3.3.8 Evolution may lead to Speciation

Question 1

What is evolution?

Answer 1

Is the change in allele frequencies in a population over time

Question 2

Name 2 methods in which evolution occurs

Answer 2

1. Natural selection
2. Genetic drift

Question 3

Describe how natural selection occurs by evolution

Answer 3

1. Selection pressures (e.g. predation, disease, competition) create struggle for survival
2. Genetic variation between individuals
3. Individuals with (beneficial) allele that makes them better adapted to selection pressure than others
4. This increases their chance of survival & more likely to reproduce and pass on this (beneficial) allele
5. Increased freqency of beneficial alleles in next generation
6. Offspring more likely to survive, reproduce and pass on their alleles
7. ∴ frequency of beneficial alleles in gene pool increases from generation to generation

Question 4

Name 4 observations Darwin made

Answer 4

• Population remain stable
• Offspring appear similar to their parents
• No two individuals are the same
• Organisms have the ability to reproduce

Question 5

What type of variation results in evolution?

Answer 5

Only genetic variation

Question 6

What is a selection pressure?

Answer 6

Anything that affects an organism’s chance of survival and reproduction

Question 7

Name 3 selection pressures

Answer 7

• Human activity / pollution
• Availability food sources
• Diseases
• Competition
• Predators
• Land clearance

Question 8

Types of natural selection affect ____ ______ in different ways

Answer 8

allele frequency

Question 9

Name the 3 types of natural selection

Answer 9

• Stablising Selection
• Directional Selection
• Disruptive Selection

Question 10

Define Stablising Selection

Answer 10

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce

(Favours the average = averages increase and extremes decrease)

Question 11

When does stablising selection occur?

Answer 11

Occurs when environment isn’t changing = reduces range of possible phenotypes

Question 12

Define Directional Selection

Answer 12

Where individuals with alleles for single extreme phenotype are more likely to survive and reproduce

Question 13

When does directional selection occur?

Answer 13

Occurs in response to environmental change

Question 14

Describe Disruptive Selection

Answer 14

Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce

Question 15

When does speciation occur?

Answer 15

When populations of same species become reproductively isolated

Question 16

Name the 2 types of speciation

Answer 16

• Allopatric Speciation
• Sympatric Speciation

Question 17

What does allopatric speciation involve?

Answer 17

Geographical isolation

Question 18

Describe allopatric speciation (6x)

Answer 18

1. Populations that are geographically separated will experience slightly environmental different conditions
2. Populations will experience different selection pressures ∴ = selection for different alleles
   
   1. Variation due to mutation
3. Selection for advantageous allele = selected organisms survive and reproduce
4. Leads to changes in allele frequency
5. Eventually, individuals from different populations changed so much = can’t breed to produce fertile offspring
   
   • Become reproductively isolated
6. 2 groups = become separate species

Question 19

Allopatric Speciation

Name 3 ways in which different changes in allele frequencies could occur

Answer 19

• Different alleles could be more favourable in new conditions and directional selection could take place
  
  • e.g. colder climate: directional selection will act on alleles for fur length = increasing frequency of allele for longer fur length
• Mutations could occur in each population
• Genetic drift

Question 20

What is sympatric speciation caused by?

Answer 20

Mutations, changes in mating patterns

Question 21

Describe how sympatric speciation could occur due to mutations

Answer 21

1. Most eukaryotic organisms = diploid
   
   1. Sometimes mutations occur that increase no. of chromosomes
   2. Known as polyploidy
2. Individuals with different no. of chromosomes can’t reproduce sexually to give fertile offspring
   
   • e.g. polyploid organism in diploid population = reproductively isolated
3. If polyploid organism reproduces asexually = new species develops

Question 22

Sympatric speciation is more common in ____ than ______

Answer 22

plants than animals

Question 23

Why does reproductive isolation occur?

Answer 23

Occurs ∵ changes in alleles and phenotypes in some individuals prevent breeding successfully with individuals without these changes

Question 24

Name 3 examples of changes that occur to individuals that leads to reproductive isolation

Answer 24

• Seasonal
  
  • Different flowering or mating seasons
  • Or become sexually active at different times of year
• Mechanical
  
  • Changes in genitalia prevent successful mating
• Behavioural
  
  • Develop courtship rituals that aren’t attractive to main populations

Question 25

What is genetic drift?

Answer 25

When chance rather than environmental factors dictates which individuals survive, breed and pass on their alleles

Question 26

Give an example of how genetic drift could lead to speciation

Answer 26

1. Individuals within population show variation in their genotypes
2. By chance, allele for one genotypes (B) is passed on to offspring more often than others
3. ∴ no. of individuals with the allele increases
4. Changes in allele frequency in 2 isolated populations could eventually lead to reproductive isolation and speciation

Question 27

Evolution by genetic drift has greater effect in ____ populations

Answer 27

smaller

Question 28

Why does genetic drift have a greater effect in smaller populations?

Answer 28

∵ chance has greater influence in smaller populations

(In larger populations, any chance variations in allele frequency tend to even out across whole population)

Question 29

Explain how an insect population can become resistant to an insectidice (3)

Answer 29

• mutations
• producing alleles giving resistance
• natural selection / leads to increased frequency of alleles in population

Question 30

Copper-tolerant grass produces its flowers eariler in the year than non-tolerant grass of the same species. Explain how this might produce two different species of grass. (5)

Answer 30

1. (populations are) reproductively isolated (due to different flowering times)
2. ∴ no flow of alleles between the populations/different (random) mutations in each group
3. different selection pressures
4. frequency of alleles will change
5. (genetic differences lead to) members of populations become unable to interbreed and produce fertile offspring

Question 31

One farmer stated that the increase in the use of Bt crop plants had caused a mutation in one of the insect species and that this mutation had spread to other species of insect. Was he correct? Explain your answer. (4)

Answer 31

• Mutations are random
• Only the rate of mutation is affected by environment
• Different species do not interbreed / do not produce fertile offspring
• So mutation/allele cannot be passed from one species to another

Question 32

There was a time lag between the introduction of Bt crops and the appearance of the first insect species that was resistant to the Bt toxin. Explain why there was a time lag. (3)

Answer 32

1. Initially one insect with favourable mutation / allele;
2. Individuals with (favourable) mutation / allele will have more offspring
3. Takes many generations for (favourable) mutation / allele to become the most common allele