BiologyCBSE Class 12 · Evolution

Natural Selection and Its Types

Updated 15 October 2026 · Fact-checked

Natural selection is the process in which individuals with heritable variations that help them survive and reproduce leave more offspring, so those variations spread. Industrial melanism in moths and antibiotic resistance are examples. Selection can be stabilising (favours the mean), directional (favours one extreme) or disruptive (favours both extremes).

Compare the dashed original curve with the filled curve after selection: it narrows, shifts one way, or splits into two peaks.

Illustration drawn for QuestionBuddy.

Understand Natural Selection and Its Types

Darwin's idea. From his voyage and observations, Darwin concluded that existing life forms share similarities with each other and with forms that lived millions of years ago, and that populations change over time. His theory rests on two key concepts: branching descent and natural selection. Alfred Wallace, working in the Malay Archipelago, reached similar conclusions.

Populations show variation, and some of it is heritable.
Resources are limited, so not all offspring survive.
Individuals whose characters suit the environment survive and reproduce more. Fitness in Darwin's sense means reproductive fitness: those who leave more progeny are 'selected by nature'.
Over generations the favoured variations increase in the population.

Example 1: industrial melanism in England.

Before industrialisation (around 1850), tree trunks were covered by whitish lichens; white-winged (light) moths were camouflaged and survived, so they outnumbered dark-winged (melanic) moths.
After industrialisation, smoke and soot killed the lichens and darkened the trunks; light moths were easily seen and eaten by predators, while dark moths were camouflaged and survived. By 1920, dark moths outnumbered light ones in industrial areas.
In rural areas without industry, melanic moths stayed rare. This shows that the better adapted form increases in a mixed population. Note that no variant is completely wiped out.
Lichens do not grow in polluted areas, so they serve as indicators of industrial pollution.

Example 2: evolution by human action. Excessive use of herbicides and pesticides has selected resistant varieties in a short time; microbes become resistant to antibiotics, and eukaryotic cells to drugs, within months or years rather than centuries.

Lamarck vs Darwin vs de Vries.

Lamarck: evolution by use and disuse of organs (e.g. giraffes stretching their necks to reach leaves and passing on longer necks). Not accepted now.
Darwin: small, heritable, directional variations sorted by natural selection; evolution is gradual.
Hugo de Vries, working on evening primrose (*Oenothera lamarckiana*), proposed mutations: large differences arising suddenly; these are random and directionless, and could cause speciation in a single step (saltation).
Modern synthetic theory: variation from mutation and recombination (plus gene flow and genetic drift), acted on by natural selection, leads to evolution; isolation helps new species form.

Giraffe's neck, explained by natural selection. Ancestral giraffes varied in neck length (heritable variation). When low leaves were scarce, those with longer necks reached more food, survived and left more offspring. Over many generations the average neck length increased. This is directional selection, not inheritance of acquired characters.

Types of natural selection (on a bell-shaped curve of a character, e.g. body size, with the mean in the middle):
1. Stabilising selection: individuals near the mean are favoured; extremes are selected against. The curve becomes narrower and taller about the same mean. Example: human birth weight (very low and very high weights have lower survival).
2. Directional selection: individuals at one extreme are favoured; the mean shifts in that direction. Examples: dark moths after industrialisation; longer necks in giraffes; antibiotic resistance.
3. Disruptive selection: individuals at both extremes are favoured; the middle is selected against. The curve may split into two peaks. Example: a population in which only the smallest and the largest seed-eaters do well because medium seeds are scarce.

Flow chart: variation (mutation, recombination) → overproduction and competition → better-suited variants survive and reproduce more (fitness) → their alleles increase → population changes (stabilising, directional or disruptive) → over long time, new species.

Draw and label: the three types of selection. Checklist: three pairs of bell curves (before and after). Stabilising: same mean, narrower peak. Directional: whole curve shifted to one side. Disruptive: curve splits into two peaks with a dip in the middle. Mark the original mean with a dotted line and shade the selected-against parts. Labels: number of individuals (y-axis), phenotype/character value (x-axis), mean, stabilising, directional, disruptive.

