Natural Selection and Its Types: ISC Class 12 Biology
Updated 15 October 2026 · Fact-checked
Natural selection is the process by which heritable variations that help individuals survive and reproduce in an environment become more common, because their bearers leave more offspring. Fitness means reproductive fitness. Acting on a bell-shaped trait distribution, selection is stabilising (favours the mean), directional (favours one extreme) or disruptive (favours both extremes).
Compare the dashed 'before' curve with the coloured 'after' curve: the mean is kept, shifted to one side, or split into two peaks.
Illustration drawn for QuestionBuddy.
Understand Natural Selection and Its Types
The idea. Darwin (and, independently, Alfred Wallace) proposed that nature 'selects' individuals whose heritable variations suit the environment. Fitness, in Darwin's sense, refers ultimately and only to reproductive fitness: the better adapted leave more progeny. Over many generations the population's characteristics change and new forms appear.
Salient features of Darwinism (learn in this order).
1. Overproduction: organisms can multiply geometrically; a population would grow exponentially if every individual reproduced maximally.
2. Limited resources: food and space are limited, so population sizes stay roughly stable except for seasonal fluctuation.
3. Struggle for existence: competition within a species (intraspecific), between species (interspecific) and with the environment.
4. Variation: no two individuals are alike, and most variations are inherited.
5. Survival of the fittest (natural selection): individuals with useful heritable variations survive and reproduce more.
6. Origin of species: useful variations accumulate over generations and new forms arise.
Malthus's contribution. Thomas Malthus argued that populations grow geometrically while food supply grows arithmetically, so not all offspring can survive. This idea of a struggle for limited resources probably influenced Darwin.
Fitness needs a genetic basis. Selection changes a population only if the favoured trait is inherited. Adaptive ability is inherited; fitness is the end result of the ability to adapt and be selected.
Examples of natural selection (ISC list).
- Long neck of the giraffe (Darwin's explanation): giraffes varied in neck length; when lower leaves were scarce, longer-necked animals reached higher leaves, survived and reproduced more; the trait accumulated. Contrast Lamarck: necks lengthened by stretching (use) and the acquired change was inherited, an idea no longer accepted.
- Industrial melanism (peppered moth, England): before industrialisation, pale lichens covered tree trunks; white-winged moths were camouflaged and dark (melanised) moths were picked out by predators. After industrialisation, soot darkened the bark and killed lichens; dark moths were camouflaged and increased while white moths were eaten. In rural areas without industry, melanic moths stayed few. No variant is completely wiped out.
- DDT-resistant mosquitoes: DDT kills susceptible mosquitoes; the few resistant variants already present survive, breed and soon make up most of the population.
- Antibiotic-resistant bacteria: an antibiotic kills sensitive cells; pre-existing resistant mutants survive and multiply, within months or years rather than centuries. These are examples of evolution by anthropogenic action.
Lederberg's replica plating experiment (1952). Joshua and Esther Lederberg grew bacteria on a master plate without penicillin. A sterile velvet pad pressed on the master plate picked up cells from every colony and printed them, in the same positions, onto a replica plate containing penicillin. A few colonies grew on the replica plate, and the colonies at the matching positions on the master plate, which had never met penicillin, were also resistant. Conclusion: resistance mutations arise at random before exposure (pre-adaptation); the antibiotic does not cause them, it only selects them. This supports Darwinian selection and rejects Lamarckian 'acquired' resistance.
Criticism of Darwinism. Darwin could not explain the origin and inheritance of variations (genetics was unknown), did not distinguish heritable from non-heritable variations, could not explain vestigial and over-specialised organs, and explained the 'survival' but not the arrival of the fittest. Neo-Darwinism filled these gaps with mutation, recombination, gene flow and genetic drift.
Types of natural selection (on a trait with a bell-shaped distribution: x-axis, trait value; y-axis, number of individuals).
| Type | Favours | Effect on curve | Example |
|---|---|---|---|
| Stabilising | the mean (average) value | peak becomes narrower and higher; mean unchanged | human birth weight: very low and very high birth weights have higher infant mortality |
| Directional | one extreme | whole curve and mean shift to one side | industrial melanism; DDT or antibiotic resistance; giraffe neck length |
| Disruptive | both extremes | curve splits into two peaks; the mean is selected against | seeds of small and large size germinate well, medium ones poorly; birds with small or large beaks where only small and large seeds are available |
NCERT's wording: stabilisation (more individuals acquire the mean character value), directional change (more individuals acquire a value other than the mean) and disruption (more individuals acquire peripheral values at both ends of the curve).
