Lecture 10 - Maintenance of genetic variation 2

Question 1

What are the features of heterozygote advantage in sickle cell anaemia?

Answer 1

S allele: causes change in the red blood cell shape (prevents symptoms of malaria)
A allele: normal blood cells
SS homozygote: usually die due to severe anaemia, but people wiith S allele quite resitant to malaria
AA homozygote: most susceptible to malaria
Genotype specific fitness: (in regions where malaria is common)
wAA: 0.9 (AA)
wAa:1 (AS)
waa: 0.2 (SS)

Question 2

What equilibrium frequencies would we expect when malaria is common?

Answer 2

w1 = 1 - s = 0.9
w2 = 1
w3 = 1-t = 0.2
Therefore s = 0.1, and t = 0.8 and 
equilibrium of q = s/(s+t)
Average allele frequency in West africa is 0.09 but we would expect lots of variation between local population

Question 3

What is segregational load?

Answer 3

Due to heterozygote advantage

• heterzygotes have higher fitness than either homozygote, yet f(Aa) cannot = 1
• homozygotes constantly emerge due to mendelian segregation

Question 4

Why do hard and soft selection exist?

Answer 4

More offspring are produced than can survive and reproduce

Question 5

What is Hard selection?

Answer 5

• results in a reduction in population size
• mortality depends on microenvironment NOT on the fitness of other genotypes
• e.g. heavy metal tolerant plants

Question 6

What is soft selection?

Answer 6

• results in no reduction in population size
• some genotypes survive or reproduce better than others
• mortality depends on the relative fitness of genotye (speed of deer running away from a predator)

Question 7

What is frequency-dependent selection? (Soft selection)

Answer 7

• fitness can depend on a genotypes frequncy in the environment
• usually being rare is an advantage

Question 8

Give an example of Frequency dependent seletion

Answer 8

Left-handedness in humans

• in interactive sports there is a higher frequency of left-handed players than in the overall population or in non-interactive sports
• rare alleles have an advantage

Question 9

What is the genotype specific fitness when frequency dependent selection is operating?

Answer 9

Genotype-specific fitness
wAA: 1 - s1 X p^2
wAa: 1 - 2 X s2 X pq
waa: 1 - s3 X q^2

Each genotype can have a different disadvantage when rare, hence three different selection coefficients

Question 10

What does the fitness of the three genotypes depend on when frequency dependent selection is operating?

Answer 10

The fitness of the three genotypes depends on allele frequencies
Each genotype can have a different disadvantage when rare, therefore we have three different selection coefficients

Question 11

How does frequency-dependent selection differ from heterozygote advantage?

Answer 11

• selection coefficients are not constant but depend on genotype frequencies
• involves soft, not hard selection
• with equal selection coefficients, at equilibrium the heterozygote has lower fitness than either homozygote

Question 12

Give two examples of frequency dependent selection

Answer 12

Apostatic selection (Negative frequency selection)
Frequency-dependent parasitism

Question 13

Give an example of apopstatic selection

Answer 13

Song thrushes and Cepea snails (negative frequency selection)

• colour polymorphisms in two species of snail
• predation by thrushes
• birds become accustomed to a particular pattern depending on the highest proportion
• snails then switch to the other pattern

Question 14

Outline frequency dependent parasitism and dissasortive mating in Perissodus microlepis

Answer 14

Perissodus microlepis

• two different jaw morphologies
• ‘Righty’ and ‘Lefty’
• frequency fluctuates around 50%
• prey know what to expect (which side attack comes from)
• disadvantagous to most common morph
• A ‘lefty’ more common to mate with a ‘righty’ to result in more diverse offspring to maintain a 50/50 distibution (dissortive mating)

Question 15

Outline genotype X genotype interactions between the snail and its parasites Potamopyrgus antipodarum

Answer 15

• local parasites have a higher experimental infection rate than foreign parasites
• infections are localised and adapted to a particular lake
• heterozygotes can be infected by both types of parasites ,

Question 16

Give an example of positive frequency dependent selection

Answer 16

Mullerian minicry in Heliconius butterflies

• two or more poisonous species, that may or may not be closely related and share one or more common predators, mimic each other’s warning signals
• there is an advantage in the shared appearance of the two prey species
• predator that learns to avoid either species in a pair of species exhibiting Müllerian mimicry therefore learns to avoid both

Question 17

Give an example of stabilising selection

Answer 17

Eurosta: galling flies
-galls are attacked by birds and parasitoid wasps
-parasitoid wasps lays eggs in galls for larvae feeding and hiding
There is an equilibrium of gall size
-the bigger the gall, parasites cannot reach the centre, small galls are preferred by wasps
-bigger more attractive to birds

Question 18

Give an example of spacial selection

Answer 18

Colour polymorphism in the rock pocket mouse Chaetodipus intermedius

• 2 colour morphs (black, sandy)
• On dark coloured rocks the black colour morph is favoured
• On light coloured rocks the sandy colour morph is favoured
• distribution reflective of this

Question 19

Give an example of a trait that is negatively correlated and said to exhibit trade offs

Answer 19

Trade off in Drosophila melanogaster and the parasite Asobara tabida

• selection for increased resistance to parasites
• more resistant flies are worse competitors

Question 20

How can genetic correlation affect selection

Answer 20

Genetic correlations can change the response to selection or be the result of selection

Question 21

What are traits genetically correlated?

Answer 21

Two mechanisms

• Pleitropy: single gene affects multiple traits, e.g. in morning glories, floral pigment also affects plant toxicity to herbivores
• Physical linkage

Question 22

Outline anti-predator behaviour in the garter snake Thamnophis ordoides (genetic correlations amongst traits)

Answer 22

Snakes display a colour and pattern polymorphism

• react to predators by fleeing or reversing
• stripes are related to their behaviour (reversing/fleeing)
• when fleeing and going forwards, they look slower than they are (optical illlusion)
• stripes and reversal are negatively correlated probably to do with linkage disequilibrium between the two traits

Question 23

What are the features of selection and evolution in correlated traits?

Answer 23

• positive selection can lead to co-adapted traits
• selection on one trait will lead to correlated evolution in the other
• traits that are negatively correlated are said to exhibit trade offs

Question 24

How can genetic correlations be measured?

Answer 24

Use same breeding designs as for additive variance, but measure two traits instead of just one
-parent/offspring regression
-sib analysis
Artificial selection experiments

Question 25

By what mechanisms can genetic variation be maintained by selection

Answer 25

Heterozygote advantage
-equilibrium determined by the fitness of the two homozygote genotypes
Fitness-dependent selection
-usually rare genotypes have higher fitness
Mutation selection balance
A balance of temporal or spacial selective forces
Genotype x environment interactions
Agonistic genetic correlations