Pearson Edexcel International A Level Biology: Climate Change, Evolution, Speciation and Sustainable Resources (YBI11)

This guide covers statements 5.16 to 5.26 of Topic 5 in the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. That includes Core Practical 12. Topics 1 to 4 are International AS (IAS) and Topics 5 to 8 are International A2 (IA2), so this material is A2. There are no tiers. Use it with the revision notes and the practice questions. See also the course hub, checklist and free diagnostics.

What this unit covers

Statement	Focus
5.16	Evidence for climate change; correlation and causation
5.17	Anthropogenic causes; greenhouse gases
5.18	Carbon cycle and reducing atmospheric carbon dioxide
5.19	(i) Extrapolation and models; (ii) limitations
5.20	Effects on plants and animals
5.21	Temperature, enzymes and Q₁₀
5.22	Core Practical 12
5.23	Evolution: mutation and natural selection
5.24	Isolation; allopatric and sympatric speciation
5.25	Controversial issues and who reaches conclusions
5.26	Reforestation, sustainable resources, biofuels

5.16 Evidence for climate change and its causes

Climate is the long-term pattern of weather. Biologists use direct measurements and proxies.

• Carbon dioxide records. Air bubbles trapped in layers of polar ice show the carbon dioxide concentration when each layer formed, and deeper layers are older. Direct atmospheric measurements have been made since the late 1950s.
• Temperature records. Thermometer records give direct measurements. Older temperatures are estimated from proxies.
• Pollen in peat bogs. Peat is waterlogged, acidic and short of oxygen, so decay is slow and pollen grains are preserved in layers. Each species has pollen with a distinctive shape, so a sample from one depth shows which plants were growing. Each plant tolerates a certain climate, so the community suggests the climate. Depths are dated, for example by radiocarbon dating.
• Dendrochronology. A tree adds one growth ring each year. Rings are wider in years with warm, wet conditions and narrower in cold or dry years. Matching ring patterns between living and older wood extends the record back thousands of years.

Worked example (dendrochronology). Eight consecutive rings measure 2.1, 1.9, 0.8, 0.7, 2.4, 2.6, 1.1 and 2.2 mm. The mean is 13.8 ÷ 8 = 1.725 mm. Rings 3, 4 and 7 average 2.6 ÷ 3 = 0.87 mm, against 2.24 mm for the other five. The tree grew poorly in those years, suggesting cold or dry conditions. Disease could also be responsible, so use many trees.

Worked example (pollen). A peat core has oak 38%, hazel 22% and grass 30% at 30 cm, but birch 52%, pine 28% and grass 15% at 100 cm. The deeper layer is older. Birch and pine suggest cooler conditions, and oak and hazel suggest a later, warmer climate. This assumes the plants had tolerances similar to the same species now.

Correlation and cause. A correlation is a relationship between two variables. It does not show that one causes the other. The figures below are illustrative.

Carbon dioxide (ppm)	315	330	345	360	375	390
Temperature anomaly (°C)	0.02	0.09	0.15	0.31	0.38	0.52

The carbon dioxide concentration rises by (390 − 315) ÷ 315 × 100 = 23.8% (to 3 s.f.), and the temperature anomaly rises with it. The correlation coefficient is r = 0.99, a very strong positive correlation, but a third factor might drive both. A causal link is supported by a mechanism (the greenhouse effect below), agreement across independent records, and models that test the mechanism.

5.17 Anthropogenic climate change and greenhouse gases

Anthropogenic means caused by human activity. The natural greenhouse effect keeps Earth warmer than it would otherwise be.

1. Incoming sunlight, mostly short-wavelength, mainly passes through the air and heats the ground and oceans.
2. The surface emits longer-wavelength infrared radiation.
3. Greenhouse gas molecules take in some of this infrared, then radiate it in all directions, partly back down.
4. Less heat escapes to space, so the surface warms.