Natural Selection and Its Types: key facts

Fitness: $fitness = reproductive success (number of surviving offspring)$; Not strength or size by itself.
Stabilising selection: $mean unchanged; variance ↓$; Peak gets taller and narrower.
Directional selection: $mean shifts to one extreme$; Industrial melanism; antibiotic and pesticide resistance.
Disruptive selection: $both extremes favoured \Rightarrow two peaks$; Can start the splitting of a population.
Sources of variation: $mutation + recombination (+ gene flow, genetic drift)$; Natural selection acts on this variation.

How to answer Natural Selection and Its Types questions

Questions use graphs, historical examples or 'explain how' cases. Always connect variation, survival, reproduction and change over generations.

1Identify the heritable variation present (light and dark moths; short and long necks; resistant and sensitive bacteria).
2Name the selecting agent (predatory birds on soot-darkened bark; scarcity of low leaves; antibiotic in the body).
3State which variants survive and reproduce more, and why (camouflage, food access, resistance).
4Conclude with the change over generations in the population (frequency of the favoured form rises).
5For graphs, compare the before and after curves: same mean and narrower → stabilising; mean shifted → directional; two peaks → disruptive.
6Contrast with Lamarck when asked: acquired characters are not inherited; only genetic variation is.

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 antibiotics

Suppose a patient stops a course of antibiotics after three days because he feels better. In his body, 1 in 10,000 bacteria carried a mutation for resistance before treatment; the antibiotic killed 99.9 per cent of the sensitive bacteria in those three days but none of the resistant ones.

Starting from 1,000,000 bacteria, how many sensitive and resistant bacteria remain after three days? What fraction of the survivors is resistant, and what type of selection is this?

Try it first, then show the solutionSolution

Resistant at start = 1,000,000 ÷ 10,000 = 100; sensitive = 999,900.
Sensitive surviving = 0.1% of 999,900 = 999.9 ≈ 1000; resistant surviving = 100.
Fraction resistant among survivors = 100 ÷ (1000 + 100) ≈ 0.09, about 9%, up from 0.01%; when they multiply again, the population will contain far more resistant bacteria.
One extreme (resistance) is favoured and the population shifts towards it: directional selection, caused by human action.

Answer: About 1000 sensitive and 100 resistant remain; about 9% are resistant (from 0.01%); directional selection.

Memory hook: Stop the course early, and you select the survivors.

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: Stay, Drift one way, Divide
Stabilising stays at the mean, Directional moves one way, Disruptive divides into two.
Use it when: Reading selection graphs.
Soot suits the sooty
Sooty trunks favoured sooty (dark) moths.
Use it when: Industrial melanism.
Lamarck lifts, Darwin sifts
Lamarck: animals lift (use) organs and pass gains on; Darwin: nature sifts existing variation.
Use it when: Comparing theories.

Natural Selection and Its Types: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Use and disuse · 1 mark · MCQBoard level

The idea of evolution by use and disuse of organs was proposed by:
(a) Darwin (b) de Vries (c) Lamarck (d) Wallace

Try it first, then show the solutionSolution

Lamarck proposed that use and disuse of organs changes them and that such acquired changes are inherited.

Answer: (c) Lamarck.

Example 2: Industrial melanism · 3 marks · Short answerBoard level

Explain how industrial melanism in England after 1850 is an example of natural selection.

Try it first, then show the solutionSolution

Before industrialisation, lichen-covered light tree trunks camouflaged white-winged moths, so predators ate more dark moths; white moths were more common (1 mark).
After industrialisation, soot darkened the trunks and lichens died; dark moths were now camouflaged and survived, while white ones were eaten (1 mark).
Dark moths left more offspring and became more common in industrial areas but not in rural areas: the better-adapted form increased, showing natural selection (1 mark).

Answer: Camouflage against changing bark colour decided survival; dark moths increased where trunks darkened.

Example 3: Reading selection curves · 3 marks · Short answerHarder board level

Three graphs show a population's body size before and after selection. In graph A the mean is unchanged but the peak is narrower; in B the curve has moved to the right; in C there are now two peaks. Name the type of selection in each and give one example of B.

