas are
monomorphic in
respect to
plumage.
Polymorphic Variation
Flower colour in lupins.
• Members of a
population may
show two or more
variations for a
particular trait.
Monogenic Traits
• Monogenic traits are due
to the action of a single
gene with two or more
allelic forms.
• These traits show
discontinuous variation the members of the
population can be grouped
into a few discrete and
non-overlapping classes.
• E.g. blood types
Polygenic Traits
• Polygenic traits are due to the actions of many genes (and
their allelic forms). These traits show continuous variation
(e.g. height).
Causes of variation
Read “identifying causes of variation” on page
471 and come up with a “conclusion”. In five
minutes, I’ll get some of you to share yours
with the class
What are the causes of variation in a
population?
Interactions with the
environment
 Identical twins = same genotype,
different phenotype
 Arrowleaf plants – different
shaped leaf in soil c.f. water
 Gender in honey bees
determined by food during larva
stage
 Snowshoe hare – Colour changes
in seasons
 Gender determined plumage in
birds
What are the causes of variation in a
population?
Genetic:
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Mono or polygenes (see pages 474-476)
Chromosome number
Sexual reproduction – recombination of alleles
Mutations – new genetic variation
The Gene Pool
• The complete set of genetic
information carried by all the
individuals of a population.
• It is not only all the genes, but
all the alleles present in the
population.
• Individuals within the
population will have some but
not all of the available alleles.
Allele Frequencies
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B = 14, b = 14
Allele Frequencies- your turn to
calculate.
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Allele Frequencies can stay constant
• Allele frequency is a measure of the relative frequency of an allele in a
population.
• Allele frequencies will remain unchanged from generation to generation if:
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There is random mating
Matings are fertile
There are no mutations
The population is closed – there is no migration
The population is large
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A population where the allele frequencies remain constant, generation after
generation is non-evolving (small catch... biological populations usually always
changing...)
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Important note – you don’t need to know about how to apply the H-W principal
anymore (page 481-82
Change Agents
• Allele frequencies are subject to change due
to various “change agents”
• A population in which the allele frequencies
are changing over generations is said to be an
evolving population.
Change agents: environmental factors
• Environmental factors act on
phenotypes
• Phenotypes may be selected
for and give the organism a
“selected advantage”. Such
organisms have a higher
“fitness value” .
• Phenotypes may also be
“selected against”.
• Selection factors can be (1)
natural or (2) artificial.
I think I will be
selected against in
this environment!
Natural selection is a mechanism for
gradual change over time
• Dark phenotype now has a selective advantage – its alleles
will increase in frequency
What is biological fitness?
• The ability to reproduce and pass on genes to the next
generation.
• Those individuals which are said to have a higher fitness are more
adapted to a particular environment so are able to survive,
reproduce and make a greater contribution to the gene pool of
the next generation – have a selective advantage
• Those individuals which are said to be less fit are less adapted to
a particular environment and contribute less to the gene pool in
the next generation.
• In a changing environment fitness is variable - an organism’s
fitness in one environment can be completely different in
another environment.
• Selecting agent causes difference in fitness between phenotypes
Activity and homework questions
• Activity 13.1 part B & Activity 13.2 (to be handed in)
• Chapter 13 quick check qu 1-10
• Chapter 13 chapter review qu 1-5
• Read “Case studies in selection” on pages 485-86
and summarise key points
• Chapter 13 quick check qu 11-13
• Chapter 13 chapter review qu 6-8
Reflection
• What questions from the start of the lesson
did you have an incorrect answer for?
Summarise the correct answers in your own
words:
What is:
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a population?
a variation and what causes them?
a gene pool?
an allele frequency?
biological fitness?
2. Population Genetics
EL:
Selection
• Two types: natural and artificial
– Natural selection = action of selecting agents (e.g.
climate, food predation etc) on populations in the
wild leading to differential reproduction (i.e. one
phenotype producing more viable offspring)
• See page 487 for examples
– Artificial selection = action of humans selecting
particular individuals from a population to breed
Change agents: gene flow
• The movement of individuals between
populations may result in very rapid changes
in allele frequencies
Immigration may increase
the variety of alleles for a
particular trait
Emigration may decrease the variety of
alleles present in the population if the
emigrant group is not representative of
the whole population
Change Agents: genetic drift
• the change in the frequency of an allele in a population
• Chance events can change allele frequencies, particularly in
small populations.
