L2 Variation and polymorphism

Question 1

What are the three main types of selection?

Answer 1

Directional, disruptive and stabilising

Question 2

What is directional selection?

Answer 2

Shifts the overall population by forming an extreme

Question 3

Give an example of directional selection

Answer 3

Malaria parasite, shows a trend towards drug resistance once they have been introduced, driven by directional selection in a mutation for resistance

Question 4

What is divergent selection?

Answer 4

Favours variants of opposite extremes, and intermediates are selected against

Question 5

What is disruptive selection?

Answer 5

Divergence within the same population

Question 6

Give an example of divergent selection

Answer 6

In great tits, melanin based patten on neck, female preference differs with males in urban and country populations, may be due to difference in resources.

Question 7

What is stabilising selection?

Answer 7

Acts against extreme phenotypes, intermediates are selected for

Question 8

Give an example of stabilising selection?

Answer 8

birth size most favourable in the middle

Question 9

What species can you see all three types of selection acting at once? Explain

Answer 9

Spadefoot toads, see two different morphs, carnivorous and omnivorous. Divergence is seen as intermediates are selected against as are less effect feeders
Directional is seen because carnivores are actually favoured over omnivores.
Stabilising is seen because although selected against under normal conditions, when occurring with spade foot bombifrons, the intermediates are favoured.

Question 10

What does selection often do to variation?

Answer 10

Reduces it, however variation is necessary for selection

Question 11

What is polymorphism?

Answer 11

Diversity in morphology across populations, is evidence of genetic variation

Question 12

What mechanisms are present to maintain variation for selection to act on?

Answer 12

Diploidy
gene flow/migration
mutation
balancing selection

Question 13

What is diploidy?

Answer 13

Alleles of low fitness can be kept in the gene pool without being expressed. Dangerous recessive mutations can hide from selection.

Question 14

What is gene flow?

Answer 14

Random changes in unselected alleles, accounts for changes in the genetic background.

Question 15

How does migration maintain variation?

Answer 15

Brings genetic background to new populations

Question 16

How does mutation maintain variation?

Answer 16

Can bring new variation into the population

Question 17

What is balancing selection?

Answer 17

Mechanism for selection that keeps allele proportions in the population at the same level, if the population is acting at max fitness

Question 18

What are the two mechanisms by which balancing selection acts?

Answer 18

Heterozygote advantage

Frequency dependent selection

Question 19

What is heterozygote advantage?

Answer 19

Case in which heterozygote genotype has a higher fitness, known as overdominance

Question 20

What are the examples of heterozygote advantage?

Answer 20

Sickle cell anaemia - increases resistance to malaria
Connexin 26 - builds thicker skin, two copies is deaf
Thalassemia - one copy can give resistance to heart disease and malaria

Question 21

Is heterozygote advantage likely to be a common method of balancing selection?

Answer 21

if an adaptation is good, should dominate the population which would wipe out heterozygotes. Suggested that every allele from mutation is fitter in heterozygote form.
May appear to be HA, but might actually be HA.

Question 22

Why might HA not be what it appears to be?

Answer 22

Need to look at what is happening in the background, may actually be in linkage disequilibrium with a beneficial allele, so the heterozygote alleles are not what is causing the benefits seen

Question 23

What is associative overdominace?

Answer 23

An increase in fitness of heterozygotes at a neutral locus because it is in linkage disequilibrium with a locus that is under selection

Question 24

What is the selection coefficient?

Answer 24

the difference between the men relative fitness if individuals of a given genotype and that of a referee genotype

Question 25

What does HA place on the population?

Answer 25

A load, is unstable because of duplication

Question 26

What is positive frequency dependent selection?

Answer 26

Survival and reproduction of any one morph increases if that phenotype form becomes more common in the environment - strength in numbers e.g. mullerian mimicry

Question 27

What is negative frequency dependent selection?

Answer 27

Survival and reproduction of any one morph decrease if that phenotype form becomes more common in the environment - better to be rare batsman mimicry , host parasite cycles

Question 28

What is frequency dependent selection

Answer 28

fitness of a phenotype depends on its frequency relative to other phenotypes in a given population