Lecture notes on different types of selection, and gene flow, including the formation of allele frequency clines and hybrid zones.

1. Lectures 15 & 16
Selection and Gene Flow
Bob Verity

2. The Drift Practical
Wednesday 6th: Mini-exam
– 11:00-11:30 OR 11:30-12:00, FB.1115a
– Bring notes on video, and demonstrate familiarity with PopG
Wednesday 6th: Practical
– 14:00-18:00, FB.1.15a
Thursday 7th: Practical
– 14:00-18:00, FB.1.15a
Friday 8th: Remedial session
– 14:00-1800, FB.115a

3. The Drift Practical

4. Lecture Outline
1) Types of Selection
2) Gene Flow
3) Allele Frequency Clines and the Formation of
Hybrid Zones

5. 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 Sold for £103,250 in 2009
greatest philosophers.”

6. 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.”

7. 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.

8. Darwin on Selection
Selection occurs at the level of
the…
Allele Population
Gene
Phenotype
Locus
Nucleotide
But genes can relate to phenotypes in various
different ways…

9. 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

10. 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

11. 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!

12. Types of Selection
Q. How can we explain the shape of this curve?

13. 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.

14. Types of Selection
Q. How can we explain the shape of this curve?

15. 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

16. 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

17. Types of Selection
Q. How can we explain the shape of this curve?

18. Types of Selection
There is a balance between having enough A alleles and having
too many!
A alleles rare: mostly A alleles common:
present in mostly present in
heterozygotes homozygotes
Selection for A Selection against A
The equilibrium frequency is the
point at which these forces balance out

19. 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

20. Types of Selection

21. Types of Selection
Q. How can we explain the shape of this curve?

22. Types of Selection
One cause of heterozygote disadvantage is the formation of
hybrids, but more on this later…
Questions?

23. Lecture Outline
1) Types of Selection
2) Gene Flow
3) Allele Frequency Clines and the Formation of
Hybrid Zones

24. 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!

25. Gene Flow
In the absence of gene flow populations tend to become genetically
differentiated from one another.

26. Gene Flow
In the absence of gene flow populations tend to become genetically
differentiated from one another.

27. 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.

28. Gene Flow
Gene flow homogenises populations, and can recover lost genetic variation

29. 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

30. 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 – a
relic population of a once more widespread
species

31. Lecture Outline
1) Types of Selection
2) Gene Flow
3) Allele Frequency Clines and the Formation of
Hybrid Zones

32. Allele Frequency Clines
• Biston betularia (the Peppered
Moth) exists in melanic and
wild-type phenotypes
• As the melanic (A) allele is
dominant: both AA and AB
individuals express the black
colouration – hence wAA = wAB
• Industrial melanism
hypothesis: selection in favour
of the melanic form post
industrial revolution

33. Allele Frequency Clines
Selection in favour of a dominant allele…

34. Allele Frequency Clines
Some evidence to support this: Mark recapture experiments found
that the fitness of the melanic morph is higher in areas where they
are prevalent.

35. Allele Frequency Clines
• The industrial revolution did not lead to
the blackening of all trees. The Delamere
Forest near Manchester and Liverpool is
relatively unaffected but the peppered
moths are predominantly melanic there.
• On the other hand the Gonodontis bidentata
(Scalloped Hazel), which are also melanic
right in the heart of the major industrial
centres, are predominantly non-melanic
in Delamere forest.
• The difference between the two species
may be explained by their dispersal rates.
HOW?

36. Allele Frequency Clines

37. Hybrid Zones

38. Hybrid Zones
The existence of this frequency cline can be explained by the reduced
fitness of heterozygotes.
HOW?

39. Hybrid Zones

40. Hybrid Zones

41. Hybrid Zones
Gene flow never gets far into the other population due to
the reduced fitness of heterozygotes

42. A Complete(ish) Picture
We can start to build up a picture of what
evolution really looks like…
• First and foremost there is genetic drift
• There may also be some selection
acting
• Gene flow homogenises allele
frequencies between populations
• Mutation introduces new
genetic variation into
populations that may have lost
it due to drift or selection
• There are still many processes
missing from this picture!