Stabilising, directional and disruptive selection, which example goes with which curve?
Farhan ShaikhDropper Sample
7 Oct 2026 · 1 min read
Short on time so I want shortcuts, but they have to be right. NCERT has the three types of natural selection with a bell curve diagram and I never remember which graph is which.
What I wrote down:
- Stabilising: more individuals acquire the mean value, the peak becomes higher and both extremes are reduced.
- Directional: individuals acquire a value other than the mean, in one direction, so the peak moves.
- Disruptive: more individuals at both ends (peripheral values), so two peaks with the mean getting smaller.
Examples I'm unsure about: industrial melanism in peppered moth (dark moths became more common after industrialisation) and also antibiotic resistant bacteria and pesticide resistant insects. Which type? Another one I know from outside the book: human birth weight, the babies with middle weight survive best.
Is my understanding right? And how can I decide the type quickly when a question describes an unknown population?
7 answers & replies
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[+][–]Kavya NaikDropperSample· 12d ago
Your three definitions are exactly right. Examples: peppered moth, DDT/herbicide/antibiotic resistance are all directional selection, the population mean shifts towards one extreme (more dark-coloured moths, more resistant individuals). Birth weight is the classic stabilising example (not in NCERT, but widely used): very small and very large babies have lower survival, medium ones do best.
Disruptive is the rare one in questions: both extremes are favoured and the intermediate form is lost, giving two peaks.
[+][–]Zoya Khan12thSample· 12d ago
A way to decide quickly is to ask "what happens to the mean and the peak height?". Mean stays and peak gets taller and narrower: stabilising. Mean moves left or right: directional. Mean falls into a valley with two peaks on either side: disruptive. Draw it in the margin in 5 seconds and the answer shows up.
[+][–]Pooja Iyer12thSample· 12d ago
Mnemonic for me: Stabilising = Stay (same average), Directional = Drift (one way), Disruptive = Divide (two groups). And all three are about what natural selection does to the frequency of phenotypes in a population. Natural selection acts on phenotype that arises from variation, and leads to adaptation of the fittest.
[+][–]Ishita GoyalDropperSample· 12d ago
Hold on, with the peppered moth, both light and dark forms existed before and after industrialisation. Isn't that two peaks, so disruptive? Genuine doubt, I've seen someone answer disruptive for it.
[+][–]Kavya NaikDropperSample· 12d ago
The light form did not become a second peak though. Before industrialisation, the light form was the majority. After, dark became the majority, so the "peak" shifted from light to dark: one direction = directional. For disruptive, light and dark would both need to survive better than intermediates (like grey) and the grey would be disappearing. Moth question is directional.
[+][–]Farhan ShaikhDropperSample· 12d agoAuthor
makes sense. I'll go with "mean shifts = directional". Thanks all, saved the 5 sec graph trick
[+][–]Siddharth RaoDropperSample· 12d ago
A trap that costs marks: don't confuse "adaptive radiation" (Darwin's finches, Australian marsupials: divergence of one ancestor into many forms in a geographical area) with disruptive selection. Adaptive radiation is about species, disruptive selection is about the distribution of one trait in one population. Also the peppered moth case NCERT uses is just to show natural selection in action: the proportion of dark moths rose with the smoke-darkened trees because they were camouflaged against predators.
