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Biology 4% exam weight

Variation, Evolution and Natural Selection

Part of the NECO SSCE study roadmap. Biology topic bio-15 of Biology.

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Variation, Evolution and Natural Selection

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your NECO SSCE paper.

Variation is the differences that exist between individuals of the same species; evolution is the cumulative change in heritable traits of a population across generations, and natural selection is Darwin’s mechanism driving that change. NECO tests this topic under Paper I objectives and Paper II essay/cognitive items at roughly 4% syllabus weight.

Must-know facts

  • Sources of variation: gene (point) mutations, chromosomal mutations (deletion, duplication, inversion, translocation, non-disjunction), and gene recombination during meiosis and fertilisation.
  • Two patterns of variation: continuous (range, e.g. height, weight) — controlled by many polygenes plus environment; discontinuous (discrete categories, e.g. ABO blood groups, tongue-rolling) — controlled by one or few genes.
  • Darwin’s five postulates: overproduction → struggle for existence → heritable variation → differential survival and reproduction → gradual accumulation of favourable variants forming new species.
  • Lamarck vs Darwin: Lamarck claimed acquired traits are inherited; Darwin showed selection acts on pre-existing random variation, a distinction NECO loves to test.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Defining the three core ideas

Variation refers to differences among individuals of the same species. These differences arise from three sources: gene mutations (random changes in DNA base sequence), chromosomal mutations (large-scale changes in chromosome structure or number), and gene recombination during meiosis and sexual reproduction, which shuffles existing alleles into new combinations. Variation can be continuous, producing a bell-shaped range of phenotypes such as human height or skin colour, or discontinuous, producing discrete categories such as ABO blood groups.

Evolution is the gradual change in heritable characteristics of a population over successive generations. The modern synthesis (neo-Darwinism) treats evolution as a change in allele frequencies within a population’s gene pool, driven by natural selection, mutation, gene flow and genetic drift.

Natural selection is the non-random differential survival and reproduction of individuals bearing advantageous heritable traits. Organisms produce more offspring than can survive, competition is intense, and those whose inherited features best match the environment leave more descendants, gradually shifting allele frequencies.

Evidence examiners expect you to cite

Line of evidenceWhat it showsClassic example
Fossil recordSequence of change over deep timeArchaeopteryx (reptile–bird link), Equus horse series
Comparative anatomyCommon ancestry vs convergent functionHomologous: human arm, whale flipper, bat wing. Analogous: insect wing vs bird wing
Comparative embryologyShared developmental plansVertebrate embryos with gill slits and tails
Comparative biochemistryMolecular kinshipCytochrome c, haemoglobin sequences; ~98–99% human–chimp DNA similarity
Observed selectionReal-time evolutionIndustrial melanism in Biston betularia; sickle-cell balanced polymorphism in West Africa

NECO tip: pair every evolutionary claim with a named example — “Archaeopteryx shows a reptile–bird transition” scores higher than “fossils show evolution”.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Lamarckism vs Darwinism — the classic comparison

FeatureLamarck’s theoryDarwin’s theory
Source of variationArises from need/usePre-existing and random
MechanismUse strengthens, disuse weakens organs; acquired traits inheritedNatural selection of favourable variants
Inheritance of acquired traitsYes (central claim)No
Germline vs somaImplies somatic change reaches gametesSomatic change does not alter germline DNA
Modern statusRejectedSupported, expanded by modern synthesis

Industrial melanism — a worked micro-example

Before the Industrial Revolution, light-coloured Biston betularia moths (form typica) were common on lichen-covered tree trunks in England because birds predatorily removed dark variants. Soot deposition from factories darkened the trunks, after which the dark form carbonaria became better camouflaged and increased in frequency. Kettlewell’s mark–release–recapture experiments confirmed differential bird predation. After Clean Air Acts reduced soot, the typica form recovered, showing that selection intensity tracked the environment.

Sickle-cell balanced polymorphism

The HbS allele produces sickle haemoglobin and is harmful in homozygotes (HbSS), causing sickle-cell anaemia. In malarial regions of West Africa, heterozygotes (HbAS) resist Plasmodium falciparum, so the allele is maintained at high frequency despite its homozygous cost — a textbook case of heterozygote advantage preserving variation.

Common pitfalls NECO marks deduct for

  • Writing that “individuals evolve”; evolution acts on populations (allele frequencies).
  • Saying natural selection creates variation; it only sorts pre-existing variation.
  • Mixing homologous (common origin) with analogous (similar function, different origin).
  • Treating Lamarck’s inheritance of acquired traits as valid for muscle, height or stretching examples.

Practice prompts

  1. State five observations that led Darwin to propose natural selection, and explain how each supports his theory.
  2. Using industrial melanism in Biston betularia, describe how pollution reversed the selection pressure and the consequence for moth population composition.

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