Diagram checklist (Figure 6.8 type). Draw three pairs of curves, each pair showing 'before' and 'after'. Label: axes (trait value; number of individuals), the original mean, the arrows of selection pressure (on both tails for stabilising, on one tail for directional, on the middle for disruptive), the new curve, and the name of each type. One clean set of three earns more than a long paragraph.
Flow chart: overproduction → limited resources → struggle → heritable variation → differential survival and reproduction → change in allele frequencies → new form.
Natural Selection and Its Types: key facts
- Darwin's chain
- 1.Overproduction
- 2.Limited resources
- 3.Struggle for existence
- 4.Heritable variation
- 5.Survival of the fittest
- 6.Origin of species
- Write the six features in this order for a 3 to 5-mark answer.
- Fitness
Fitness reproductive success, the number of fertile offspring left Not physical strength or long life Needs a heritable (genetic) basis - Not strength or long life.
- Three types
Stabilising favours the mean; curve narrows, mean unchanged Directional favours one extreme; mean shifts Disruptive favours both extremes; curve splits into two peaks - Read which phenotype classes survive best in the data.
- Replica plating logic
Master plate no penicillin, all colonies grow Velvet pad copies every colony to the same position Replica plate penicillin, only resistant colonies grow Conclusion resistance existed before exposure; penicillin only selects it - Mutation first, selection after.
How to answer Natural Selection and Its Types questions
ISC asks the selection type from data or a graph, an example with explanation (melanism, resistance, giraffe), Darwin's postulates, and the meaning of replica plating.
- 1For data or graph items, find which classes survive or reproduce best: middle only (stabilising), one end (directional), both ends (disruptive).
- 2Write the exact term: 'disruptive', never 'dispersive' or 'diversifying' alone.
- 3For an example, give the chain: variation already present → environment changes → one variant survives and reproduces more → its frequency rises.
- 4Say the variation was pre-existing and heritable; the environment selects, it does not create the variant.
- 5For replica plating, name master plate, velvet, penicillin plate and the matching-position result, then the conclusion.
- 6Draw the three curves whenever a 'types' question carries 3 marks or more.
Natural Selection and Its Types in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Directional selection by an antibiotic
Suppose a patient stops an antibiotic course after three days because the fever has gone, and a week later the infection returns and does not respond to the same drug.
Explain, using natural selection, why the infection came back resistant.
Try it first, then show the solutionSolution
- The bacterial population already contained a few resistant mutants before treatment began.
- The short course killed most sensitive cells but left the resistant and partly affected cells alive.
- With competitors removed, resistant cells multiplied rapidly; the population is now mostly resistant.
- This is directional selection: the curve shifted towards resistance.
Answer: The drug selected pre-existing resistant bacteria, which multiplied once sensitive ones were killed; completing the full course reduces the chance of survivors.
Memory hook: The drug chooses, it does not create.
Tricks to remember Natural Selection and Its Types
Short memory aids. Say each one aloud once, then use it on a question the same day.
S-D-D by where the arrows hit
Stabilising: arrows hit both tails. Directional: arrows hit one tail. Disruptive: arrow hits the middle.
Use it when: Naming the type from a graph.
Disrupt splits
Disruptive selection disrupts the single peak into two; it is not 'dispersive'.
Use it when: Avoiding the 2024 spelling error.
O-L-S-V-S-O
Overproduction, Limited resources, Struggle, Variation, Survival of fittest, Origin of species.
Use it when: Salient features of Darwinism.
Natural Selection and Its Types: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Seed sizes and germination · 3 marks · Short answerBoard level
In a plant population, tiny seeds and very large seeds produce far more seedlings than seeds of intermediate size. Name the type of natural selection, state its effect on the distribution of seed size, and sketch the expected curve after many generations.
Try it first, then show the solutionSolution
- Both extremes are favoured and the mean is selected against: disruptive selection (1 mark).
- The single bell-shaped curve becomes two peaks, one at small and one at large size (1 mark).
- Sketch: original single curve; arrow pointing down on the middle; new two-peaked curve with labelled axes (1 mark).
Answer: Disruptive selection; the distribution becomes bimodal.
Example 2: Industrial melanism · 3 marks · Short answerBoard level
Moth collections from an English industrial town showed mostly white-winged moths before industrialisation and mostly dark-winged moths after it. Account for this change through natural selection, and predict the moth colours in a nearby unpolluted village.
Try it first, then show the solutionSolution
- Before industrialisation, pale lichens covered tree trunks; white-winged moths were camouflaged, dark moths were seen and eaten by predatory birds, so white moths were more common (1 mark).
- Soot killed lichens and darkened bark; now dark moths were camouflaged and white moths were eaten, so melanic moths increased (1 mark).
