Topic 7B: Populations and Evolution

Question 1

What is a gene pool?

Answer 1

• All the alleles of all the individuals in a population at any one time

Question 2

What are allele frequencies?

Answer 2

• The number of times an allele occurs within a population

Question 3

What is the Hardy-Weinberg principle?

Answer 3

• Allele frequencies will remain the same from one generation to the next

Question 4

What conditions are there in Hardy-Weinberg?

Answer 4

• No selection
• No mutation
• No migration
• Large population size
• Mating is random

Question 5

What is the first equation?

Answer 5

p + q = 1

Question 6

What is the first equation used for?

Answer 6

• Predicting allele frequencies

Question 7

What do p and q mean?

Answer 7

• p = frequency of the dominant allele
• q = frequency of the recessive allele

Question 8

What is the second equation?

Answer 8

p2 + 2pq+ q2 = 1

Question 9

What is the second equation used for?

Answer 9

• Predicting genotype and phenotype frequencies

Question 10

What do p2, 2pq and q2 mean?

Answer 10

• p2 = frequency of homozygous dominant genotype
• 2pq = frequency of heterozygous genotype
• q2 = frequency of homozygous recessive genotype

Question 11

How does variation occur?

Answer 11

• Mutation
• Crossing over of chromatids
• Independent segregation
• Random fertilisation

Question 12

Describe the general process of evolution

Answer 12

• Variation arises due to mutation
• Something acts as a selection pressure
• Individuals with favourable alleles more likely to survive and reproduce
• Pass on the favourable allele to offspring
• Over time the frequency of the favourable allele in the population increases

Question 13

When does stabilising selection occur?

Answer 13

• In stable environmental conditions

Question 14

What happens in stabilising selection?

Answer 14

• Average phenotype makes individuals more likely to survive and reproduce
• Is selected for
• Extremes selected against

Question 15

What happens to mean and SD in stabilising selection?

Answer 15

• Same mean
• Smaller SD

Question 16

When does directional selection happen?

Answer 16

• When an environmental change has occurred

Question 17

What happens in directional selection?

Answer 17

• One extreme phenotype makes individuals more likely to survive and reproduce
• Is selected for
• Average and other extreme selected against

Question 18

What happens to mean and SD in directional selection?

Answer 18

• New mean
• Same SD

Question 19

What happens in disruptive selection?

Answer 19

• Both extreme phenotypes selected for
• Environment favours more than one phenotype

Question 20

Define species

Answer 20

• Organisms that can breed together to produce fertile offspring

Question 21

What is speciation?

Answer 21

• Development of a new species from an existing species

Question 22

How does speciation occur?

Answer 22

• Initially a single population of a species
• Splits to 2 populations that cannot interbreed
• Different environments have different selection pressures
• Each population has variation, different mutations and so different alleles arise
• Some individuals are better adapted to new conditions
• Allele frequencies change in the 2 populations
• Eventually allele differences are so great, the 2 populations cannot interbreed
• 2 different species made

Question 23

What is allopatric speciation?

Answer 23

• Geographically isolated
• e.g. by river, mountain range etc

Question 24

What is sympatric speciation?

Answer 24

• No geographical isolation
• Reproductively isolated
• Seasonal changes / temporal shift –> breed at different times (nocturnal) or breed at different times in the season
• Behavioural changes –> changed courtship behaviours
• Mechanical changes –> differences in genitalia / stamens

Question 25

What is genetic drift?

Answer 25

• Change in allele frequencies due to chance

Question 26

What size population does genetic drift have a greater effect on and why?

Answer 26

• Smaller
• Less individuals to start with - loss of some has a greater impact