Lecture 8 - Selection

Question 1

What are the different types of selection?

Answer 1

Directional
Stabilising
Disruptive

Question 2

What are the prerequisites for selection?

Answer 2

• variation in fitness between genotypes

- variation needs to have a genetic basic (be heritable)

Question 3

What are the features of selection?

Answer 3

• non-random process
• can lead to adaptation
• simplest form: reduces genetic variation

Question 4

What are the features of adaptation?

Answer 4

• difficult to show that it is the response to selection (mathematical model)
• often a slow process, longer than a human life span

Question 5

What examples are there of adaptation and selection?

Answer 5

• the increase of Methacillin-resistant Staphylococcus aureus in samples from hospitals
• the peppered moth Biston bistularia

Question 6

Outline the MRSA example of adaptation and selection

Answer 6

MRSA
-MRSA infections now respond only to very advanced antibiotics that were never meant to be a first-line defense
-number of MRSA and MSSA samples from hospitals has dramatically increased
-rate of MRSA infections recorded at U.S. academic hospitals doubled between 2003 and 2008
Peppered moth Biston bistularia
-

Question 7

Outline the peppered moth example of adaptation and selection

Answer 7

• Before the industrial revolution in Britain, most peppered moths were of the pale variety
• camouflaged against the pale birch trees mutant black coloured moths were easily spotted and eaten by birds
• white variety had an advantage, and they were more likely to survive to reproduce.
• Airborne pollution in industrial areas blackened the birch tree bark with soot
• the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators
• black variety had an advantage, and they were more likely to survive and reproduce
• Over time, the black peppered moths became far more numerous in urban areas than the pale variety.

Question 8

What factors does the response to selection depend upon?

Answer 8

Heritability
Strength of selection (dependent upon fitness differences between genotypes)
Allele frequencies
Dominance of the allele 
Population size

Question 9

What are the two types of fitness?

Answer 9

Absolute fitness

Relative fitness

Question 10

What is the definition of absolute fitness?

Answer 10

Absolute fitness is the genotype specific rate of increase that predicts the absolute number of individuals

Question 11

What is the definition of relative fitness?

Answer 11

Relative fitness is the fitness relative to one chosen genotype, normally relative to the fittest genotype

Question 12

What does the basic selection model assume?

Answer 12

Diploid organisms
One locus with two alleles
Sexual reproduction
Non overlapping generations 
Random mating
Viability selection
Population in Hw equilibrium 
An ideal population (infinite size, no population structure, no mutation)

Question 13

What are the symbols for relative fitness and absolute fitness?

Answer 13

AA: w1
Aa: w2
aa: w3

Average fitness of a population is w bar

Question 14

What do the genotype and allele frequencies after selection depend upon?

Answer 14

The initial allele frequencies
The fitness of each genotype
The average fitness in a population

Question 15

What are the formula for genotype frequencies after selection?

Answer 15

(P^2 x w1) / w bar (total fitness)

Question 16

What are two implications on the change of allele frequencies?

Answer 16

No change when p=0 or q=0, ie when there is only one allele in the population
No change when all the genotypes have the same fitness

Question 17

Give an example of the change in allele frequencies

Answer 17

The scarlet tiger moth callimlrpha Dominula

• the medionigra morph reduces spotting on the forewing
• only found in the cothill population
• the morph declined rapidly in the cothill population

Question 18

Aside from being expressed in terms of w, how else can relative fitness also be expressed?

Answer 18

In terms of a selection coefficient s

S is the difference between 1 and the relative fitness value of the genotype

Question 19

What is the genotype specific fitness of AA, Aa, and aa when selection is against a recessive phenotype and when selection is against a dominant phenotype? (Fitness in terms of s)

Answer 19

Selection against a recessive phenotype
AA: 1
Aa: 1
aa: 1 - s
Selection against a dominant phenotype
AA: 1 - s 
Aa: 1 - s
aa: 1

Question 20

How does selection act?

Answer 20

Selection interacts with other processes e.g. Genetic drift

And is more efficient in large populations