Try it first, then show the solutionSolution

A: stabilising selection (individuals near the mean favoured) (1 mark).
B: directional selection (one extreme favoured; mean shifts) (1 mark).
C: disruptive selection (both extremes favoured). Example of B: increase of dark moths after industrialisation, or antibiotic resistance (1 mark).

Answer: A stabilising, B directional (e.g. industrial melanism), C disruptive.

Example 4: Saltation · 2 marks · Very short answerHarder board level

Who proposed the idea of saltation, and from work on which plant? Explain why saltation conflicts with Darwin's view of how evolution proceeds.

Try it first, then show the solutionSolution

Hugo de Vries, from work on the evening primrose; saltation is speciation caused by a single-step, large mutation (1 mark).
De Vries' mutations are large, sudden, random and directionless, so evolution would occur in jumps; Darwin's variations are small, heritable and directional, so evolution is gradual (1 mark).

Answer: Hugo de Vries (evening primrose); single-step large mutations (saltation) versus Darwin's gradual evolution by small directional variations.

Example 5: Disruptive selection on a polygenic trait · 3 marks · Short answerHOTS / competency

Suppose a polygenic trait (e.g. beak depth) shows a bell-shaped distribution in a bird population. A drought leaves only very small and very large seeds for several generations. (a) Predict the change in the distribution. (b) Name the type of selection. (c) Why could this lead towards two species over a long time?

Try it first, then show the solutionSolution

(a) Birds with intermediate beaks find little suitable food and leave fewer offspring; the distribution develops two peaks (small and large beaks) with a dip in the middle (1 mark).
(b) Disruptive selection (1 mark).
(c) If the two groups also begin to mate mostly within their own type, genetic differences accumulate between them and they may become reproductively isolated, forming two species (1 mark).

Answer: Two-peaked distribution; disruptive selection; could split the population into two species.

Example 6: Giraffe's neck, two ways · 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 what type of selection does it represent?

Try it first, then show the solutionSolution

Lamarck: giraffes stretched their necks to reach high leaves; the stretched necks (acquired characters) were passed to offspring (1 mark).
Darwin: giraffes varied naturally in neck length; when low food was scarce, longer-necked individuals got more food, survived and reproduced more, passing on the trait; over generations, the average neck lengthened (1 mark).
Darwin's explanation is accepted, since acquired characters are not inherited; it is directional selection (1 mark).

Answer: Lamarck: inheritance of acquired stretching (rejected); Darwin: selection of naturally longer necks (accepted, directional).

Where students lose marks in Natural Selection and Its Types

The slips examiners see most often, and what to write instead.

Mistake: Writing a Lamarckian explanation
Everyday language says animals 'adapt' on purpose.
Wrong working
Moths turned dark to hide on sooty trees.
Right working
Dark moths already existed; they survived better on sooty trees and left more offspring.
Do this: Selection acts on existing heritable variation; individuals do not change on purpose.
Usually costs: 1-2 marks.
Mistake: Saying light moths disappeared
Over-statement.
Wrong working
All white moths were eliminated.
Right working
White moths decreased but were not wiped out.
Do this: Use 'frequency increased/decreased'.
Usually costs: ½-1 mark.
Mistake: Defining fitness as strength
'Survival of the fittest' is misread.
Wrong working
Fitness means the strongest survive.
Right working
Fitness means reproductive fitness: leaving more offspring.
Do this: Always say 'reproductive fitness'.
Usually costs: 1 mark.
Mistake: Confusing disruptive with directional
Both move individuals away from the mean.
Wrong working
Disruptive selection shifts the mean to one side.
Right working
Directional: one extreme favoured, mean shifts. Disruptive: both extremes favoured, two peaks.
Do this: Count the peaks after selection.
Usually costs: 1 mark.

Exam tips

In 'explain how' answers, use the four-step chain: variation, selecting agent, differential reproduction, change in frequency.
Mention lichens as pollution indicators in industrial melanism answers.
Draw small before/after curves for the types of selection; label the mean.
Use the words 'heritable', 'reproductive fitness' and 'over generations'.
Keep Lamarck, Darwin and de Vries distinct: use and disuse; natural selection of small variations; large random mutations (saltation).

