2. Announcements
•
The workshop is this week. The assessment completed
during the workshop will count for 20% of your score in
this course.
•
Please go through the PopG tutorial, you will be tested
upon entering the workshop.
•
Please bring a pencil (and eraser)
•
Make sure you know your Student ID number
•
Don’t forget:
–
–
–
Tuesday 12 Nov. 14:00-17:00, FB 1.15a à C800, F850, Z100
Thursday 14 Nov. 15:00-18:00, FB 115a à C300, C400, C431
Friday 15 Nov. 14:00-17:00, FB 1.23 à C100
5. Summary of Week 6 Lectures
1) Defined our terms
–
Gene, Locus, Allele, Genotype, Phenotype, Gamete, Zygote,
Dominant, Recessive
2) Introduced genetic drift
–
–
–
Stochastic change in allele frequencies
Can lead to fixation or loss of alleles.
Stronger in small populations, eg. in founder events.
3) Touched on Selection
–
–
Selection occurs against a background of drift
Related to ‘fitness’ of a particular genotype
Any Questions?
6. Darwin on Selection
In 1859 Darwin rocked the foundations of modern science with
the publication of his seminal work “On the Origin of Species by
Means of Natural Selection”
“When on board H.M.S. “Beagle”,
as a naturalist, I was much struck
with certain facts in the distribution of
the inhabitants of South America, and
in the geological relations of the present
to the past inhabitants of that
continent. These facts seemed to me to
throw some light on the origin of
species – that mystery of mysteries, as
it has been called by one of our
greatest philosophers.”
Sold for £103,250 in 2009
7. Darwin on Selection
Darwin looked at selection, both artificially and in the wild, and
concluded that it could lead to systematic changes over long
timescales.
“That most skillful breeder, Sir John Sebright,
used to say, with respect to pigeons, that ‘he
would produce any given feather in three years,
but it would take him six years to obtain a
head and beak’”
“I can see no good reason to doubt that female
birds, be selecting, during thousands of
generations, the most melodious or beautiful
males, according to their standard of beauty,
might produce a marked effect.”
8. Darwin on Selection
Darwin was unaware of Gregor
Mendel’s work on heredity, and as
such many of the details of Darwin’s
theory were wrong (see “Pangenesis”).
However, the central principles of
evolution by natural selection hold
true to this day.
We can use our rigorous notation
from earlier lectures to obtain a more
up-to-date perspective on selection.
9. Darwin on Selection
Selection occurs at the level of
the…
Gene
Locus
Allele Population
Phenotype
Nucleotide
But genes can relate to phenotypes in various
different ways…
10. Types of Selection
If an allele is dominant then the heterozygote has the same phenotype as the
homozygote.
A is dominant
If an allele is recessive then the heterozygote has the same phenotype as the
other homozygote.
A is recessive
11. Types of Selection
If A is dominant then the heterozygote has the same fitness as the
homozygote
wAA = 1
wAB = 1
wBB = 0.8
If A is recessive then the heterozygote has the same fitness as the other
homozygote
wAA = 1
wAB = 0.8
wBB = 0.8
12. Types of Selection
Recall the picture of drift + selection from earlier lectures…
Don’t be seduced by the smoothness of these lines – drift is still occurring in
the background!
14. Types of Selection
When A is at high frequency B is rare, and
therefore B is most often present in
heterozygotes.
From a fitness point of view there is nothing
to differentiate AA from AB individuals, and
so there is very little phenotypic variation
for selection to operate on.
This is the same reason it is difficult
to eliminate deleterious recessive
alleles from a population, for example
in Ellis-van Creveld syndrome.
16. Types of Selection
Even when the A allele is at high
frequency the B allele is always ‘visible’
From a fitness point of view selection is
always acting to drive out B alleles
Dominant disorders can be driven out
of a population more easily than
recessive disorders, and hence there
are less of them around.
Marfan syndrome
17. Types of Selection
Other types of selection include heterozygote advantage (overdominance)…
wAA = 0.8
wAB = 1
wBB = 0.8
and heterozygote disadvantage (underdominance)…
wAA = 1
wAB = 0.8
wBB =1
19. Types of Selection
There is a balance between having enough A alleles and having
too many!
A alleles rare: mostly
present in
heterozygotes
Selection for A
A alleles common:
mostly present in
homozygotes
Selection against A
The equilibrium frequency is the
point at which these forces balance out
20. Types of Selection
A classic example of heterozygote advantage is sickle-cell anemia.
21. Types of Selection
A classic example of heterozygote advantage is sickle-cell anemia.