- In rural areas without pollution, bark stayed pale and melanic moths remained few (1 mark).
Answer: Predators selected the moth form that contrasted with the bark; dark moths rose only in polluted areas.
Example 3: Replica plating · 3 marks · Short answerHarder board level
Describe Lederberg's replica plating experiment. What did it prove about the origin of antibiotic resistance?
Try it first, then show the solutionSolution
- Bacteria were grown into colonies on a master plate without penicillin; a velvet pad transferred cells from every colony, in the same positions, to a plate containing penicillin (1 mark).
- Only a few colonies grew on the penicillin plate; the colonies at the matching positions on the master plate, never exposed to penicillin, were also found to be resistant (1 mark).
- So resistance mutations arose by chance before contact with the antibiotic; penicillin only selected them (pre-adaptation), supporting Darwin and not Lamarck (1 mark).
Answer: Resistance pre-exists as random mutation; the antibiotic selects it.
Example 4: Birth weight and selection · 3 marks · Short answerHarder board level
Infant survival is highest at an intermediate birth weight and falls for very light and very heavy babies. (a) Name the type of selection. (b) How will the variance and the mean of birth weight change over generations? (c) Why does this kind of selection not produce new forms?
Try it first, then show the solutionSolution
- (a) Stabilising selection (1 mark).
- (b) The mean stays the same; the spread (variance) narrows and the peak becomes taller (1 mark).
- (c) It removes extremes and keeps the population at the existing optimum, so it maintains rather than changes the character (1 mark).
Answer: Stabilising; same mean, narrower distribution; it preserves the existing form.
Example 5: Darwin versus Lamarck on the giraffe · 3 marks · Short answerHarder board level
Explain the long neck of the giraffe according to Lamarck and according to Darwin. Which explanation is accepted and why?
Try it first, then show the solutionSolution
- Lamarck: ancestors stretched their necks to reach leaves on tall trees; the acquired longer neck was passed on (use and disuse, inheritance of acquired characters) (1 mark).
- Darwin: ancestral giraffes varied in neck length; when food was scarce, longer-necked individuals reached higher leaves, survived and left more young; the trait accumulated over generations (1 mark).
- Darwin's explanation is accepted: changes acquired in body cells during life are not inherited; only heritable variation in the germ line passes on (1 mark).
Answer: Lamarck: inheritance of acquired stretching; Darwin: selection of pre-existing heritable variation (accepted).
Example 6: A lake with two kinds of food · 4 marks · Short answerHOTS / competency
Suppose a fish population feeds on both small soft prey and large hard-shelled snails. Fish with slender jaws handle small prey well, fish with heavy jaws crush snails well, and fish with intermediate jaws do poorly at both. (a) Predict how jaw form will change. (b) If the snails disappear, what type of selection will follow? (c) How could this situation lead to two species?
Try it first, then show the solutionSolution
- (a) Disruptive selection: slender and heavy jaws increase, intermediates decline; jaw form becomes bimodal (1 mark).
- (b) Only slender jaws now pay off: directional selection towards slender jaws (1 mark).
- (c) If fish of each jaw type tend to mate with similar fish (for example because they feed in different parts of the lake), gene flow between the two groups falls (1 mark).
- Differences accumulate until the groups cannot interbreed: speciation (1 mark).
Answer: (a) Disruptive. (b) Directional. (c) Disruptive selection plus reduced interbreeding can split one population into two species.
Where students lose marks in Natural Selection and Its Types
The slips examiners see most often, and what to write instead.
Mistake: Writing 'dispersive' for disruptive selection
In 2024 examiners reported that for seeds whose small and large sizes germinated best, candidates wrote 'dispersive' instead of disruptive.
Do this: Learn the three exact terms: stabilising, directional, disruptive.
Usually costs: 1 mark.
Mistake: Describing selection without diagrams
Examiners (2024) suggested using diagrams to explain the types; answers in words alone often mix up the curves.
Do this: Draw before-and-after curves with the arrows of selection pressure.
Usually costs: 1 to 2 marks in 3-mark answers.
Mistake: Saying the environment causes the useful variation
Writing that DDT or antibiotics 'make' insects or bacteria resistant is Lamarckian.
Do this: Say the variation existed already; the chemical selected it.
Usually costs: The explanation mark.
Mistake: Defining fitness as strength
Fitness in Darwinism means reproductive success, not physical strength or longevity.
Do this: Write 'leaves more fertile offspring'.
Usually costs: 1 mark.
Exam tips
- For any example of selection, write four links: variation, change in environment, differential survival and reproduction, change in population.
- Name the moth Biston betularia only if you are sure of the spelling; 'peppered moth' is enough.
