Mark Scheme Answers

Question 1

Suggest one advantage of fish occupying different depths of lake (2)

Answer 1

• Reduces interspecific competition
• Food source/prey/oxygen

Question 2

Suggest why there are different values of biomass in each quadrat (2)

Answer 2

• Quadrats from different parts of field
  -Biotic/abiotic factors are different

Question 3

Explain how producing shorter stems increases the harvest index (1)

Answer 3

It increases the proportion of crops that is used

Question 4

Why are nests sampled at random (3)

Answer 4

• To eliminate bias
• Produce representative data
• Allows use of statistical tests

Question 5

When a correlation coefficient is obtained and produces a negative value, what does a negative value indicate? (1)

Answer 5

When one variable increases, the other variable tends to decrease

Question 6

Suggest two reasons why freshwater habitats hydrilla has rapidly become the dominant species (2)

Answer 6

• No/few consumers/pests/pathogens
• Outcompetes/better competitor for resources/light/CO2/abiotic factor/ideal niche

Question 7

Suggests two economic consequences of the spread of hydrilla in water systems (4)

Answer 7

• Cost of control/removal
• Cost of restoring habitat/conservation
• Loss of income from fishing
• Loss of income from boating/tourism/recreation

Question 8

The diversity of animal species is higher at E than A. Explain why? (2)

Answer 8

• Greater variety of food
• More variety of habitats

Question 9

Suggest two reasons for conserving woodlands (5)

Answer 9

• Conserving/protecting habitats/niches
• Conserving/protecting (endangered) species/maintains/increases biodiversity
• Reduces global warming/greenhouse effect/climate change/remove/take up carbon dioxide
• Source of medicines/chemicals/woods
• Reduces erosion/eutrophication

Question 10

The seeds produced by this species of tree did not grow successfully in a climax community. Suggest why? (3)

Answer 10

• Small size
• Too little food in seed to become established
• Not enough photosynthesis

Question 11

Suggest why the biodiversity of plant species in a climax community (woodland) may decrease (1)

Answer 11

Climax community is normally woodland, so the tree canopies may block light from the species growing below. Only plant species able to synthesize in low light intensity survive, reducing species diversity

Question 12

Suggest why incubating at higher temperatures does not produce valid results when using dry biomass to measure? (1)

Answer 12

• Heat can cause the loss of other substances

Question 13

Give an example of a null hypothesis (1)

Answer 13

Bacteriophage has no effect on the number of living bacteria

Question 14

Suggest why human ATP synthase is not inhibited and bacterial synthase is inhibited (1)

Answer 14

ATP synthase has a different tertiary structure to bacterial ATP synthase

Question 15

Suggest why scientists released more transgenic males (contain a lethal gene that kills offspring’s before reproduction) every week in developing a new method to control A. aegypti (mosquitoes) (1)

Answer 15

• To maintain the number/competition as they die/ have a short life span

Question 16

Suggest one reason why farmers would want the calves to be all of the same sex (3)

Answer 16

• Females for milk
• Males for meat
• Male and female for breeding

Question 17

Male cats with tortoiseshell (co-dominant alleles) do not usually occur. Explain why (1)

Answer 17

• Only one allele/ Y chromosome does not carry allele/gene

Question 18

Some cat breeders select for polydactyly (dominant). Describe how this would affect the frequencies of the homozygous genotypes for this gene in their breeding populations over time (1)

Answer 18

Homozygous alleles will increase and homozygous recessive will decrease

Question 19

When is a chi squared value test used? (1)

Answer 19

When using categorical data/ comparing frequencies

Question 20

What is the format answer for linked genes (4)

Answer 20

• State what genes are linked e.g. (GN and gn linked)
• What offspring’s do the parents produce? (GgNn individual produces mainly GN and gn gametes
• Fewer homozygous genes individuals
• Crossing over produces some / few Gn and gN gametes