Greenhouse gases include carbon dioxide, methane, water vapour and nitrous oxide. Methane traps more heat per molecule than carbon dioxide, but carbon dioxide is far more abundant. Human activities increase these gases (the enhanced greenhouse effect):

• Burning fossil fuels releases carbon dioxide that was locked away for millions of years.
• Deforestation removes trees that take up carbon dioxide, and their burning or decay releases it.
• Cattle, rice paddies and landfill release methane from anaerobic microbes.
• Fertilisers release nitrous oxide through microbial reactions.

5.18 Using the carbon cycle to reduce atmospheric carbon dioxide

Photosynthesis removes carbon dioxide from the atmosphere. Respiration, combustion and decomposition return it. Reduce the atmospheric level by cutting inputs or increasing removal:

• Burn less fossil fuel, using renewable energy.
• Reforestation, so photosynthesis stores carbon as wood.
• Protect forests, peat bogs and wetlands so stored carbon stays locked away.
• Capture carbon dioxide from power stations and store it underground.
• Reduce methane by capturing it from landfill and manure, and by managing livestock and rice paddies.

Worked example. A planting scheme covers 600 ha, and the trees take up 6.5 tonnes of carbon per hectare per year. Carbon taken up = 600 × 6.5 = 3,900 tonnes of carbon per year. Carbon dioxide has a relative formula mass of 44 and carbon has 12, so the mass of carbon dioxide is 3,900 × 44 ÷ 12 = 14,300 tonnes per year. Uptake slows as the forest matures.

5.19 Extrapolation, models and their limitations

Extrapolating means extending a trend beyond the measured data. Climate models use measured data, known processes and assumed future emissions to predict temperature and rainfall.

Worked example. The carbon dioxide concentration was 355 ppm in 1990 and 390 ppm in 2010. The mean rate of increase is 35 ÷ 20 = 1.75 ppm per year. Extrapolating at a constant rate: 390 + 1.75 × 30 = 442.5 ppm in 2040. If the rate is rising, this underestimates.

Limitations (5.19 ii):

• Climate has many interacting variables and feedback loops.
• Models depend on assumptions about future human behaviour, such as emissions and population.
• Past data may be incomplete, and different models give different predictions.
• Extrapolation assumes the trend continues.

5.20 Effects of climate change on plants and animals

Changing rainfall patterns and changes in seasonal cycles alter the conditions organisms live in.

• Distribution. Species move towards the poles or up mountains when their tolerance range is exceeded. Species that cannot move may be lost locally.
• Development. Temperature changes the rate of enzyme-controlled development, so insect larvae may develop faster, and eggs may hatch earlier.
• Lifecycles and seasons. Plants may flower earlier. Migrating birds may arrive earlier. A mismatch arises if insects emerge earlier but nesting birds do not, so chicks lack food.
• Rainfall. Drought reduces plant growth and food supplies, and flooding may drown roots.

5.21 Temperature, enzymes and Q₁₀

Temperature raises the rate of an enzyme-controlled reaction because particles gain kinetic energy, giving more frequent collisions and more with enough energy to react. Above the optimum, hydrogen bonds in the tertiary structure break, the active site changes shape and the enzyme denatures. Plants, ectothermic animals and microorganisms have metabolic rates that follow environmental temperature, so warming speeds their growth and development until the optimum is passed.

The temperature coefficient is Q₁₀ = (rate at T + 10 °C) ÷ (rate at T). A Q₁₀ of 2 means the rate doubles for each 10 °C rise. For another interval, Q₁₀ = (R₂ ÷ R₁)^(10 ÷ (T₂ − T₁)).

Worked example 1. A beetle larva takes 40 days to reach the pupa at 15 °C and 19 days at 25 °C. Rate is proportional to 1 ÷ time, so Q₁₀ = 40 ÷ 19 = 2.1 (2 s.f.).

Worked example 2. Oxygen uptake of a lizard is 0.8 units at 14 °C and 2.6 units at 26 °C. The interval is 12 °C. Q₁₀ = (2.6 ÷ 0.8)^(10/12) = 3.25^0.8333 = 2.7 (2 s.f.).

