Evolution

Question 1

Explain how a genetic bottleneck event may lead to a decrease in genetic diversity [2]

Answer 1

1. Few random surviving individuals
   » reduced population size
2. Some alleles lost to GENETIC DRIFT
   » few remaining alleles not representative of the original gene pool (underrepresented)

Question 2

Explain why morphological data alone may not accurately represent the degree of relatedness [3]

Answer 2

1. Morphological structures may be due to CONVERGENT EVOLUTION
2. where 2 unrelated organisms (no RCA / evolved independently)
3. faced similar selection pressures (niches) in similar environments
4. led to independent selection of similar analogous structures adapted to that environment

Question 3

Explain how biogeography supports DTE [4]

Answer 3

DESCENT FROM COMMON ANCESTOR

1. All species descended from a common ancestor
2. which originated in the supercontinent

DISPARATE DISTRIBUTION

3. Could disperse across continents which were close together
4. Continental drift followed&raquo_space; populations isolated

MODIFICATION FROM COMMON ANCESTOR

5. Oceans were barriers that disrupted gene flow
6. Different selection pressures&raquo_space; natural selection&raquo_space; speciation

Question 4

Explain how amphibians evolved from fish [4]

Answer 4

1. VARIATION&raquo_space; some could breathe air better / sturdier fins
2. Strong SELECTION PRESSURE to move to land
   » predators/competition for food at sea
   » new niches on land
3. Such fish could survive&raquo_space; reproduce&raquo_space; pass down advantageous alleles
4. New species, Tiktaalik evolved
5. Further selection pressure on land
   » for the ability to move and breathe on land

Question 5

Explain the significance of Tiktaalik [2]

Answer 5

1. It was a TRANSITIONAL FOSSIL

2. Supports DTE by illustrating descent from a common ancestor with modification

Question 6

Explain how NS led to a decrease in mastodon population size towards the end of the Ice Age [3]

Answer 6

1. Warmer climate acted as SELECTION PRESSURE
2. SELECTIVE ADVANTAGE: thin fur allowed for heat loss
3. Higher chance of reaching reproductive age and producing FERTILE, VIABLE OFFSPRING with advantageous alleles

Question 7

Explain why mtDNA genes are preferred [4]

Answer 7

1. No recombination/crossing-over
2. Differences in mtDNA solely due to the accumulation of mutations
3. Faster mutation rate
4. Discernable differences between DNA of organisms

Question 8

Explain the consequences of having a large population that descended from a small number of individuals [3]

Answer 8

1. Low genetic variation
2. High homozygosity
3. Increase the chances of harmful recessive alleles being expressed
4. Could face extinction if environment changes
   » cannot adapt to new selection pressures

Question 9

Explain why a combination of antibiotics reduces the risk of developing ABR [2]

Answer 9

1. Ensures all bacteria are killed
2. Bacteria that are resistant against one antibiotic are likely susceptible to antibiotic
   » chances of mutations occurring in 2 genes in the same bacterium that make it resistant is very small
3. Antibiotics have different sites of action

Question 10

Explain how ABR develops [4]

Answer 10

1. Patient does not complete the course of antibiotics
2. Some susceptible bacteria survive
3. Replicate quickly&raquo_space; increased chance of mutation in allele coding for ABR
4. Allele introduced via conjugation
5. Antibiotic acts as SELECTION ADVANTAGE&raquo_space; resistant bacteria selected for
6. SELECTION PRESSURE
7. INCREASE IN ALLELE FREQUENCY (survive > reproduce > pass down allele)

Question 11

Explain change in hind limb length on source island and founder islands [5]

Answer 11

1. Founder islands - decrease - directional selection
2. Source island - constant - stabilising selection

+ explain NS on founder islands

Question 12

Explain speciation process on founder islands [3]

Answer 12

1. GEOGRAPHICAL ISOLATION&raquo_space; DISRUPT GENE FLOW
2. DIFFERENT SELECTION PRESSURES
3. Variation in phenotypes
   » Individuals with favourable traits/better adapted had a selective advantage&raquo_space; selected for
   » increase frequency of favourable alleles (survive, reproduce, pass down alleles)
4. Sub-populations evolve independently
   + accumulate different MUTATIONS
   + subjected to GENETIC DRIFT and NATURAL SELECTION
   » allele frequencies change
5. Forms REPRODUCTIVELY ISOLATED species via ALLOPATRIC SPECIATION

Question 13

Explain the relationship between generation time and rate of evolution [3]

Answer 13

1. (inverse relationship)
2. Less reproduction / DNA replication per unit time
3. Fewer mutations occur per unit time
4. Fewer changes in allele frequency

Question 14

Suggest why the tuatara remained largely unchanged [3]

Answer 14

1. Well adapted
2. Environment&raquo_space; selection pressures stayed constant
3. Low rate of mutations
4. No separation of the population for speciation

Question 15

Explain why small founder populations experience greater genetic change

Answer 15

1. Genetic change = change in allele frequency
2. Founder’s effect - fewer representative alleles
   » each random mating is more significant;
   » each individual forms a larger proportion of the gene pool
   » easier to lose allele from the gene pool

Question 16

Suggest why % ABR infections fell [2]

Answer 16

1. Fewer doses of antibiotics used&raquo_space; less selective pressure
2. Non-resistant strains have a SELECTIVE ADVANTAGE
   » resources not used for ABR&raquo_space; diverted to growth and reproduction