OCR A-Level Biology: Biodiversity, Classification and Evolution — Complete Revision Guide (H420)
OCR A-Level Biology: Biodiversity, Classification and Evolution — Complete Revision Guide
Biodiversity, Classification and Evolution forms the second half of Module 4 of the OCR A-Level Biology A (H420) specification, sitting alongside communicable diseases and immunity. It is one of the most data-rich and quantitatively examined courses on the H420 path, bringing together the three intellectual pillars that organise all of modern biology: how we measure life's variety (biodiversity and sampling), how we group it (classification and phylogeny), and how we explain it (variation, natural selection and speciation). Both Paper 1 (Biological Processes) and Paper 3 (Unified Biology) of H420 reliably set extended-response items on natural selection, speciation and the comparison of the five-kingdom and three-domain classification systems, alongside calculation-driven items on Simpson's index, the Hardy-Weinberg equation and chi-squared testing.
Course 7 of 12 on the LearningBro OCR A-Level Biology learning path consolidates the evolutionary and ecological framework on which the rest of the path's biology depends. It draws on biological molecules for the molecular phylogenetic markers used in modern taxonomy, on nucleic acids, enzymes and biological reactions for the rRNA-based work that produced the three-domain system, and it sits adjacent to genetics and inheritance (which develops allele-frequency change in detail) and to cloning, biotechnology and ecosystems (which extends the in-situ and ex-situ conservation framework introduced here).
Guide Overview
The Biodiversity, Classification and Evolution course is structured as twelve lessons that move from biodiversity measurement and conservation through classification and phylogeny to the mechanism of evolution and the statistical tests used to test evolutionary hypotheses.
- Biodiversity Levels
- Sampling Techniques
- Simpson's Index of Diversity
- Genetic Diversity and Allele Frequencies
- Factors Affecting Biodiversity
- Conservation: In Situ and Ex Situ
- Classification: Taxonomic Hierarchy
- Five Kingdom and Three Domain Systems
- Phylogeny and Cladistics
- Evidence for Evolution
- Variation, Natural Selection and Adaptation
- Speciation and Statistical Testing
OCR H420 Specification Coverage
This course covers OCR H420 Modules 4.2 (Biodiversity) and 4.3 (Classification and Evolution) in full. The specification organises this material around three principal areas — quantitative biodiversity, hierarchical classification with phylogenetic reasoning, and evolutionary mechanism — each mapped here to one or more lessons (refer to the official OCR specification document for exact wording).
| Sub-module | Topic area | Primary lesson(s) |
|---|---|---|
| 4.2.1 | Levels of biodiversity (habitat, species, genetic) | Biodiversity Levels |
| 4.2.1 | Sampling techniques (quadrat, transect, Lincoln index) | Sampling Techniques |
| 4.2.1 | Simpson's Index of Diversity | Simpson's Index of Diversity |
| 4.2.1 | Genetic diversity within and between populations; allele frequency | Genetic Diversity and Allele Frequencies |
| 4.2.1 | Factors affecting biodiversity (deforestation, agriculture, climate change) | Factors Affecting Biodiversity |
| 4.2.1 | In-situ and ex-situ conservation; CITES and Rio convention | Conservation: In Situ and Ex Situ |
| 4.3.1 | Taxonomic hierarchy; binomial nomenclature | Classification: Taxonomic Hierarchy |
| 4.3.1 | Five-kingdom vs three-domain systems; molecular evidence | Five Kingdom and Three Domain Systems |
| 4.3.1 | Phylogeny; cladograms; molecular clocks | Phylogeny and Cladistics |
| 4.3.2 | Evidence for evolution (fossil, comparative anatomy, molecular) | Evidence for Evolution |
| 4.3.2 | Variation (continuous, discontinuous); natural selection (directional, stabilising, disruptive) | Variation, Natural Selection and Adaptation |
| 4.3.2 | Speciation (allopatric, sympatric); Hardy-Weinberg; chi-squared | Speciation and Statistical Testing |
Module 4.2 and 4.3 content is examined across all three H420 papers, with Paper 1 and Paper 2 reliably setting an extended-response item on natural selection, speciation or the comparison of classification systems, and Paper 3 favouring quantitative items on Simpson's index, Hardy-Weinberg p-squared plus 2pq plus q-squared equal to one, and chi-squared goodness-of-fit testing of Mendelian inheritance ratios.
