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Learn7402Paper 1 · Paper 1: Topics 1–4Genetic information, variation and relationships between organismsClassification and biodiversity
Genetic information, variation and relationships between organisms · Subtopic 2 of 3

Classification and Biodiversity

Organisms are classified into three domains (Bacteria, Archaea, Eukarya) and a hierarchy: domain, kingdom, phylum, class, order, family, genus, species; named binomially (Homo sapiens). Eukarya kingdoms: protoctists, fungi, plants, animals. Biodiversity includes ecosystem, species (richness and evenness) and genetic diversity. Measure with random sampling (quadrats at random coordinates) or systematic sampling (transects across a gradient). Mobile animals: mark–release–recapture, N=n1×n2m2N=\frac{n_1\times n_2}{m_2}. Simpson's index D=1−∑(nN)2D=1-\sum\left(\frac nN\right)^2 (0 to 1; higher = more diverse). Spearman's rank tests correlation between two variables: rs=1−6∑d2n(n2−1)r_s=1-\frac{6\sum d^2}{n(n^2-1)}.

Cambridge 9700 Paper 4Edexcel YBI11 WBI12AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the three-domain system and taxonomic hierarchy.
  • Describe sampling methods.
  • Calculate Simpson's index and population estimates.
  • Use Spearman's rank correlation.
Pause and think

Two fields both have 4 species. Why might one still be more diverse?

Section 2 of 14

Core idea

Start hereCounting life

How do you count every snail in a field? You don't: you sample, mark and estimate.

The big idea

Classification organises life into domains and ranks; sampling and indices let us measure biodiversity fairly.

The three domains

Based on rRNA and cell structure.

Worked exampleSimpson's index

Counts 10, 20, 30, 40.

Goal
D.
Strategy
1 − Σ(n/N)².
  1. ∑(nN)2=0.01+0.04+0.09+0.16=0.30\sum\left(\frac nN\right)^2=0.01+0.04+0.09+0.16=0.30
    N = 100.
  2. D=1−0.30=0.70D=1-0.30=0.70
    Subtract.
Answer0.70.
✓ CheckBetween 0 and 1 ✓.

Summary

  • Domain to species.
  • Sampling methods.
  • Simpson's D, Lincoln index, Spearman's.
Key words
Species richness
Number of species in an area.
Species evenness
How evenly individuals are spread among species.
HistoryWoese's domains. Carl Woese compared ribosomal RNA in 1977 and discovered that "bacteria" were really two very different groups: Bacteria and Archaea.
Fun factUndiscovered species. Scientists estimate around 8.7 million eukaryotic species exist, and most have never been described.
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