- Give replica plating's conclusion in one line: mutation precedes selection.
- In data items, look at which phenotype classes do best before naming the type.
- Use 'heritable variation' rather than 'variation' alone.
Practice questions on Natural Selection and Its Types
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · MCQBoard level
When selection pushes the average value of a trait steadily towards one end of its range, it is called:
(a) stabilising (b) directional (c) disruptive (d) artificialHint
One extreme favoured.
Answer
(b) directional.
Q2 · 2 marks · Short answerBoard level
Who proposed natural selection independently of Darwin, and what does 'fitness' mean in Darwinism?
Hint
A naturalist in the Malay Archipelago.
Answer
Alfred Wallace. Fitness means reproductive fitness: the ability to leave more fertile offspring than others.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): DDT-resistant mosquitoes appeared within a few years of large-scale spraying.
Reason (R): DDT causes mutations for resistance in the mosquitoes that it does not kill.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.Hint
Does the chemical create or select the variant?
Answer
(c). Resistance mutations already existed; DDT only selected the resistant individuals.
Q4 · 3 marks · Short answerHarder board level
State two criticisms of Darwinism and explain how neo-Darwinism answers one of them.
Hint
Origin of variation.
Answer
Darwin could not explain the origin and inheritance of variations; he did not distinguish heritable from non-heritable variation (also: vestigial organs, arrival of the fittest). Neo-Darwinism explains the origin of variation by mutation and recombination, and their spread by gene flow, drift and selection.
Q5 · 2 marks · Short answerHarder board level
How did the work of Malthus influence Darwin's theory?
Hint
Geometric versus arithmetic increase.
Answer
Malthus argued that populations grow geometrically while food grows arithmetically, so many offspring must die. Darwin applied this to all organisms: overproduction with limited resources leads to a struggle for existence, in which the better adapted survive.
Q6 · 2 marks · Short answerHarder board level
Why are resistant bacteria seen within months or years, whereas a similar change in a fish population would take very much longer?
Hint
Life span and generation time.
Answer
The rate of appearance of new forms is linked to life span: bacteria divide within hours and become millions quickly, so a selected variant soon outgrows the rest; fish take years per generation, so the same change needs a vast time.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a weed population on a farm is sprayed with the same herbicide every season. In the first year 99% die; by the fifth year only 30% die. (a) Explain the change. (b) Name the type of selection. (c) Suggest two practices that slow this change.
Hint
Survivors breed.
Answer
(a) The few naturally resistant weeds survived each spraying and reproduced, so the resistant proportion rose year by year. (b) Directional selection. (c) Rotate herbicides with different modes of action; combine with mechanical weeding or crop rotation, so no single pressure is applied every season.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a bird species on an island eats seeds. In a drought year only large, hard seeds remain. Birds with deep beaks survive better; next year the average beak depth of chicks is greater.
(a) Name the type of selection. (1)
(b) What condition must beak depth satisfy for the chicks' average to change? (1)
(c) Sketch-describe how the beak-depth curve changes. (1)Hint
Heritability.
Answer
(a) Directional selection. (b) Beak depth must be heritable (have a genetic basis); otherwise survivors' offspring would not differ. (c) The bell-shaped curve shifts towards greater beak depth; its shape stays similar but the mean moves to the right.
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.Which type of selection keeps the mean and narrows the curve?Show answer
Stabilising selection.
2.Give an example of directional selection.Show answer
Industrial melanism in the peppered moth (also DDT or antibiotic resistance).
3.What did replica plating show?Show answer
Antibiotic resistance mutations exist before exposure; the antibiotic only selects them.
4.Whose essay on population influenced Darwin?Show answer
Thomas Malthus.
5.What does disruptive selection favour?Show answer
Both extremes of a trait over the average.
6.What is meant by fitness in Darwinism?Show answer
Reproductive fitness: leaving more progeny.
Natural Selection and Its Types: frequently asked questions
What is natural selection?
The process by which heritable variations that improve survival and reproduction in an environment become more common over generations, because their bearers leave more offspring.
What are the three types of natural selection?
Stabilising (favours the average), directional (favours one extreme, so the mean shifts) and disruptive (favours both extremes, so the distribution splits into two peaks).
Is industrial melanism directional selection?
Yes. In polluted areas the dark form was favoured and the population shifted towards it.
Why is antibiotic resistance an example of evolution?
The antibiotic kills sensitive bacteria; pre-existing resistant mutants survive and multiply, so the heritable make-up of the population changes within a short time.
Can disruptive selection lead to new species?
It can contribute: if the two extreme groups also stop interbreeding, their differences accumulate and they may become separate species.