Starter
Which of the following is true of polyploid plants? They
have:
A – reduced yield and the diploid chromosome number
B - increased yield and the diploid chromosome number
C – reduced yield and sets of chromosomes greater than diploid
D – increased yield and sets of chromosomes greater than diploid.
Key Area 1.7
Evolution
Learning Outcomes
Key Area 1.7
Evolution and Inheritance
Learning Outcomes
1. Define ‘evolution’
2. Describe the process of vertical inheritance
3. Describe the process of vertical inheritance as a result of
sexual reproduction
4. Describe the process of vertical inheritance as a result of
asexual reproduction
5. Describe the process of gene transfer
6. Explain how HGT led to rapid evolutionary change
7. State which type of gene transfer is safe and explain why
8. Describe the process of gene transfer from prokaryotes
into eukaryotes
9. Describe the process of gene transfer from viruses to their
host cell
Evolution
• Evolution is the change that occurs in
organisms over generations as a result
of genomic variations.
• These variations take the form of
changes in the frequencies of certain
genetic sequences.
Introduction to Evolution
Vertical Inheritance
• Genetic sequences are transferred
vertically from parent down to
offspring as a result of sexual or
asexual reproduction.
Vertical Inheritance
Sexual Reproduction
• In sexual reproduction, the parents are
normally genetically different and
produce offspring that vary further in
genetic makeup.
Vertical Inheritance
Asexual Reproduction
• In asexual reproduction, a single parent with a
certain genome produces offspring with
exactly the same genome and no variation
occurs among the successive generations.
• Vertical inheritance occurs among eukaryotes
and prokaryotes.
Horizontal Transfer of Genetic
Material
• Prokaryotes can transfer genetic material
from one cell to another horizontally.
• A much faster method of obtaining a
beneficial gene than waiting to evolve by
natural means!
• The cells involved may not even belong to
the same species.
Rapid Evolutionary Change in
Prokaryotes
• During the early stages of prokaryotic
evolution the rate of gene loss was high.
• To compensate, there was a high rate of
horizontal gene transfer (HGT).
• This led to a rapid spread of new genetic
sequences which promoted the build up of
larger genomes and allowed rapid evolutionary
change to occur among prokaryotes.
Horizontal Gene Transfer (HGT)
• There is no guarantee that a genetic
sequence gained horizontally will be an
advantage.
• It may useless or even harmful, which
makes HGT a risky evolutionary strategy.
• Vertical gene transfer is much safer as
the genes have been “tried and tested” by
the parents.
Tree of Life
• As organisms reached a certain level of
multicellular complexity, the importance
of gaining sequences by HGT decreased.
• As the role of vertical inheritance
increased in importance, distinct
lineages began to emerge with their own
sets of specific genes, eventually giving
rise to the “tree of life”.
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MRSA
• HGT still takes place in modern prokaryotes.
• A current example among certain bacteria is the
exchange of plasmids carrying the genes that are
resistant to antibiotics.
• This has resulted in the emergence of strains of
bacteria that are resistance to all known antibiotics.
HGT from prokaryotes into eukaryotes
• Some bacteria can transfer genetic material
horizontally to the genomes of eukaryotes.
• For example:
Agrobacterium tumefacians infects wounded
plant cells with a plasmid which integrates a
sequence of its DNA into the genome of the
host plant cell.
HGT from viruses to host cells
• Some viruses are able to transfer their DNA
horizontally into the genome of their host cell.
• For example, the herpes virus reproduces
within the host cells and destroys them,
however, it is able to integrate its DNA into
the genome of other healthy cells and remain
dormant as a provirus until the person’s state
of health drops.
• HIV behaves in the same way.
You should now be able to . . .
1. Define ‘evolution’
2. Describe the process of vertical inheritance
3. Describe the process of vertical inheritance as a result of sexual
reproduction
4. Describe the process of vertical inheritance as a result of asexual
reproduction
5. Describe the process of gene transfer
6. Explain how HGT led to rapid evolutionary change
7. State which type of gene transfer is safe and explain why
8. Describe the process of gene transfer from prokaryotes into
eukaryotes
9. Describe the process of gene transfer from viruses to their host
cell
Starter
• Number you and your partner ‘1’ and ‘2’.
• Number 1’s should write a brief description of
vertical gene inheritance on their show me
board.
• Number 2’s should write a brief description of
horizontal gene transfer on their show me
boards.
• Swap boards and evaluate each others
comments.
Outcomes Covered
Key Area 1.7
Selection
Learning Outcomes
1. Describe the process of natural selection
2.Describe the process of sexual selection
Selection
Selection is the process by
which the frequencies of some
DNA sequences increase in a
population because they have
been selected for and some
decrease because they have
been selected against.
Natural Selection
• Organisms produce more offspring than the environment
can support.
• Many offspring die before reaching reproductive age for
various reasons.
• Members of a species show variation.
• Those offspring better adapted (have a selective
advantage) survive, reproduce and pass on favourable
characteristics.
• The process is sometimes referred to as ‘survival of the
fittest’.
Natural Selection
Natural Selection Example: DDT
• DDT is a poisonous chemical which has been
widely used against many insects, including
mosquitoes which carry malaria and yellow
fever.
• Within just a few years of use, mutant forms
of insects resistant to DDT had ‘appeared’.
• The mutants are able to make an enzyme that
renders the chemical harmless.
• They had not arisen in response to DDT!
