How are organisms classified, and how do we measure and compare biodiversity?
Classification and biodiversity: the three domains and the taxonomic hierarchy; phylogeny; the species concept; measuring biodiversity using the index of diversity; and genetic diversity.
A focused answer to the Eduqas Component 2 statement on classification and biodiversity. Covers the three domains and taxonomic hierarchy, phylogeny, the species concept, the index of diversity calculation, and genetic diversity.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
Eduqas wants you to describe the three domains and the taxonomic hierarchy, explain phylogeny and the species concept, calculate and interpret the index of diversity, and explain genetic diversity. This opens Component 2's theme of how organisms are related.
The taxonomic hierarchy and the three domains
Classification arranges organisms into nested groups (taxa): domain, kingdom, phylum, class, order, family, genus, species. Each species has a two-part binomial name (genus then species, for example Homo sapiens), written in italics.
Phylogeny and the species concept
Phylogeny is the evolutionary history and relationships between organisms, often shown as a branching phylogenetic tree where closely related species share a recent common ancestor. Modern classification aims to be phylogenetic (reflecting evolutionary relationships).
Measuring biodiversity
Biodiversity can be considered at the level of habitats, species and genes. Species diversity combines species richness (the number of species) with evenness (their relative abundance), measured by the index of diversity:
where is the total number of all organisms and is the number of each species. A higher means greater diversity (more species, more evenly spread), and a more diverse community is generally more stable.
Genetic diversity is the number of different alleles in a population's gene pool; greater genetic diversity means a population is more likely to adapt to change. It can be assessed from the proportion of polymorphic genes or by DNA analysis.
Examples in context
Example 1. Why monocultures are unstable. A farmer's field of one crop has very low species diversity (a low index), so a single pest or disease can devastate it, whereas a diverse natural community is more resilient, a favourite Eduqas application.
Example 2. Reclassifying with molecular data. Organisms once grouped by appearance have been moved when DNA sequencing revealed their true relationships, showing why phylogeny based on molecular evidence is more reliable than morphology alone.
Try this
Q1. Write the eight levels of the taxonomic hierarchy in order, from the largest group down. [2 marks]
- Cue. Domain, kingdom, phylum, class, order, family, genus, species.
Q2. State the three domains. [1 mark]
- Cue. Bacteria, Archaea and Eukarya.
Q3. Explain why a high index of diversity often indicates a stable ecosystem. [2 marks]
- Cue. Many species, evenly spread, means more interdependence and alternative food sources, so the community is less affected by the loss or fluctuation of any one species.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksExplain why classification systems have changed over time to use the three-domain system, referring to the evidence used.Show worked answer →
Classification was originally based on observable features (morphology), but similar features can evolve independently (convergent evolution), which can be misleading.
Newer evidence from molecular biology, such as comparing DNA base sequences, RNA (especially ribosomal RNA) and protein (amino acid) sequences, gives a more objective measure of how closely related organisms are.
This molecular evidence showed that the prokaryotes split into two fundamentally different groups, the Bacteria and the Archaea, leading to the three-domain system (Bacteria, Archaea and Eukarya).
Markers reward the limitation of using features alone, the use of molecular (DNA/RNA/protein) evidence, and the splitting of prokaryotes into Bacteria and Archaea giving three domains.
Eduqas 20214 marksA woodland sample contained 3 species of ground plant with 8, 5 and 2 individuals. Calculate the index of diversity using the formula D equals N(N minus 1) divided by the sum of n(n minus 1), where N is the total number of organisms and n is the number of each species.Show worked answer →
Total number of organisms N equals 8 plus 5 plus 2 equals 15, so N(N minus 1) equals 15 times 14 equals 210.
For each species, n(n minus 1): 8 times 7 equals 56; 5 times 4 equals 20; 2 times 1 equals 2. The sum is 56 plus 20 plus 2 equals 78.
D equals 210 divided by 78 equals 2.69 (to 3 significant figures).
Markers reward the correct N(N minus 1), the correct sum of n(n minus 1), and the final value of about 2.69.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)