Lectures 15 & 16Selection and Gene Flow

Bob Verity

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

The Drift Practical

Lecture Outline

1) Types of Selection

2) Gene Flow

3) Allele Frequency Clines and the

Formation of Hybrid Zones

Darwin on SelectionIn 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

Darwin on SelectionDarwin looked at selection, both artificially and in the wild, and concluded that it could lead to systematic changes over long timescales.

“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.”

“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’”

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.

Darwin on Selection

Selection occurs at the level of the…

GeneAllele

Phenotype

Population

LocusNucleotide

But genes can relate to phenotypes in various

different ways…

Types of SelectionIf an allele is dominant then the heterozygote has the same phenotype as the homozygote.

If an allele is recessive then the heterozygote has the same phenotype as the other homozygote.

A is dominant

A is recessive

If A is dominant then the heterozygote has the same fitness as the homozygote

If A is recessive then the heterozygote has the same fitness as the other homozygote

Types of Selection

wAA = 1 wAB = 1 wBB = 0.8

wAA = 1 wAB = 0.8 wBB = 0.8

Types of SelectionRecall 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!

Types of Selection

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

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.

Types of Selection

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

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

Other types of selection include heterozygote advantage (overdominance)…

and heterozygote disadvantage (underdominance)…

Types of Selection

wAA = 0.8 wAB = 1 wBB = 0.8

wAA = 1 wAB = 0.8 wBB =1

Types of Selection

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

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 thepoint at which these forces balance out

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

Types of Selection

Types of Selection

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

Types of Selection

One cause of heterozygote disadvantage is the formation of hybrids, but more on this later…

Questions?

Lecture Outline

1) Types of Selection

2) Gene Flow

3) Allele Frequency Clines and the

Formation of Hybrid Zones

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 can be one-directional or multi-directional

• Movement of individuals does not necessarily imply movement of genes!

Gene flow describes the processes by which individuals genes (or alleles) move from one population to

another.

Gene Flow

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

Gene Flow

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

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.

Gene Flow

Gene flow homogenises populations, and can recover lost genetic variation

Gene FlowMany 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

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

Lecture Outline

1) Types of Selection

2) Gene Flow

3) Allele Frequency Clines and the

Formation of Hybrid Zones

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

Allele Frequency Clines

Selection in favour of a dominant allele…

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.

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?

Allele Frequency Clines

Hybrid Zones

Hybrid Zones

The existence of this frequency cline can be explained by the reduced fitness of heterozygotes.

HOW?

Hybrid Zones

Hybrid Zones

Hybrid Zones

Gene flow never gets far into the other population due to the reduced fitness of

heterozygotes

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!