TOPIC 7 MARK SCHEME SHIT

Question 1

Use your knowledge of succession to explain the increase in biomass during the first 20 years (3)

Answer 1

1. Pioneer species
2. Change in abiotic conditions/less hostile/more habitats/niches
3. Increase in number/amount/diversity of species/plants/animals

Question 2

Suggest reasons for conserving woodlands

Answer 2

1. Conserving/protecting habitats/niches
2. Conserving/protecting species/maintains/increases diversity
3. Reduces global warming/climate change
4. Source of medicines/chemicals/wood
5. Reduces erosion/eutrophication

Question 3

Explain the increase in the diversity of birds as the woodland developed (3)

Answer 3

1. Greater variety/diversity of plants/insects/more plant/insect species
2. More food sources/more varieties of food
3. More habitats/niches

Question 4

ALLOPATRIC SPECIATION 8 POSSIBLE POINTS

Answer 4

1. Geographical isolation
2. Separate gene pools / no interbreeding / gene flow (between populations);
   Accept: reproductive isolation
3. Variation due to mutation;
4. Different selection pressures / different abiotic / biotic conditions / environments / habitats;
5. (Different/advantageous) ALLELE(S) passed on/selected
6. Different(ial) reproductive success / selected organisms (survive and) reproduce
7. Leads to change / increase in allele frequency
8. (Eventually different species) cannot (inter)breed to produce fertile offspring

Question 5

Mark release recapture (4)

Answer 5

1. Capture / collect sample, mark and release;
2. Method of marking does not harm lizard / affect survival / make it more visible to predators;
3. Leave sufficient time for lizards to (randomly) distribute (on island) before collecting a second sample;
4. (Population =) number in first sample × number in second sample divided by number of marked lizards in second sample / number
   recaptured.

Question 6

Use your knowledge of gene linkage to explain these results (4)

Answer 6

1. GN and gn linked
2. GgNn individual produces mainly GN and gn gametes
3. Crossing over produces some/few Gn and gN gametes
4. So few(er) Ggnn and ggNn individuals

Question 7

Reasons why observed phenotype ratios are often not the same as expected ratios

Answer 7

1. Random fertilisation of gametes
2. Small sample size
3. Lethal alleles/genotypes
4. Linked genes
5. Epistasis

Question 8

Why use chi squared test?

Answer 8

Categorical data

Question 9

Explain results in table - question hasn’t said if genes are linked (3)

Answer 9

1. Genes are linked
2. No crossing over
3. Only GL and gl (gametes produced) / No Gl and no gL (gametes produced)

Question 10

What is a gene pool? (1)

Answer 10

All the ALLELES present in a population

Question 11

Explain why mutations cannot be spread from one species to another (4)

Answer 11

1. Mutations are spontaneous/random
2. Only the rate of mutation is affected by environment
3. Different species do not interbreed/produce fertile offspring
4. So mutation/gene/allele cannot be passed from one species to another

Question 12

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

Answer 12

1. Initially one/few/ insects 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 (of this gene)

Question 13

Describe how the behaviour of a species could result in sympatric speciation (3)

Answer 13

1. NOT geographically isolated
2. (Leading to) reproductive isolation/gene pools kept separate
3. Changes in ALLELE FREQUENCIES
4. Can’t breed/mate to produce fertile offspring

Question 14

An island possesses two species of palm tree which have arisen via sympatric speciation. The two species diverged from each other after the island was formed 6.5 million years ago. The flowering times of the two species are different.

Using this info, suggest how these two species of palm tree arose by sympatric speciation (5)

Answer 14

1. Occurs in the same habitat/environment/population
2. Mutation/s cause different flowering times
3. Reproductive separation/isolation OR no gene flow OR gene pools remain separate
4. Different ALLELE/S passed on/selected OR change in ALLELE frequency
5. DISRUPTIVE selection
6. Eventually different species cannot (inter)breed to produce fertile offspring

Question 15

Lactose is the main sugar in milk and is hydrolysed by the enzyme lactase. Lactase is essential to newborn mammals as milk is their only source of food. Most mammals stop producing lactase when they start feeding on other food sources. Humans are an exception to this because some continue to produce lactase as adults. The ability to continue producing lactase is known as lactase persistence (LP) and is controlled by a dominant allele. A number of hypotheses based on different selection pressures have been put forward to explain LP in humans.