Biodiversity Levels and Sampling
The biodiversity levels lesson develops the three nested scales the specification requires candidates to distinguish. Habitat (or ecosystem) diversity counts the variety of habitats in a region; species diversity combines species richness (the number of species present) with species evenness (the relative abundance of each); genetic diversity is the variety of alleles within and between populations of one species. The three scales are causally linked — habitat diversity is the substrate for speciation, species diversity supports the food-web complexity that ecosystem function requires, and genetic diversity is the raw material on which natural selection acts.
The sampling techniques lesson develops the standard field methods: random quadrat sampling (using coordinates from a grid) for sessile organisms, systematic line and belt transects for environmental gradients (across a sand-dune succession or up a rocky-shore zonation), and the mark-release-recapture Lincoln index for mobile populations, where the estimated population size equals the product of the first and second sample sizes divided by the number of recaptured marked individuals. Candidates must identify the appropriate technique for a given study system and recognise the assumptions underlying the Lincoln index — no births, deaths or migration during the sampling interval, no mark loss, no behavioural change from being marked, and random mixing of marked individuals back into the population. Random sampling is preferred over opportunistic sampling because it minimises observer bias.
Simpson's Index of Diversity
The Simpson's Index lesson is the quantitative core of the biodiversity sub-module and a routine source of AO2 calculation marks. The OCR formulation is
D=1−∑(Nn)2
where n is the number of individuals of a given species and N is the total number of individuals across all species. Values approach zero in communities dominated by one species and approach one in evenly-distributed multi-species communities. The Index combines richness and evenness in a single quantity, and is to be preferred over a raw species count when communities of different evenness are being compared. Candidates must calculate D from a frequency table, comment on the value relative to a comparison habitat, and justify why a higher value indicates greater ecological stability.
Genetic Diversity and Factors Affecting Biodiversity
The genetic diversity lesson develops the third biodiversity scale at the population-genetics level. Allele frequency — the proportion of all gene copies at a locus accounted for by a particular allele — is the operative variable in evolutionary change; any rise or fall in allele frequency between generations is, by definition, evolution. Genetic diversity is reduced by genetic bottlenecks (population crashes that retain only a fraction of the original allele pool), founder effects (small samples colonising a new area), inbreeding in small populations and intensive artificial selection in domesticated species. The full mechanistic treatment of allele-frequency change sits ahead in genetics and inheritance.
The factors affecting biodiversity lesson develops the principal anthropogenic drivers — habitat destruction (deforestation, drainage of wetlands, urbanisation), monoculture and intensive agriculture, pollution, overexploitation of harvested species, introduction of invasive species, and climate change. Examiners reward candidates who can identify a named driver, trace its mechanism (loss of habitat heterogeneity, eutrophication, selective predation pressure) and propose a specific intervention. The H420 specification requires candidates to discuss these drivers in academic register, paraphrasing textbook consensus on extinction rates and biodiversity counts rather than citing specific contested figures.
Conservation: In Situ and Ex Situ
The conservation lesson develops the two complementary conservation frameworks. In-situ conservation protects species in their natural habitats through legally designated reserves (national parks, sites of special scientific interest, marine protected areas), habitat restoration and active species management. Ex-situ conservation removes individuals to controlled environments — captive breeding programmes in zoos, plant species in botanic gardens, seed banks for crop and wild plant genetic resources — to maintain populations that cannot be sustained in the wild and ultimately to reintroduce them. The lesson contrasts the relative strengths of each approach: in-situ preserves co-evolved community context but is vulnerable to ongoing habitat pressures, while ex-situ buys time but risks domestication and loss of fitness for the original environment.
