The principles of classification and phylogeny, the three domains and five kingdoms, biodiversity, and how to measure species diversity.

What this dot point is asking

WJEC wants you to explain the principles of classification and phylogeny, describe the taxonomic hierarchy and the binomial system, outline the three domains and five kingdoms, and define and measure biodiversity, including using the index of diversity.

The classification hierarchy

A species is a group of organisms with similar features that can interbreed to produce fertile offspring. This definition has limits: it cannot be applied to organisms that reproduce asexually, or to extinct species known only from fossils, which is one reason classification is debated and revised.

Phylogeny, domains and kingdoms

The three domains are Archaea, Bacteria and Eukarya, distinguished mainly by differences in ribosomal RNA, membrane lipids and cell biochemistry. Within the eukaryotes, the five kingdoms are Prokaryotae, Protoctista, Fungi, Plantae and Animalia. Classification is revised as new evidence appears, especially from DNA sequencing and amino acid (protein) sequencing: more closely related organisms share more similar sequences, and molecular clocks can estimate how long ago lineages diverged.

Biodiversity and the index of diversity

Biodiversity is the variety of living organisms in an area, including the number of species (species richness), the evenness of their abundance, and the genetic variety within species.

Examples in context

Example 1. Reclassifying with molecular data. Giant pandas were long debated as either bears or raccoons based on appearance. DNA and protein sequence comparisons settled it: pandas group firmly within the bear family Ursidae. This shows how molecular evidence can override misleading physical similarities, the exact point WJEC tests on revised classification.

Example 2. Monitoring river health with diversity indices. Ecologists sample river invertebrates and calculate an index of diversity to judge pollution. A clean river has many species evenly spread and a high index; a polluted river is dominated by a few tolerant species and scores low. The index turns a species count into a single comparable number for monitoring over time.

Try this

Q1. State the correct order of the taxonomic hierarchy from kingdom to species. [1 mark]

• Cue. Kingdom, phylum, class, order, family, genus, species.

Q2. Explain why a community with many species but one very common species may have a low index of diversity. [2 marks]

• Cue. The index weights the evenness of individuals; dominance by one species lowers $D$ even when the species number is high.

Q3. A sample contains $6$ of species P and $6$ of species Q. Calculate the index of diversity using $D = 1 - \sum (n/N)^2$. [3 marks]

• Cue. $N = 12$; each fraction squared is $(6/12)^2 = 0.25$; sum $= 0.5$; $D = 1 - 0.5 = 0.5$.