Natural selection (lec 5)

Question 1

Population structure effect on variation

Answer 1

-migration between populations (gene flow)
-isolation of populations makes them more distinct
size of population and amount of gene flow will determine evolutionary outcomes

Question 2

Disruptive selection

Answer 2

• diversifying

- homozygotes fitter than heterozygotes or multiple variants favoured

Question 3

Positive selection:

Answer 3

• eg. capsid proteins of viruses
• difficulty developing vaccine due to changing coats to avoid immune system (red queen)
• mutations in the sites that the host immune system recognise are under positive selection
• recognition sites thus change v rapidly

Question 4

Purifying selection

Answer 4

eg capsid proteins of viruses

• some parts of virus can’t tolerate changes as they are necessary to maintain structure or functions
• these regions are under purifying selection, show few changes

Question 5

In reality most genes probably experience a mix of both:

Answer 5

-purifying and positive selection

Question 6

Balancing selection

-e.g. sickle cell anaemia gene for malarial resistance

Answer 6

• multiple variants are favoured
• results in maintenance of high levels of polymorphism
• often because different alleles are favoured in different circumstances or because theres an advantage to being different (frequency-dependent selection)

Question 7

Frequency-dependent selection (balancing selection)

Answer 7

e. g. MHC

- heterozygotes have an advantage

Question 8

Inbreeding vs outcrossing (reproductive advantage)

Answer 8

• inbreeding increases homozygosity
• recessive alleles persisting in populations brought out into the phenotype due to inbreeding
• inbreeding depression - reduced fitness

Question 9

Inbreeding could be an advantage to not breaking up

Answer 9

co adapted gene complexes

-after a period of inbreeding, negative deleterious mutations purged so no inbreeding depression, as long as environment remains the same

Question 10

Outbreeding can lead to

Answer 10

heterozygote advantage
-also called heterosis or hybrid vigour
eg, Soay sheep in Hebrides
-higher fitness sheep have higher levels of heterozygosity

Question 11

Outbreeding depression=

Answer 11

• organisms well adapted to an environment mate with individuals adapted to another environment
• eg conservation genetics and captive breeding

Question 12

Purging

Answer 12

-individuals that sirvive have gotten rid of the nastiest deleterious recessive mutations

Question 13

Assortative mating

Answer 13

-can ensure that similarly adapted individuals mate

Question 14

Dis-assortative mating

Answer 14

-can ensure individuals mate with genetically different individuals to increase genetic diversity

Question 15

Kin recognition

Answer 15

-behavioural recognition of close relatives

Question 16

inbreeding avoidance

Answer 16

-recognition associated with MHC

Question 17

Self-incompatibility in plants

-type of frequency-dependent selection

Answer 17

• genetically controlled

- pollen with similar genotype to that expressed on stigma not allowed to fertilise

Question 18

Hardy-Weinberg equilibrium HWE 1908

Answer 18

• forces affecting variation are tested

- many assumptions used to develop models to test hypotheses about polymorphisms in populations

Question 19

genotype frequencies

Answer 19

p2 + 2pq + q2 = 1

Question 20

Allele frequencies

Answer 20

p + q = 1