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4.10.6Additional lessons

Types of Natural Selection — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 89 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 5Three Modes of Selection

The pattern of selection depends on which phenotypes are most fit.

Stabilising
  • Stabilising Individuals near the average are favoured and both extremes are selected against, so variation narrows around the mean.
Directional
  • Directional One extreme is favoured, so the mean of the trait shifts in that direction over generations.
Disruptive
  • Disruptive Both extremes are favoured over intermediates, so the distribution can become two peaks.
Normal distribution
  • Normal distribution These effects are described by the shape of the bell curve of a continuous trait before and after selection.
TypeFavoured phenotypeEffect on meanEffect on spread
StabilisingIntermediateNo shiftDecreases
DirectionalOne extremeShiftsLittle change
DisruptiveBoth extremesLittle overall shiftIncreases

Part 2 of 5Examples in Order

1
Stabilising: human birth mass stays near the mean because very small babies are at risk of cold and very large babies at risk during birth.
2
Directional: bacteria exposed to an antibiotic evolve higher resistance as the most resistant survive and reproduce.
3
Disruptive: in some African finches, birds with small or large beaks feed efficiently on soft or hard seeds, while intermediates feed poorly.
4
Over time disruptive selection can split a population into two forms and may contribute to speciation.

Part 3 of 5Drawing and Describing Curves

Drawing and Describing Curves
Before selection
  • Before selection The original distribution is a symmetrical bell curve with the mean in the middle.
After stabilising
  • After stabilising The curve becomes taller and narrower while the mean stays in the same place.
After directional
  • After directional The whole curve moves toward the favoured extreme, often with a similar shape.
After disruptive
  • After disruptive The curve flattens and develops two peaks at the extremes with a dip in the middle.

Part 4 of 5Worked Example

Question

  • Mean beak depth in a finch population is 9.0 mm before a drought and 9.6 mm afterwards, with a similar spread.
  • Calculate the percentage shift in mean and name the type of selection.

  1. Shift = 9.6 − 9.0 = 0.6 mm.
  2. Percentage change = 0.6 ÷ 9.0 × 100 = 6.7%.
  3. The mean moved in one direction while the spread stayed similar, so selection favoured one extreme.

Answer A 6.7% increase in mean beak depth, showing directional selection.

Part 5 of 5Exam Checklist & Tips

4.10.6 – Key Points for the Exam

  • Stabilising selection Selection against both extremes, which maintains the average phenotype.
  • Directional selection Selection favouring one extreme and shifting the mean.
  • Disruptive selection Selection favouring both extremes over intermediates.
  • Continuous variation Variation with a range of values, usually controlled by many genes.
  • Mean The average value of a trait in a population.

Common Mistakes

  • Describing directional selection as the loss of variation; the main effect is a shift in the mean.
  • Confusing disruptive with stabilising selection.
  • Drawing curves that move without labelling the axes of trait value and number of individuals.
  • Writing that selection acts on genotypes directly rather than on phenotypes.

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