5.22 Core Practical 12: temperature and development

Aim. Investigate how temperature affects the development of organisms, such as seedling growth rate or brine shrimp hatch rates, taking the ethical use of organisms into account.

Method (brine shrimp).

1. Use one batch of salt solution, with the same volume and 100 eggs in each tube.
2. Stand tubes in water baths at 15, 20, 25, 30 and 35 °C, with the same light.
3. After a fixed time (such as 24 hours), count the hatched nauplii.
4. Repeat each temperature at least three times.

Results. Hatched out of 100: 15 °C: 12, 15, 9; 20 °C: 31, 28, 34; 25 °C: 58, 61, 55; 30 °C: 83, 79, 86; 35 °C: 40, 44, 37. The means are 12, 31, 58, 83 and 40 (rounded), so the hatch percentage rises to 30 °C and falls at 35 °C because of denaturation. Between 20 and 30 °C, Q₁₀ = 82.7 ÷ 31.0 = 2.7 (2 s.f.).

Seedlings. Keep species, seed number, depth, water and light constant and measure length after a fixed time.

Control variables: number and batch of organisms, salinity or water volume, light, time and pH. Ethics: use the smallest number needed, keep organisms in suitable conditions, avoid unnecessary suffering, and dispose of them humanely. Plant seedlings raise fewer ethical issues than animals.

5.23 Evolution through mutation and natural selection

Evolution is a change in allele frequency in a population over generations.

1. Mutation is a random alteration to the DNA base sequence, and it produces new alleles. A few give an advantage in a given environment.
2. Organisms vary and produce more offspring than survive.
3. A selection pressure (a predator, a disease, an antibiotic, a change in climate) means some individuals survive and reproduce more than others.
4. Those individuals pass on the advantageous allele, so its frequency rises across generations.

Worked example. In a bacterial culture the resistance allele has frequency 0.001. An antibiotic kills 90% of susceptible cells and no resistant cells. Fraction resistant after one round, assuming regrowth to the same size: 0.001 ÷ (0.001 + 0.999 × 0.1) = 0.0099. After a second round, 0.091. After a third, 0.50.

5.24 Isolation and speciation

A species is a group whose members interbreed and produce fertile offspring. Gene flow is the movement of alleles between populations by interbreeding. Isolation reduces gene flow. Each population then has different mutations and selection pressures, so allele frequencies diverge. Once the populations can no longer produce fertile offspring together, they are separate species.

• Allopatric speciation: a physical barrier such as a sea, mountain range or river divides the populations.
• Sympatric speciation: populations live in the same area, but reproductive isolation arises. Examples: different flowering times, a switch to a different host or food source, courtship differences, and polyploidy in plants (extra chromosome sets cause instant isolation).

5.25 Controversial issues and who reaches the conclusions

Scientific evidence on questions such as how to reduce climate change, or how far humans are responsible, can be interpreted differently.

• Funding: a study paid for by an interested industry may select data or stress certain results.
• Peer review and replication add confidence.
• Values affect which actions people choose, even when they accept the same evidence.
• Check sample size, controls, and whether the conclusion goes beyond the data.

5.26 Reforestation, sustainable resources and biofuels

Human needs (food, timber, fuel, land) conflict with conservation. Effective management meets needs without destroying the resource.

• Reforestation restores habitat, takes up carbon dioxide and reduces soil erosion. Single-species plantations support less biodiversity than natural woodland, and young trees take decades to store much carbon.
• Sustainable forestry replants and fells selectively, so the forest regenerates.
• Biofuels (ethanol from sugar crops, biodiesel from plant oils) can be considered close to carbon neutral, because the carbon dioxide released on burning was taken up by the crop. Against that: growing them can replace food crops or forest, and fertilisers and machinery use energy.

Common errors

• Saying that a correlation proves causation.
• Using Q₁₀ as a difference, not a ratio.
• Saying individuals evolve; populations do.
• Saying mutations happen “to” help; they are random.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, February 2021, Topic 5, statements 5.16 to 5.26, published by Pearson Education Limited.