Question 21

When comparing variation in size between two groups of organisms, it is often considered more useful to compare standard deviations rather than ranges. Explain why. (4)

Answer 21

• S.D shows spread about mean whereas range only shows highest and lowest values
• Range is influenced by a single outlier/ anomaly
• S.D shows if differences are likely to be significant
• Overlap, not significantly significant/ no overlap significantly different

Question 22

Lemmings are small mammals which live in the Arctic. Lemmings often live in isolated populations. From time to time some lemmings move and join other populations. Explain how this movement is important in maintaining genetic variability in lemming populations (1)

Answer 22

Increases gene pool/ Migrants bring in new alleles

Question 23

The pesticide is able to pass easily through cell membranes. Suggest why (3)

Answer 23

• Pesticide is lipid soluble
• Membranes contain lipid/ phospholipid/ diffuses directly across phospholipid bilayer
• Pesticide fits into receptors

Question 24

Scientists placed male and female snails of forms T and M into an aquarium. They recorded how many form T males mated with form T females and how many mated with for M females.

The scientists found that the probability of a form T male mating with a form T female was greater than 90%. They interpreted this result as evidence that speciation was taking place.

Explain why (4)

Answer 24

• T males mainly breed with T females and not M so they are reproductively isolated from M
• Different selection pressures in their true environment
• Different alleles are passed onto offspring
• Changing allele frequency in T (compared to M)

Question 25

What is the template for allopatric speciation (7)

Answer 25

* Geographical isolation * Different abiotic/environmental conditions/ different selection pressures * Separate gene pools/ no interbreeding * New alleles are formed through mutation * advantageous alleles passed onto offspring * Change in allele frequency in each population * over a long period of time, different populations are unable to reproduce to produce a fertile offspring - leads to disruptive selection

Question 26

Give the answer template for the formation of resistant alleles (6)

Answer 26

* Pesticides/ named factor creates a selection pressure * Allele for resistance already present in some insect/named animal due to mutation - Insects/named animal with resistance allele have a selective advantage - More likely to survive and reproduce more offspring/differential reproductive success - Offspring inherit allele which increases frequency of resistance allele - Overtime the proportion of insects in the population with the allele increases

Question 27

Give the answer template for sympatric speciation (7)

Answer 27

- Sympatric speciation occurs in the same habitat/not geographically isolated - Within the population, mutations have caused the formation of different alleles (specify difference) by chance - 2 populations are formed with separate gene pools which are reproductively isolated - Both populations reproduce passing on advatageous alleles to their offspring - Allele frequency of both populations increases - Overtime the 2 populations can no longer interbreed to produce fertile offsprings - Leads to disruptive selection

Question 28

A forest was cleared to make more land available for agriculture. After the forest was cleared, the species diversity of the insects in the area had decreased. Explain why? (4)

Answer 28

- Decrease in the variety of plants/ fewer plant species - Decrease in the variety of food/ fewer food sources - Fewer habitats/niches - Pesticides kill unwanted plants

Question 29

Evolutionary relationships between different primates can be found by comparing their proteins and DNA. All primates produce a species-specific type of haemoglobin. An antibody against human haemoglobin could be used to compare evolutionary relationships between different primates. How? (3)

Answer 29

-Same haemoglobin that bonds to the same antibody have a similar ancestry - Agglutination -Forms a precipitate, the thickness of precipitates can be measured

Question 30

Why was a statistical test necessary for analyzing the results of the investigation (3)

Answer 30

- To ensure that the DIFFERENCE/CORRELATION is statistically significant - To accept/reject the null hypothesis - Determines the probability of the DIFFERENCE/CORRELATION being due to chance

Question 31

Male fruit flies are more likely than female fruit flies to have white eyes Explain why (2)

Answer 31

* Males have one allele * Females need two recessive alleles/females must be homozygous recessive

Question 32

What causes the selection pressure (1)

