Describe the role of isolation in natural selection.

Explain the transfer of energy through an ecosystem. Support your explanation with a labelled diagram.

For a named endangered species, discuss why it is endangered and why it should be conserved.

natural selection leads to survival of the fittest;
leading to adaptation of species/populations to new environmental conditions;
isolation occurs when members of a population are prevented from interbreeding due to geographical barriers / eg, mountain formations/continental drift/island formation;
isolation occurs when members of a population are prevented from interbreeding due to reproductive barriers / eg behavioural/seasonal/ecological/mechanical differences/incompatibilities;
the isolated groups within the population may then be subject to different selective pressures/natural selection;
so that the separate groups evolve increasingly different characteristics and become unable to reproduce fertile offspring between them;
in this way natural selection can lead to formation of new species/speciation;

award [1 max] for a good example of natural selection due to isolation

[4 max]

energy enters (most) ecosystems from the sun / energy enters (some) ecosystems from geothermal vents;
plants/(archae)bacteria/autotrophs convert this energy into biomass/glucose/chemical energy / plants use photosynthesis to convert light energy into chemical energy;
energy is transferred from the autotrophs to consumers by feeding/consumption/food chain;
energy is lost as heat (entropy) in respiration (as it is transferred along a food chain);
this heat is transferred to atmosphere where it is absorbed/reflected back to earth/lost to space;
this illustrates the second law of thermodynamics(–increasing entropy in a system);
only 10% of the available energy in one trophic level is passed onto the next/higher trophic level;
chemical energy is transferred to decomposers via waste / death;      [4 max]

award up to [2 max] for a diagram which shows at least producers, consumers and decomposers with flows going the right direction (accept plant/herbivore/carnivore/detritivore or named examples of each);
an energy source (sun/geothermal) and losses (heat / decomposers);

[6 max]

Why it is endangered [5 max]. Award [3 max] if only one of the following categories is addressed:
eg Tiger

Biological/ecological factors:
as a top carnivore it requires a large ecosystem/food production base / typically having relatively low population size;
as a mammal with relatively long gestation/low reproduction rate population growth is slow;

Human activities:
direct killing/hunting/poaching by humans / invasive predator leads to reduction in population;
fragmentation of habitat through human land use leads to lack of gene flow/inbreeding/low genetic diversity;
loss of habitat/prey species through pollution/hunting/habitat disturbance leads to increased competition for resources eg prey/shelter/mates;

Conservation difficulties:
tiger habitat may cross national boundaries making cooperative management/international protection agreements difficult;
protection undermined by corruption/black market demand for tiger parts;
low employment may promote poaching / local poverty limits effectiveness of enforcement/protection;

Why it should be preserved [5 max]. Award [3 max] if only two or fewer of the following categories are addressed:

Ethical reasons:
because it has intrinsic value/biorights / humans have no rights to destroy species;
many religions/local cultures attach particular spiritual/cultural significance to the tiger;
…although not all cultures/value systems will acknowledge spiritual/intrinsic values;
we owe it to future generations to pass on the same environmental heritage we received;

Economic reasons:
each species in ecosystem contributes to the overall natural capital/sustainable resources/goods and services the system provides;
tigers provide a significant source of income from ecotourism;

Aesthetic reasons:
tigers are beautiful/unique/interesting and make the world more pleasurable;
all species add to the diversity/quality of human experience;

Ecological reasons:
as top carnivores, tigers have an important role in maintain balances within their ecosystem / may be a keystone/umbrella species;
all species add to the diversity of a system, making it more stable/resistant to change;

Reasons for other species may also include:
genetic variation in plant and animal species provides the means for sustaining and improving production in farms, forestry, animal husbandry and fisheries;
many organisms contain chemicals which may be of medical or commercial value;

Award [1 max] for an explicit and valid conclusion.
Award [7 max] for marking points above [6 max] if no named species, and [1 max] for a clear conclusion.

[8 max]

Most candidates gave a reasonable description of natural selection and isolation. This indicates good progress as natural selection questions in the past have been poorly answered. Gaining full marks for this question was generally the stronger candidates.

The candidates all made an attempt at the required diagram, indicating they read the question carefully. Most gained one mark, for the sun and heat; however a significant majority did not include decomposers and so did not gain the second mark. This question was well answered and even very weak candidates wrote enough to gain a few points. Some answers were very eloquent and indicate the candidates have a strong grasp of energy transfer.

Nearly all candidates named an appropriate endangered species, mostly animals but some plants. The answers all included both parts of the question, why endangered and why it should be conserved. The stronger answers made sure to include a range of reasons for each. Some weaker candidates gave generic answers about the species being important for the ecosystem, but no details. The conclusion mark was difficult for most candidates.