# Types of Natural Selection > Types of natural selection for NEET Biology: directional selection with industrial melanism, stabilising selection with heterozygote advantage, genetic drift. - Canonical URL: https://prepelephant.com/topics/neet-ug/biology/selection-types-evolution - Exam / course: NEET-UG · Subject: Biology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Types of Natural Selection", PrepElephant, https://prepelephant.com/topics/neet-ug/biology/selection-types-evolution ## Direct answer Natural selection is differential survival and reproduction of variants, and NEET tests it through named outcomes rather than definitions. Directional selection shifts a trait in one direction when the environment changes — the textbook case is industrial melanism in England's peppered moth, where soot-blackened trunks let dark moths escape bird predation while the light form declined. Stabilising selection favours the intermediate and holds a trait steady — the classic case is the heterozygote advantage of sickle-cell haemoglobin in African malarial regions, where both homozygotes are selected against and the heterozygote persists. Disruptive selection favours both extremes over the mean. Genetic drift (including the founder effect), mutation, gene flow and recombination are the other forces that change allele frequencies. ## What you must remember - **Directional selection:** one extreme phenotype is favoured; the peppered moth (*Biston betularia*) of industrial England is NCERT's showcase, with dark melanic forms replacing light ones on soot-darkened trees. - **Industrial melanism reversed:** after clean-air legislation reduced soot, the light form reportedly recovered — evidence that selection tracks environment, not progress. - **Stabilising (balancing) selection:** intermediate genotype is favoured; HbA/HbS heterozygotes in malarial Africa survive better than either homozygote, maintaining the sickle allele at low frequency. - **Disruptive selection:** both extremes are favoured at the cost of intermediates — rare in exam stems but completes the triplet. - **Hardy-Weinberg context:** gene migration, genetic drift, mutation, genetic recombination and natural selection are the five factors that disturb allele-frequency equilibrium. - **Genetic drift and founder effect:** random changes in allele frequencies in small populations; when a few individuals seed a new population, the group carries an unrepresentative sample of alleles. - **de Vries and saltation:** mutation can cause large, single-step change; Hugo de Vries' saltation contrasts with Darwin's gradual accumulation of small variations. - **Fitness redefined:** evolutionary fitness means reproductive success — leaving more offspring — not physical strength. ## Replaying the peppered moth Pre-industrial England had lichen-pale tree trunks, and the light peppered moth resting on them was nearly invisible to birds; the rare dark mutants were eaten and remained rare. Industry then blackened the trunks with soot and killed the lichens, and the logic reversed: on dark bark the light moths stood out and the melanic form blended, so birds culled the pale majority. Within decades the population's composition had flipped, dark forms dominating industrial districts — nothing about the moths' genes "tried" anything; predation simply redrew survival odds, generation after generation, in one direction. When air-quality laws returned cleaner trunks in the twentieth century, observers reported the light form climbing back — selection is loyal to conditions, not to favourites, which is the examiner's favourite twist. The sickle-cell story shows the same engine running in stabilising mode. In African malarial regions, HbA/HbA homozygotes risk death from falciparum malaria; HbS/HbS homozygotes risk severe sickle-cell anaemia; the heterozygote, only mildly affected, resists malaria best of all. Selection therefore holds the S allele at intermediate frequencies — an equilibrium that would puzzle anyone expecting selection to eliminate a "bad" allele. Read together, the two cases teach the lesson NEET wants: selection is a filter with no agenda, its direction set by whatever the environment currently rewards, and allele frequencies are its ledger. Drift, mutation and migration also write in that ledger, but only selection consistently adapts populations to their conditions. ## Where students slip The first slip is mechanical: calling industrial melanism an example of genetic drift or of Lamarckian inheritance — it is natural selection, with bird predation as the agent. The second is the heterozygote case: students mark the sickle-cell equilibrium as directional selection because a phenotype is favoured; the pattern — intermediate genotype persisting while both extremes are penalised — is stabilising or balancing. The third is vocabulary: "survival of the fittest" is often misread as physical fitness; in evolutionary biology fitness counts offspring contributed to the next generation. Finally, saltation questions: de Vries proposed mutation-driven, single-step speciation, contrasted with Darwin's gradualism — a statement pair that appears almost every year in some form. ## Frequently asked questions ### What type of selection does industrial melanism demonstrate? Directional selection — the environment shifted, and one extreme (the melanic form) was favoured over the other across generations. ### Why does the sickle-cell allele persist in African populations? Because heterozygotes (HbA/HbS) resist malaria better than normal homozygotes while avoiding severe sickle disease — stabilising selection maintains the allele. ### Which five factors can disturb Hardy-Weinberg equilibrium? Gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. ### What is the founder effect? A small group splitting off to establish a new population carries an unrepresentative sample of the original gene pool, and chance alone fixes its allele frequencies. ### What did Hugo de Vries mean by saltation? Evolution through large, single-step mutations producing new species suddenly, rather than Darwin's gradual accumulation of small variations.