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4.10.3Additional lessons

Selection Pressures: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 86 of 101

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Reading the Data

Data questions on selection ask for a trend, a cause and an evaluation.

Identify the trend
  • 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
  • Name the pressure Link the change to a plausible selective agent, such as predation, drought or an antibiotic.
Heritability
  • Heritability Check that the trait is genetic, because only inherited variation can respond to selection.
Control
  • Control Look for comparisons, such as treated and untreated groups, that show the pressure is the likely cause.
YearDark moths (%)Light moths (%)
Year 11090
Year 106040
Year 20955

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
  • Sample size Larger samples reduce the effect of chance and give more reliable estimates of trait frequency.
Alternative causes
  • Alternative causes Migration or genetic drift may change allele frequencies, so these should be excluded where possible.
Time scale
  • Time scale Short data series may show random fluctuation, whereas long series show consistent direction.
Statistics
  • 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?

  1. Year 1: 10% of 200 = 0.10 × 200 = 20 moths.
  2. Year 20: 95% of 200 = 0.95 × 200 = 190 moths.
  3. 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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