Drought and finch beak depth – selection on a quantitative trait, R = h²S
BiologyEvolutionAges 17–18
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Sign in to playA population of ground finches varies continuously in beak depth, and part of that variation is inherited (heritability h²). In a drought the small soft seeds run out and only large hard seeds remain, so deep-beaked birds survive better; the histogram shows the survivors, the selection differential S, and how far the offspring shift, R ≈ h²S. Students change h² and the strength of selection, let the rains return, and follow mean beak depth over the generations.
Lesson: Darwin's theory of evolution: variation, inheritance and natural selection; directional selection on a quantitative trait (Darwin's finches)
What it shows
Natural selection needs three things: variation in a trait, inheritance of that variation, and differences in survival or reproduction. This simulation follows ground finches whose beak depth varies continuously. In a drought only large, hard seeds are left, so deep-beaked birds survive better. The histogram compares all birds with the survivors, giving the selection differential S, and then shows the offspring, whose mean shifts by the response R. On average R = h²S, the breeder's equation, so the shift depends on how much of the variation is inherited.
How to use
Watch one season at a time with Next generation: grey bars are all birds, orange bars the survivors, the green line the offspring. Compare predicted and actual R in the readout. Change h² (this starts a new population) and Strength of selection, then press Rains return to see what happens when small seeds become plentiful again.
Parameters you can change
- Heritability h² of beak depth 0–1
- Birds per generation N 50–1000 birds
- Initial mean beak depth 7.5–11 mm
- Initial standard deviation of beak depth 0.4–1.5 mm
- Strength of selection (scarcity of small seeds) 0–1
- Start with a drought season
Questions to explore
- When h² = 0 the survivors still have deeper beaks, so why do the offspring not change?
- For the same selection differential S, how does the response R change if h² is doubled?
- After many rainy years, how does mean beak depth change, and why more slowly than in a drought?
