Classification & Biodiversity

Chapter Summary

• Taxonomy classifies organisms into a hierarchy: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species (no overlap between groups).
• The three-domain system (Archaea, Eubacteria, Eukarya) replaced the old idea that all prokaryotes were one group, based on molecular phylogeny (RNA/DNA/protein comparisons).
• The five kingdoms (Prokaryotae, Protoctista, Fungi, Plantae, Animalia) sit under the domains — four of them (Protoctista, Fungi, Plantae, Animalia) are all Eukarya.
• Species are named using the binomial system (Genus species, italicised, genus capitalised).
• Human activity (habitat destruction, overexploitation, hunting, agriculture, climate change) is reducing global biodiversity.
• Biodiversity is assessed at three levels: ecosystem/habitat diversity, species diversity, and genetic diversity.
• Species richness (number of species) is simple but can be misleading — species abundance and an index of diversity give a fuller picture.
• Random sampling with quadrats, sweep nets, pitfall traps, and kick-sampling is used to estimate distribution and abundance.
• A niche is unique to one species; adaptations (anatomical, behavioural, physiological) let species fit their niche.
• The Hardy-Weinberg equation estimates allele/genotype frequencies in a population when no evolution is occurring.
• Allele frequencies change due to mutation, natural selection, genetic drift (bottlenecks, founder effect), which can lead to reproductive isolation and speciation.
• Seed banks and zoos play conservation roles, each with real advantages and limitations.

1. The Three Domains of Life

Why do we classify organisms at all?

Imagine trying to talk about biology without any grouping system — every single one of the ~8.7 million species on Earth would need its own totally separate conversation. Classification (called taxonomy) exists to make life easier to study, understand, and communicate about. It groups organisms based on evolutionary relationships — how closely related they are by descent from a common ancestor. This approach is called phylogenetic classification.

The system is hierarchical, meaning it's organised in nested levels called taxa (singular: taxon). Crucially: there is no overlap between groups at the same level — an organism belongs to exactly one species, one genus, one family, and so on. Smaller taxa nest inside bigger ones, like Russian dolls.

The pattern to understand, not just memorise: as you go from Domain down to Species, groups contain fewer organisms but those organisms are more similar to each other. A Domain contains billions of wildly different organisms; a Species contains only organisms that can interbreed to produce fertile offspring.

Why molecular evidence changed everything