Natural Selection Example
• A tiny number of resistant mutants just
happened to be present within the
natural insect population.
• When the DDT spray was applied the
non-resistant insects died.
• The resistant mutants had a selective
advantage and multiplied.
• Natural selection took place.
Selection Against Deleterious
Sequences
• A deleterious genetic sequence can code
for an inferior version of a characteristic.
• The individual will be poorly adapted to the
environment.
• As a result, it will leave fewer offspring.
• In turn, fewer copies of the gene will be
passed on and a non-random reduction in
its frequency will occur.
• A frequency that is lethal will disappear
much more quickly.
Sexual Selection
• Females tend to be selective and choose
one high quality male to breed with.
• Male animals must compete for a sexual
partner.
• Sexual selection is a non-random
process that results in the increase in
frequency of DNA sequences that
increase the reproductive success.
Male-to-Male Competition
• Males compete aggressively with one
another for territories and access to
females.
• They may also use “weapons” such as
antlers.
Female Choice
• The female may selects her mate based
on the traits he displays.
• Female Choice
Investigating Sexual Selection in
Brine Shrimps
• Practical Technique
Starter
Learning Outcomes
1. Describe and explain the outcome of stabilising selection.
2. Describe and explain the outcomes of directional
selection.
3. Describe and explain the outcomes of disruptive
selection.
Types of Selection for a
Quantitative Trait
• A polygenic trait, such as seed mass, is
quantitative.
• Natural selection can affect the frequency
of a quantitative trait within a large
population in one of three ways:
1. Stabilising selection
2. Directional selection
3. Disruptive selection
Stabilising Selection
• Leads to a reduction in genetic diversity
without a change in the mean value.
• Maintains the status quo for the best
adapted genotypes.
Directional Selection
• Favours a version of the characteristic
that was initially a less common form.
• Results in a progressive shift in the
population’s mean value for the trait.
Disruptive Selection
• Extreme versions of a trait are
favoured at the expense of the
intermediates.
• Results in the population becoming split
into two distinct groups each with its
own mean value.
You should now be able to . . .
1. Describe and explain the outcome of
stabilising selection.
2.Describe and explain the outcomes of
directional selection.
3.Describe and explain the outcomes of
disruptive selection.
Starter
Learning Outcomes
1. Define the term ‘gene pool’.
2. Define the term ‘genetic drift’.
3. Describe the ‘founder’ effect.
4. Explain the term ‘neutral mutation’.
5. Describe the effect of genetic drift on neutral
mutations.
Genetic Drift
• A gene pool is the total of all the different
genetic sequences present in a population.
• The random increase of decrease in frequency
of genetic sequences (due to sampling error)
is called the genetic drift.
• Random genetic drift creates the potential
for evolutionary change.
Founder Effect
• A small group of organisms (splinter
group) is isolated from the rest of the
population and “finds” a new population.
Founder Effect
The original
population has 4
alleles of the gene.
The splinter group
only have 3 and
they occur in
different
frequencies.
After several
generations, the
genetic makeup of
the new population
is very different to
the original and its
members become
distinctive.
Neutral Mutations
• A neutral mutation is a change to the sequence
that has no effect on the amino acid produced.
• E.g. GCC changed to GCA – both still code for
alanine.
• Neutral mutations do not affect how well adapted
the individual becomes to the environment so are
not involved in natural selection.
Neutral Mutations
• Neutral mutations are affected by
genetic drift.
• Most of the changes that occur in the
frequencies of these neutral genetic
sequences are the result of random
genetic drift acting as a mechanism of
evolution.
You should now be able to. . .
1. Define the term ‘gene pool’.
2.Define the term ‘genetic drift’.
3.Describe the ‘founder’ effect.
4.Explain the term ‘neutral mutation’.
5.Describe the effect of genetic drift on
neutral mutations.
Starter
Learning Outcomes
1. Define the term ‘species’.
2. Define ‘speciation’.
3. Describe the sequence of events in allopatric
speciation and the type of barrier involved.
4. Describe the sequence of events in sympatric
speciation and the barriers involved.
5. Explain the formation of hybrid zones and the
consequences of a barrier being introduced.
Species
• A species is a group of organisms that
are able to interbreed with one another
to produce fertile offspring.
Speciation
• Speciation if the formation of new
biological species brought about by
evolution as a results of isolation,
mutation and selection.
Allopatric Speciation
• Gene flow is interrupted by a
geographical barrier.
• Examples of these are a river, mountain
range, desert or sea.
Allopatric Speciation
Sympatric Speciation
• When two (or more) populations live in
close proximity to one another in the same
environment, but become genetically
isolated.
• Isolation is by a behavioural or ecological
barrier (or polyploidy in plants).
• Examples of ecological barriers are
temperature or pH.
• Sympatric speciation is promoted by
disruptive selection.
Sympatric Speciation
Hybrid Zones
• A region can be occupied by several
populations of an organism which vary in
their ability to interbreed form hybrid
zones.
• These five closely related species
belong to the same “ring” species.
THINK. . .PAIR. . .SHARE
• What would happen if a barrier was
introduced that separated hybrid zone
‘B’ and ‘C’?
You should now be able to . . .
1. Define the term ‘species’.
2.Define ‘speciation’.
3.Describe the sequence of events in
allopatric speciation and the type of
barrier involved.
4.Describe the sequence of events in
sympatric speciation and the barriers
involved.
5.Explain the formation of hybrid zones and
the consequences of a barrier being
introduced.