International conservation frameworks — CITES regulating trade in endangered species, the Rio Convention on Biological Diversity, the Countryside Stewardship Scheme as a UK example — are examined as the institutional architecture that delivers conservation in practice. Cross-links into cloning, biotechnology and ecosystems develop the in-situ and ex-situ framework further into the ecosystem-management and biotechnology-assisted conservation context.
Classification: Taxonomic Hierarchy and the Three Domains
The taxonomic hierarchy lesson develops the nested ranks — domain, kingdom, phylum, class, order, family, genus, species — that organise biological diversity into a single hierarchical scheme. The binomial nomenclature paraphrased from Linnaeus's eighteenth-century system gives each species a two-part Latin name (genus and specific epithet, italicised with the genus capitalised). The biological species concept paraphrased from Mayr's mid-twentieth-century formulation defines a species as a group of organisms capable of interbreeding to produce fertile offspring; the morphological and phylogenetic species concepts handle the cases (asexual organisms, extinct taxa) where the biological concept does not apply.
The five-kingdom and three-domain lesson contrasts the older Whittaker five-kingdom system (Animalia, Plantae, Fungi, Protoctista, Prokaryotae) with the three-domain system (Archaea, Bacteria, Eukarya) paraphrased from Woese's work on ribosomal RNA sequence comparison in the late 1970s and 1980s. The molecular evidence showed that the prokaryotic kingdom contained two deeply divergent lineages — bacteria and archaea — that differ in membrane lipid chemistry, RNA polymerase structure, ribosomal RNA sequence and cell-wall composition, and that archaea are in some respects more closely related to eukaryotes than to bacteria. Candidates are reliably asked to explain why the three-domain system replaced the five-kingdom system at the highest taxonomic level, citing the molecular evidence over morphological grouping.
Phylogeny, Cladistics and Evidence for Evolution
The phylogeny and cladistics lesson develops the reconstruction of evolutionary relationships from shared derived characters (synapomorphies). Cladograms are branching diagrams in which each node represents a common ancestor and each branch represents a lineage; sister taxa share a more recent common ancestor than either does with any outgroup. Molecular clocks use the rate of neutral mutation in non-coding or synonymous sites to estimate divergence times. Cross-links into nucleic acids, enzymes and biological reactions develop the rRNA-based taxonomy that underwrites the three-domain system.
The evidence for evolution lesson develops the three principal lines: the fossil record (transitional forms, sequence of appearance consistent with phylogenetic prediction, biostratigraphic dating), comparative anatomy (homologous structures inherited from a common ancestor and modified by divergent selection — the pentadactyl limb being the textbook example — versus analogous structures arising from convergent selection on unrelated lineages), and molecular evidence (sequence comparison of conserved proteins such as cytochrome c, and of ribosomal RNA). The convergent triangulation of these three independent lines is what gives the modern evolutionary synthesis its empirical weight.
Variation, Natural Selection and Speciation
The variation, natural selection and adaptation lesson develops the central mechanism. Continuous variation is polygenic, environmentally modulated and normally distributed (height, mass); discontinuous variation is typically governed by one or a few loci and falls into discrete categories (blood group, eye colour in simple cases). Natural selection, paraphrased from the framework set out by Darwin in 1859 and independently by Wallace in the same period, operates whenever there is heritable variation in a trait that affects reproductive success: individuals with favourable phenotypes leave more offspring, the favourable alleles rise in frequency, and the population adapts. The three modes — directional selection (one extreme favoured, mean shifts; antibiotic resistance, peppered moth industrial melanism), stabilising selection (the mean favoured, variance reduced; human birth weight) and disruptive selection (both extremes favoured at the expense of the mean; can precipitate speciation) — must be distinguished, sketched and exemplified.
