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Evolution happens when populations change over time, and the engine of change is natural selection acting on variation. This lesson ties together the origin of variation (genetic and environmental), the mechanism by which favourable variants spread (natural selection in its three modes), and the adaptations that result. OCR A-Level Biology A specification 4.2.2 (h)–(j) requires you to describe variation, explain how it relates to natural selection, and recognise the anatomical, behavioural and physiological adaptations of organisms to their environments.
Key Definitions:
- Variation — the differences between individuals of the same species.
- Interspecific variation — variation between different species.
- Intraspecific variation — variation within the same species.
- Continuous variation — traits that vary along a smooth gradient (e.g. height, mass).
- Discontinuous variation — traits falling into distinct categories (e.g. blood group).
- Natural selection — differential survival and reproduction of individuals best adapted to their environment.
- Adaptation — any feature that enhances an organism's survival and reproduction in its environment.
Variation is the raw material of evolution. Without differences between individuals, natural selection has nothing to act on.
Both are real and measurable; the two are connected because intraspecific variation is the starting point from which new species arise.
flowchart TD
A[Variation] --> B[Continuous]
A --> C[Discontinuous]
B --> B1[Polygenic traits]
B --> B2[Normal distribution]
B --> B3[Strong environmental influence]
C --> C1[Usually one or few genes]
C --> C2[Distinct categories]
C --> C3[Little environmental influence]
Continuous variation — traits that form a smooth gradient:
Discontinuous variation — traits that fall into distinct categories:
Exam Tip: OCR mark schemes want you to state clearly: continuous = polygenic + environmental influence + normal distribution; discontinuous = one/few genes + little environment + distinct categories. Use these exact phrases.
Natural selection was proposed in 1858 by Charles Darwin and independently by Alfred Russel Wallace in a joint paper to the Linnean Society of London. Darwin expanded it the following year into On the Origin of Species (1859). The theory has four essential observations and two inferences:
Observations:
Inferences:
In the 20th century, natural selection was combined with Mendelian genetics (Darwin did not know about Mendel's work) to produce the modern synthesis — the framework of population genetics in which evolution is defined as a change in allele frequencies over generations.
This sequence is the standard mark-scheme answer for "describe how natural selection works". Memorise it.
Natural selection can push a population in three different ways:
flowchart TD
A[Natural Selection] --> B[Directional]
A --> C[Stabilising]
A --> D[Disruptive]
B --> B1[One extreme favoured]
C --> C1[Mean favoured, extremes selected against]
D --> D1[Both extremes favoured, mean selected against]
One extreme of the phenotype range is favoured, shifting the mean of the population over generations.
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