– The sickle-cell allele (HbS) is autosomal recessive; meaning only
homozygotes are affected
– However, HbS also confers partial resistance to malaria, meaning in
certain parts of the world the heterozygote has the highest fitness
Historical distribution of malaria and HbS allele
24. Types of Selection
One cause of heterozygote disadvantage is the formation of
hybrids, but more on this later…
Questions?
25. Announcements
•
The workshop is this week. The assessment completed
during the workshop will count for 20% of your score in
this course.
•
Please go through the PopG tutorial, you will be tested
upon entering the workshop.
•
Please bring a pencil (and eraser)
•
Make sure you know your Student ID number
•
Don’t forget:
–
–
–
Tuesday 12 Nov. 14:00-17:00, FB 1.15a à C800, F850, Z100
Thursday 14 Nov. 15:00-18:00, FB 115a à C300, C400, C431
Friday 15 Nov. 14:00-17:00, FB 1.23 à C100
27. Gene Flow
So far we have only looked at the effects of drift and selection within a single
panmictic population. To understand how evolution works across different
populations we must talk in terms of “gene flow”.
Gene flow describes the processes by which individuals genes (or
alleles) move from one population to another.
• Gene flow can be one-directional
or multi-directional
• Movement of individuals does
not necessarily imply movement
of genes!
28. Gene Flow
In the absence of gene flow populations tend to become genetically
differentiated from one another.
29. Gene Flow
In the absence of gene flow populations tend to become genetically
differentiated from one another.
30. Gene Flow
In the absence of gene flow populations tend to become genetically
differentiated from one another.
This is mainly visible in neutral loci, which are evolving under drift alone.
31. Gene Flow
Gene flow homogenises populations, and can recover lost genetic variation
32. Gene Flow
Many populations are isolated, experiencing limited or zero gene flow.
In this case we expect drift to lead to differentiation between
populations.
Smaller numbers of differences are expected between close branches,
larger differences between more distant branches
33. Gene Flow
• Branching patterns can also be
constrained by geographic
boundaries within species. In
this case, as before, drift leads
to differentiation between
distinct populations.
• Patterns reflect the consequences of the
spread of populations since the last ice age
(ending 10,000 years ago), at the height of
which most of Europe was inhospitable for
the species that currently inhabit it.
• Populations were restricted to refugia, and
they become a relic population of a once
more widespread species
35. Mutation
Consider the following questions…
1) What is mutation?
2) What are some ways of classifying mutation?
3) How does mutation interact with drift and selection?
36. Mutation
Mutation
• The processes producing genetic variation
• The original source of all genetic variation
• A permanent structural alteration in DNA
In most cases, DNA changes either have no effect or cause harm, but
occasionally a mutation can improve an organism's chance of
surviving and passing the beneficial change on to its descendants.
37. Mutation
Point Mutation
One base exchanged for another
Insertion
Extra base pair(s) inserted
Deletion
Base pair(s) lost
Frameshift
Applies to insertions and deletions. Anything
which changes the amino acid sequence being
coded for
38. Mutation
There are also some larger mutational events
that can occur, including…
• Large-scale deletion/insertion events
• Duplication
• Inversion
• Translocation
and some very large…
• Polysomy
• Whole genome duplication
39. Mutation
Without some process generating variation, eventually all alleles will become
either fixed or lost over enough time
41. Mutation
• Each gene copy experiences mutation at a rate μ
• In a population of 2N genes this is a total mutation rate of 2Nμ
• The chance of any one new allele going to fixation is 1/(2N)
• Therefore…the probability of a new mutant allele going to fixation under
drift alone is 1/(2N) * 2Nμ = μ
The rate of substitution is independent
of the population size
43. Sneak Peek: The Workshop
• What happens when you make a population bigger? smaller?
(with drift or drift and selection)
• What about if you change the fitness of one genotype or
another (aka change selection pressures)?
• What about if there is mutation? or migration?
44. Mini Revision Session
Short questions…
1. Define the terms Dominant and Recessive.
2. How are relative and absolute fitness calculated?
3. Is genetic drift stronger or weaker in a small population? Why?
Longer questions…
1. Explain how random sampling from a finite population leads to stochastic
changes in allele frequencies.
2. Why do we expect many more carriers of recessive deleterious alleles than
affected individuals?
3. For an allele that is at a high frequency in a population, would selection be
able to bring it to fixation faster if it is dominant or recessive? Why?
45. Mini Revision Session
I will not be providing model answers for all of the mini-revision session
questions, as it is far more important that you think about these
questions yourselves! All of the answers are contained within the lecture
notes of the past two weeks – once you familiarise yourself with this
material these questions should seem fairly straightforward.
Now is a good opportunity to ask me any questions
you have before the workshop