The speciation and statistical testing lesson develops two modes of speciation. Allopatric speciation occurs when a geographical barrier physically isolates two subpopulations, divergent selection (and genetic drift in small isolated populations) drives genetic differentiation, and reproductive isolation evolves as a by-product until interbreeding is no longer possible even if the populations rejoin. Sympatric speciation occurs without geographical isolation, typically through behavioural, temporal or polyploid reproductive isolation acting within a single geographical range — the polyploid origin of many flowering-plant species being a clear example.
The same lesson develops the two principal statistical tests the OCR specification requires. The Hardy-Weinberg principle — paraphrased from independent contributions by Hardy and Weinberg in 1908 — gives genotype frequencies under the null model of no evolution as
p2+2pq+q2=1
where p and q are the allele frequencies at a diallelic locus. The principle assumes large population, random mating, no mutation, no migration and no selection; any deviation from these assumptions produces evolutionary change. Candidates use the equation to calculate allele and genotype frequencies from observed data, including the frequency of heterozygous carriers of a recessive condition from the observed frequency of affected homozygotes. The chi-squared test compares observed and expected frequencies under a null hypothesis (a Mendelian 3:1, 9:3:3:1 or 1:1 ratio); candidates must calculate the statistic, count degrees of freedom (number of categories minus one), compare against the critical value at the 5 percent significance level and accept or reject the null hypothesis with a sentence of biological interpretation.
Linking to the Other Courses
Biodiversity, Classification and Evolution is the most synoptic course on the H420 path. The molecular phylogeny in the phylogeny and cladistics lesson rests on the protein and nucleic acid biochemistry developed in biological molecules and nucleic acids, enzymes and biological reactions. The natural-selection mechanism feeds directly into the antibiotic-resistance content of communicable diseases and immunity, where mutation-and-selection at the bacterial population level is the textbook example. The allele-frequency mathematics of the Hardy-Weinberg principle is developed in greater inheritance-pattern detail in genetics and inheritance. The conservation-management content of the in-situ and ex-situ lesson is extended into ecosystem and biotechnology frameworks in cloning, biotechnology and ecosystems. The chi-squared statistical testing introduced here is the canonical test for the Mendelian inheritance ratios that genetics develops, and the broader statistical toolkit is consolidated in the final exam-preparation course on the path. Even the cardiac-cycle and dissociation-curve content of exchange and transport is given evolutionary context here, because the four-chamber double circulation is itself an evolutionary adaptation to high metabolic rate.
Required Practicals / PAGs
This course anchors two of the OCR H420 Practical Endorsement PAGs:
- PAG 3 (Sampling techniques in the field) — random quadrat sampling, line and belt transects, mark-release-recapture for mobile populations, anchored in the sampling techniques lesson and in the Simpson's Index calculation lesson. PAG 3 quantitative output (raw counts, calculated index values, Lincoln index population estimates) is reliably examined on Paper 3.
- PAG 11 (Research skills — planning and presenting biological investigations) — provides the methodological framework for designing and presenting biodiversity, evolution and population-genetics investigations, anchored across the factors affecting biodiversity lesson, the conservation lesson, and the speciation and statistical testing lesson.
Examiners use PAG 3 in particular as the practical context for Simpson's Index and Lincoln index calculations on Paper 3; PAG 11 underwrites the experimental-design extended-response items on the same paper.
Closing
Biodiversity, Classification and Evolution is the conceptual capstone of Module 4 and one of the most quantitatively examined courses on the OCR H420 specification. The biodiversity-measurement toolkit (Simpson's Index, Lincoln index), the three-domain classification framework, the three lines of evidence for evolution, the three modes of natural selection and the two modes of speciation, together with the Hardy-Weinberg and chi-squared statistical tests, return across essentially every series of OCR Biology papers. Start with the Biodiversity, Classification and Evolution course and work through the twelve lessons in sequence; build a flashcard deck for the named statistical formulae and the canonical worked examples (peppered moth for directional selection, polyploid plants for sympatric speciation, the Galapagos finches for allopatric divergence) and drill Simpson's Index and Hardy-Weinberg calculations until they are automatic. The Module 5 physiology and Module 6 genetics content that follows then become a series of consequences of the evolutionary and ecological framework established here.