4.10.3Additional lessons
Selection Pressures: Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 86 of 101
Part 1 of 5Reading the Data
Data questions on selection ask for a trend, a cause and an evaluation.
Identify the trend
- State what changes over time in the trait frequency and quote values from the graph or table with units.
Name the pressure
- Link the change to a plausible selective agent, such as predation, drought or an antibiotic.
Heritability
- Check that the trait is genetic, because only inherited variation can respond to selection.
Control
- Look for comparisons, such as treated and untreated groups, that show the pressure is the likely cause.
| Year | Dark moths (%) | Light moths (%) |
|---|---|---|
| Year 1 | 10 | 90 |
| Year 10 | 60 | 40 |
| Year 20 | 95 | 5 |
Part 2 of 5Interpreting a Moth Data Set
1
In a polluted woodland the proportion of dark moths rises from 10% to 95% over twenty years.
2
Calculate the change: 95 − 10 = 85 percentage points, and the relative increase is 85 ÷ 10 = 850%.
3
Explain: soot darkened tree bark, so dark moths were better camouflaged and birds took fewer of them.
4
Evaluate: the data show a correlation, so predation experiments and the genetic basis of colour are needed to be sure.
Part 3 of 5Evaluating Evidence
Evaluating Evidence
Sample size
- Larger samples reduce the effect of chance and give more reliable estimates of trait frequency.
Alternative causes
- Migration or genetic drift may change allele frequencies, so these should be excluded where possible.
Time scale
- Short data series may show random fluctuation, whereas long series show consistent direction.
Statistics
- A statistical test, such as a chi-squared test, shows whether a difference is likely to be due to chance.
Part 4 of 5Worked Example
Question
- Dark moths form 10% of a sample of 200 in year 1 and 95% of a sample of 200 in year 20.
- How many dark moths were sampled in each year?
- Year 1: 10% of 200 = 0.10 × 200 = 20 moths.
- Year 20: 95% of 200 = 0.95 × 200 = 190 moths.
- Increase in number = 190 − 20 = 170.
Answer 20 in year 1 and 190 in year 20.
Part 5 of 5Exam Checklist & Tips
4.10.3 – Key Points for the Exam
- Percentage point change The difference between two percentages.
- Correlation A relationship between two variables that does not by itself prove cause.
- Camouflage Colouring that helps an organism avoid detection.
- Sample size The number of individuals measured.
- Reliability The extent to which repeated measurements give consistent results.
Common Mistakes
- Quoting a trend without numbers from the data.
- Confusing a percentage point change with a percentage change.
- Claiming that correlation proves cause without suggesting a test.
- Ignoring that pollution may have changed other things in the habitat as well as bark colour.
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