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This is a fishy story
Genetic Drift: The bottleneck effect
• Occurs when there is a severe
reduction in the population
size caused by intense natural
selection or disaster.
• Surviving members may be an
unrepresentative sample of
the original population.
• Loss of genetic variability,
increases risk of extinction
• e.g. Helmeted honeyeater
Genetic drift: The Founder effect
• A small unrepresentative
sample leaves to colonise a
new region
• Captive breeding programs
in zoo represent a founder
effect. Care must be taken
to avoid inbreeding and to
maximise genetic diversity
This is less crowded but
do we have enough
genetic variability
Smarties population dynamics
• Divide your smarties on your plate into two populations
• Swap smarties between populations → what does this
demonstrate?
• Now isolate red and orange smarties → what does this
demonstrate?
• Put all the smarties on the plate together again and eat all
except red and orange smarties → what does this demonstrate?
• close your eyes and eat half. What do you have left over? →
what does this demonstrate?
The biological species concept
• Organisms that interbreed and
produce viable and fertile
offspring in the natural
environment are members of
the same species.
• Speciation is the process that
results in a new species.
There are several forms of
speciation.
• In unit 4 the focus is on
allopatric speciation- this is
based upon geographical
isolation mechanism
An interbreeding population of one species will show
phenotypic variation and be subject to selection pressures
Allopatric speciation
Hey! Where have
these mountains
come from? We
are cut off
• The population becomes
divided by a physical barrier,
or just a large distance.
• This can happen when a
population migrates or is
dispersed or when the
geography changes
catastrophically (earthquakes,
volcanoes, changes in sea
level) or gradually (erosion,
continental drift).
Its got colder! I
need a new
mating partner
Allopatric speciation
• Each fragmented population
will be subject to selection
pressures that may differ from
those experienced by the
original population.
• Different selection pressure
will act on different
phenotypes changing the
frequencies of alleles.
• Mutations will continue to
occur and may be selected for.
Allopatric speciation
• If the physical barrier was
removed, the population would
have accumulated enough
change and be so different that
they have become reproductively
isolated due to:
– Changes in courtship
– Differences in breeding season
– Biochemical changes in ova and
sperm
– Structural incompatibility (e.g. big
dog, little dog)
– Sterile hybrids (e.g. mule)
• they can no longer interbreed
and have become two new
distinct species
Extinction
• Species become extinct when
they lack the genetic diversity
to withstand environmental
selection pressures – they fail
to adapt.
• Since all environments change
eventually, it is the fate of any
one species to become extinct
– including our own!
• Human intervention through
climate change, habitat
destruction and other factors
is causing the largest mass
extinction in the history of the
Earth
Activity and homework questions
• Activity 13.3 part B(to be handed in)
• Chapter 13 quick check qu 16-21
• Chapter 13 Biochallenge 2
• Chapter 13 chapter review qu 10, 13-14
Reflection
• What questions from the start of the lesson
did you have an incorrect answer for?
Summarise the correct answers in your own
words:
What is:
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gene flow?
genetic drift?
a genetic bottle neck?
speciation and extinction?
3. Population Genetics
EL:
The ‘story’ in mtDNA
• mtDNA is inherited only
through the maternal line
allowing the tracing of direct
genetic descent free of
recombination.
• All the mitochondria in your
cells are clones of your
mother! Whereas genes in
nuclear DNA are inherited
from both parents.
• We can be more certain about
the inheritance of mtDNA.
Why use mtDNA for population
genetics studies
• Read pages 494 – 495 and summarise the
advantages of using mtDNA in population
genetics studies
• Read pages 496-498 and answer:
– Quick check qu 22 & chapter review question 16
Reflection
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?