Answer 32

The resistant allele

Question 33

Describe how you would use the hardy-Weinberg equation to calculate the frequency of people who are healthy but carriers (heterozygotes) of the allele (4)

Answer 33

* Find the value of q first * Find the square root of q2 * Use p+q/ determine the frequency of both alleles * Use of 2pq to find carriers/heterozygotes

Question 34

Explain the advantage of presenting results as ratios? (2)

Answer 34

- for valid comparison - Number/sample size may vary

Question 35

A large size results in snow geese being adapted for breeding in colder conditions. Explain how? (knowledge application) (2)

Answer 35

- Small surface area to volume ratio - Lose less heat to the environment

Question 36

how to find the degrees of freedom? (1)

Answer 36

total number of groups minus 1

Question 37

Give an example of a null hypothesis (1)

Answer 37

There is no significant difference between the observed and the expected number of guinea pigs/ any difference is due to chance

Question 38

Describe one way in which scientists could find out whether cichlids from two different populations belong to the same species (2)

Answer 38

- Breed together - if fertile offspring then same species

Question 39

The experimenters conducted the x2 analysis and found that p<0.001. interpret the test statistic in relation to the null hypothesis, using the terms probability and chance in your answer (3)

Answer 39

- There is less than 0.01% probability that the difference is due to chance - reject the null hypothesis - There is a significant difference between observed numbers of offspring and expected number of offsprings

Question 40

Many species of cichlids are similar in size and apart from their colour, in appearance. Suggest how the variety of colour patterns displayed by these cichlids may help, maintain the fish as separate species (2)

Answer 40

- selection of mate is dependent on colour pattern - maintains separate gene pools

Question 41

How can resistance suddenly appear in an isolated population of rats which has never been exposed to warfarin (1)

Answer 41

Mutation / (spontaneous) change in a gene / change in DNA;

Question 42

Give two assumptions that must be made when using the Hardy-Weinberg equation. (5)

Answer 42

- No selective advantage / All genotypes equally fertile; - Large population; - Random mating; (IGNORE ‘random fertilisation’) - No mutation; - No emigration / immigration;

Question 43

During the last 700 000 years there have been long periods when the water level was much lower and Lake Malawi split up into many smaller lakes. Explain how speciation of the cichlids may have occurred following the formation of separate, smaller lakes. (5)

Answer 43

- isolation of two populations; - variation already present due to mutations; - different environmental conditions / selection pressures leading to - selection of different features and hence different alleles; - different frequency of alleles; separate gene pools / no interbreeding;

Question 44

The actual percentage of heterozygotes is likely to be lower in future generations than the answer to part (c)(i). Explain why (1)

Answer 44

Affected individuals (usually) do not reproduce / die during childhood / do not pass on allele / genetic screening;

Question 45

Describe how you would use quadrats to determine whether a particular plant species has a clustered or a random distribution. (3)

Answer 45

- use of random numbers to place quadrats; - number of individuals counted in large number of quadrats; - little variation random, large variation - clustered;

Question 46

Less competition for....

Answer 46

- nutrients - Water

Question 47

Application

Answer 47

- Importance of maintaining the gene pool

Question 48

The ecologists captured insects from a number of sites on the island. Suggest how they decided where to take their samples. (2)

Answer 48

1. Draw grid over (map of) area; 2. Select squares/coordinates at random;

Question 49

Give two assumptions made when using the mark-release-recapture method (2)

Answer 49

1. No emigration/immigration; 2. No losses to predation; 3. Marking does not affect survival; 4. Birth rate and death rate equal; 5. (In this case) all belong to one population;

Question 50

Suggest the advantage of using the fluorescent powder in this experiment. (2)

Answer 50

1. Only glows brightly with UV, so doesn’t make insects more visible; 2. So doesn’t affect/increase predation; OR 1. Glows brightly with UV making visible; 2. So makes it easy to pick out labelled insects;