One hypothesis for LP in humans suggests that the selective pressure was related to some human populations farming cattle as a source of milk. Describe how farming cattle as a source of milk could have led to an increase in LP. (4)

Answer 15

1. LP/allele due to mutation
2. Milk provides name nutrient (ignore sugar & lactose)
3. Individuals with LP more likely to survive AND reproduce/individuals with advantageous ALLELE more likely to survive AND reproduce
4. Directional selection
5. Frequency of ALLELE increases (in the offspring/next generation)

Question 16

Yaramul is a town in a historically isolated region of the Andes Mountains. The
population of this town has the highest frequency of the E280A mutation in the world. The origin of the E280A mutation in this population has been traced back to a common ancestor in the 17th century. Natural selection has not reduced the frequency of the E280A mutation in the population.

Suggest & explain two reasons why there is a high frequency of the E280A mutation in Yaramul (2)

Answer 16

1. Isolated so inbreeding / low genetic diversity / small gene pool;
2. Allele inherited (through generations) from (common) ancestor;

Question 17

There are nine subspecies of giraffe. These subspecies evolved when populations of giraffe were separated for long time periods. Each subspecies has distinct coloured skin markings. Some biologists have suggested that up to six of
these subspecies should be classified as different species.

Explain how different subspecies of giraffe may have evolved from a common ancestor. (5)

Answer 17

1. No interbreeding/gene pools are separate/geographical isolation
2. Mutation linked to different markings/colours
3. Selection survival linked to markings/colours
4. Adapted organisms breed/differential reproductive success
5. Change/increase in allele frequency/frequencies

Question 18

Suggest why the mark release recapture method can produce unreliable results in very large habitats (1)

Answer 18

Less chance of recapturing organism / unlikely fish distribute randomly/evenly

Question 19

Describe and explain how succession occurs (4)

Answer 19

1. (Colonisation by) pioneer species
2. Pioneers/species/organisms change the environment/habitat/conditions
3. (Environment becomes) less hostile/more suitable for other SPECIES/less suitable for previous SPECIES
4. Change/increase in DIVERSITY/BIODIVERSITY
5. (to) climax community

Question 20

Describe how you could estimate the size of a population of sundews in a small marsh (5)

Answer 20

1. Use a grid/divide area into squares/sections
2. Method of obtaining random coordinates/numbers eg. calculator/computer/random numbers table/generator
3. Count number/frequency in a quadrat/section (Accept % cover in quadrat/section)
4. Large sample of quadrats AND calculate mean/average (per quadrat/section)
5. Valid method of calculating total number of sundews (eg. mean number of plants per quadrat x # of quadrats in marsh)

Question 21

Explain how data in the table (species, # of days after flooding, mean number of organisms under each # of days after flooding) provides evidence of succession (5)

Answer 21

1. (Overall, data show an) increase in species richness/species diversity/# number of living organisms
2. _______ is a pioneer
3. (Pioneers cause) named change of environment eg. provide food for other species
4. New species/example from data colonise once there is a change
5. Original pioneer (named) decline/outcompeted/eaten as succession continues

Question 22

Give two features of a climax community (2)

Answer 22

1. Same species present (over long time) / stable community
2. Abiotic factors (more or less) constant (over time)
3. Populations stable (around carrying capacity)

Question 23

Other than hunting, suggest two reasons why populations might show very low levels of genetic diversity (2)

Answer 23

1. Population might have been very small/genetic bottleneck
2. Population might have started with small number of individuals/by one pregnant female/founder effect
3. Inbreeding