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NCERT's Figure 6.8 shows three bell curves and three labels, and most students memorise the shapes. The trouble starts when a question gives you a graph, a table of survivors or a field study and asks which type of selection it shows. This post shows you how to read the curves, works through one sample population under all three types, and links each type to a real study.
Where is this in the syllabus?
Types of natural selection are in Chapter 6, Evolution, of the NCERT Class 12 Biology textbook (2026-27 reprint). The NEET (UG) 2026 syllabus lists "Natural Selection with examples, types of natural selection" and the CBSE Biology curriculum for 2026-27 has the same phrase. For the rest of the chapter, including Hardy-Weinberg problems, see our evolution guide.
What do the three types mean?
NCERT defines them by what happens to individuals in the curve:
Stabilising: "more individuals acquire mean character value". The middle is favoured, both tails lose out, and the curve gets narrower around the same peak.
Directional: "more individuals acquire value other than the mean character value". One tail is favoured, so the peak moves that way.
Disruptive: "more individuals acquire peripheral character value at both ends of the distribution curve". Both tails are favoured, the middle loses out, and the curve can split into two peaks.
To read any graph, ask two questions in order. Did the peak move? If yes, the selection is directional. If no, did the curve get narrower or wider? Narrower is stabilising; wider, or two humps, is disruptive.
Worked example: one population, three outcomes
These are sample numbers for practice, not real data. A population of 100 birds is sorted by beak depth:
8 mm: 10 birds, 9 mm: 20, 10 mm: 40, 11 mm: 20, 12 mm: 10.
The mean is (80 + 180 + 400 + 220 + 120) / 100 = 10.0 mm. Now apply three different survival patterns.
Survival 20%, 50%, 80%, 50%, 20% (middle favoured). Survivors: 2, 10, 32, 10, 2, total 56. Mean: (16 + 90 + 320 + 110 + 24) / 56 = 560 / 56 = 10.0 mm. The mean has not moved, but 32 of 56 survivors (57%) are now in the middle class, up from 40%. The curve is narrower: stabilising.
Survival 10%, 30%, 50%, 70%, 90% (deep beaks favoured). Survivors: 1, 6, 20, 14, 9, total 50. Mean: (8 + 54 + 200 + 154 + 108) / 50 = 524 / 50 = 10.48 mm. The peak has shifted towards deeper beaks: directional.
Survival 80%, 40%, 10%, 40%, 80% (both extremes favoured). Survivors: 8, 8, 4, 8, 8, total 36. Mean: (64 + 72 + 40 + 88 + 96) / 36 = 360 / 36 = 10.0 mm. The mean is unchanged again, but the middle class has fallen to 4 of 36 (11%). The spread has widened and the middle is a dip: disruptive.
Cases 1 and 3 have the same mean. Only the spread separates them, which is why a question that reports only the average cannot tell you the type.
What does each type look like in a real population?
Directional. After a drought on Daphne Major in the Galápagos, Boag and Grant (1981) found that survival of Darwin's finches "was nonrandom. Large birds, especially males with large beaks, survived best because they were able to crack the large and hard seeds that predominated in the drought." One end of the beak-size range was favoured, so the average moved.
Disruptive. In the African finch Pyrenestes, birds come in small-billed and large-billed forms that feed differently on soft and hard seeds. Smith (1993) reported long-term field support for earlier suggestions "that disruptive selection is acting on bill size", with the forms occupying distinct adaptive peaks.
Stabilising. The goldenrod gall fly is a good case because two enemies pull in opposite directions. Parasitoid wasps attack larvae in small galls, and birds attack larger galls. In a study across 16 populations over four generations, Weis, Abrahamson and Andersen (1992) found that these opposing attacks "resulted in stabilizing selection in half the population generations". The other half shows something useful too: the type of selection can change from year to year in the same species.
The peppered moth in NCERT is worth placing as well. In polluted areas, dark moths became more common than white ones. One end of the colour range was favoured, so it is directional.
Which mistakes cost marks?
Calling stabilising selection "no selection". Individuals at both tails are being removed every generation; the mean just stays put.
Reading one shaded tail as disruptive. Disruptive needs both tails favoured. One tail is directional.
Judging by the mean alone. Stabilising and disruptive selection can leave the mean exactly where it was, as in the worked example.
Mixing up which curve is which in Figure 6.8. NCERT labels them (a) stabilising, (b) directional and (c) disruptive. Draw them from memory and check.
A practice routine
Sketch the three curves, before and after, from memory. Under each, write the one-line NCERT definition.
Make your own sample population with five classes, choose a survival pattern and calculate the new mean and the share in the middle class. Ask a friend to name the type from your numbers alone.
Match each real study above to its type without looking, then explain in one sentence what was being favoured.
Mix these with Hardy-Weinberg questions so that you have to decide which idea a question is testing.
If you want to see a distribution shift rather than read about it, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Today, cover the answers in the worked example and recompute the three survivor means yourself.