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 · Assertion-reasonBoard level
Assertion (A): The population of melanic moths increased in industrial areas after the Industrial Revolution.
Reason (R): In industrial areas, lichen-covered trees were replaced by soot-covered trees, against which dark moths were camouflaged.
(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
Camouflage.
Answer
(a).
Q2 · 1 mark · MCQBoard level
Variations caused by mutations, according to de Vries, are:
(a) small and directional (b) random and directionless (c) acquired during life (d) always harmful
Hint
Contrast with Darwin.
Answer
(b) random and directionless.
Q3 · 3 marks · Short answerHarder board level
Suppose a survey of moths in an industrial town found 30 dark moths out of 400 in an early year and 340 dark moths out of 400 many years later, while a nearby village showed 28 and 32 dark moths out of 400 in the same years. Calculate the percentages and explain the difference between town and village.
Hint
Percentage = count/400 × 100.
Answer
Town: 30/400 = 7.5% → 340/400 = 85%. Village: 28/400 = 7% → 32/400 = 8%. In the town, soot darkened tree trunks, so dark moths were camouflaged and survived predation better; their frequency rose sharply (directional selection). In the village, lichen-covered light trunks remained, so dark moths stayed rare. Light moths were not completely wiped out in the town.
Q4 · 2 marks · Very short answerHarder board level
Why is the rapid appearance of pesticide-resistant insects called 'evolution by anthropogenic action'? Which type of selection is involved?
Hint
Who applied the selecting agent?
Answer
Humans applied the pesticide, which killed sensitive insects and allowed rare resistant variants to survive and reproduce; resistant varieties spread within months or years. Because the selecting agent is human-made, it is evolution by anthropogenic action. It is directional selection.
Q5 · 4 marks · Case-basedHOTS / competency
Read and answer.
Suppose a graph shows the distribution of a polygenic trait in a population as a single bell-shaped curve. After many generations of a certain kind of selection, the curve becomes much narrower and taller around the same mean.
(a) Name this type of selection. (1)
(b) Give one human example. (1)
(c) Explain why the variation decreased while the mean stayed the same, and state whether evolution has stopped. (2)
Hint
Extremes selected against.
Answer
(a) Stabilising selection. (b) Human birth weight: babies of intermediate weight have the highest survival. (c) Individuals at both extremes survive or reproduce less, so their alleles decline, while those near the mean are favoured; the mean is unchanged but variance falls. Evolution has not stopped: selection is still acting, maintaining the existing optimum; if the environment changes, the same population can respond.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Some organisms are better adapted to survive in an otherwise hostile environment.
Reason (R): Adaptive ability is inherited and has a genetic basis.
(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
Selection acts on heritable variation.
Answer
(a). Heritable (genetic) adaptations let some survive and pass the ability on.

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.What does fitness mean in Darwin's theory?Show answer
Reproductive fitness: leaving more offspring.
2.Which moths increased after industrialisation in England?Show answer
Dark-winged (melanic) moths.
3.What can lichens indicate?Show answer
Industrial pollution (they do not grow in polluted areas).
4.Which type of selection favours both extremes?Show answer
Disruptive selection.
5.On which plant did de Vries work?Show answer
Evening primrose (Oenothera lamarckiana).
6.Which type of selection keeps the mean and narrows the curve?Show answer
Stabilising selection.

Natural Selection and Its Types: frequently asked questions

What is natural selection?

The process by which individuals with heritable variations suited to the environment survive and reproduce more, so those variations become more common over generations.

What are the types of natural selection?

Stabilising (favours the mean), directional (favours one extreme, shifting the mean) and disruptive (favours both extremes, giving two peaks).

How is industrial melanism an example of natural selection?

When soot darkened tree trunks, dark moths were camouflaged and survived predation better, so their proportion rose; in unpolluted areas light moths stayed common.

How is antibiotic resistance an example of evolution?

Antibiotics kill sensitive bacteria, while rare resistant variants survive and multiply, so resistant populations appear within months or years.

What is the difference between Darwin's and de Vries' views?

Darwin saw evolution as gradual selection of small directional variations; de Vries proposed large, random mutations that could form new species in one step (saltation).