Topic 3: Classification and Biodiversity — Edexcel A-Level Biology B
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Topic 3: Classification and Biodiversity explained
Topic 3: Classification and Biodiversity covers 3 specification points, including Classification i Know that the classification… and Natural selection i Understand how evolution….
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Open a point below for its explanation and revision guidance.
3.1 Classification i Know that the classification system consists of a hierarchy of domain, kingdom, phylum, class, order, family, genus and species. ii Understand the limitations of the definition of a species as a group of organisms with similar characteristics that interbreed to produce fertile offspring. iii Understand why it is often difficult to assign organisms to any one species or to identify new species. iv Understand how gel electrophoresis can be used to distinguish between species and determine evolutionary relationships. v Know that DNA sequencing and bioinformatics can be used to distinguish between species and determine evolutionary relationships. vi Understand the role of scientific journals, the peer review process and scientific conferences in validating new evidence supporting the accepted scientific theory of evolution. vii Understand the evidence for the three-domain model of classification as an alternative to the five-kingdom model and the role of the scientific community in validating this evidence.
Classification arranges life in a hierarchy: domain, kingdom, phylum, class, order, family, genus, species, each level nested inside the one above. The species definition (similar features, interbreeding, fertile offspring) fails for asexual organisms, ring species and fertile hybrids, so assigning organisms or naming new species is often hard. Gel electrophoresis separates DNA fragments by size and charge, giving banding patterns that distinguish species and reveal relationships; DNA sequencing plus bioinformatics compares bases directly. Scientific journals, peer review and conferences test and validate new evidence for evolution. The three-domain model, based on molecular evidence, is an alternative to the five-kingdom model, validated by the scientific community.
3.2 Natural selection i Understand how evolution can come about through natural selection acting on variation bringing about adaptations. ii Understand how organisms occupy niches according to physiological, behavioural and anatomical adaptations. iii Understand how reproductive isolation can lead to allopatric and sympatric speciation. iv Understand that there is an evolutionary race between pathogens and the development of medicines to treat the diseases they cause.
Natural selection acts on variation within a population: individuals with adaptations better suited to the environment survive, reproduce and pass on alleles, so allele frequencies change over generations and evolution occurs. Organisms occupy niches through physiological adaptations (for example enzyme or metabolic changes), behavioural adaptations (for example migration or courtship) and anatomical adaptations (for example body shape or insulation). Reproductive isolation, whether by a geographical barrier (allopatric) or within the same area (sympatric), prevents gene flow and allows speciation. Pathogens and medicines are in an evolutionary race: resistant pathogens survive treatment and reproduce, so new medicines are continually needed.
3.3 Biodiversity i Know that biodiversity can be assessed at different scales: ● within a habitat at the species level using a formula to calculate an index of diversity: D = N(N - 1) / Σn(n - 1) ● within a species at the genetic level by looking at the variety of alleles in the gene pool of a population. ii Understand the ethical and economic reasons (ecosystem services) for the maintenance of biodiversity. iii Understand the principles of ex-situ (zoos and seed banks) and in-situ conservation (protected habitats), and the issues surrounding each method.
Biodiversity is assessed at more than one scale. Within a habitat, species diversity is quantified using the index of diversity D = N(N - 1) / Σn(n - 1), where N is the total number of individuals of all species recorded and n is the number of individuals of each individual species; a larger D indicates greater species diversity. Within a species, genetic diversity is assessed by examining the variety of alleles present in the gene pool of a population, since a broad allele range supports adaptation. Biodiversity is maintained for ethical reasons and for economic reasons through ecosystem services, such as pollination, flood control and nutrient cycling. Conservation may be ex-situ, in zoos or seed banks, or in-situ in protected habitats; each approach has strengths and issues, including cost, genetic representation and habitat protection.
Your focus
- Recall and sequence the eight levels of the classification hierarchy.
- Explain limitations of the species definition and difficulties in assigning organisms to species.
- Describe how gel electrophoresis, DNA sequencing and bioinformatics distinguish species and show evolutionary relationships.
Show all 12 objectives
- Explain how journals, peer review and conferences validate evidence, including evidence for the three-domain model.
- Explain how natural selection acting on variation produces adaptations and evolution.
- Describe physiological, behavioural and anatomical adaptations that allow niche occupation.
- Distinguish allopatric and sympatric speciation through reproductive isolation.
- Explain the evolutionary race between pathogens and the development of medicines.
- Calculate an index of diversity using D = N(N - 1) / Σn(n - 1) and interpret the value obtained.
- Describe how genetic diversity within a species is assessed from the variety of alleles in a gene pool.
- Explain ethical and economic reasons for maintaining biodiversity, including named ecosystem services.
- Compare ex-situ and in-situ conservation and evaluate an issue associated with each method.
Topic 3: Classification and Biodiversity exam tips
Marking Points
- Recall the correct order of the eight hierarchical taxa: domain, kingdom, phylum, class, order, family, genus, species.
- Explain at least two limitations of the species definition, such as asexual reproduction or fertile hybrids.
- Describe how gel electrophoresis separates DNA fragments to produce species-specific banding patterns.
- State that DNA sequencing and bioinformatics compare base sequences to determine evolutionary relationships.
- Explain how peer review, journals and conferences validate new evidence for evolution.
- Outline evidence supporting the three-domain model as an alternative to the five-kingdom model.
- Explain natural selection as variation, differential survival, reproduction and inheritance of advantageous alleles.
- Describe how physiological, behavioural and anatomical adaptations allow organisms to occupy niches.
- Distinguish allopatric speciation (geographical isolation) from sympatric speciation (isolation without a geographical barrier).
- Explain how reproductive isolation prevents gene flow and leads to speciation.
- Describe the evolutionary race between pathogens and medicines, including selection of resistant strains.
- D = N(N - 1) / Σn(n - 1) is used to calculate an index of diversity within a habitat at the species level, where N is the total number of individuals of all species and n is the number of individuals of one species.
- A higher value of D indicates a habitat with greater species diversity, so the index allows habitats to be compared numerically.
- Genetic diversity within a species is assessed by examining the variety of alleles in the gene pool of a population.
- Biodiversity is maintained for ethical reasons, such as the intrinsic value of species and habitats.
- Biodiversity is maintained for economic reasons through ecosystem services, for example pollination, flood control and nutrient cycling.
- Ex-situ conservation includes zoos and seed banks, which protect organisms or genetic material away from their natural habitat.
- In-situ conservation protects habitats in place, allowing species and ecological interactions to be conserved within the ecosystem.
- Each conservation method has issues, such as cost, limited genetic representation in ex-situ collections, and difficulty enforcing protection of in-situ habitats.
Examiner Tips
- 💡Learn the eight taxa as a fixed sequence so hierarchy questions are answered quickly and accurately.
- 💡When asked about limitations, give a named example such as asexual bacteria or a fertile hybrid rather than a vague statement.
- 💡Link each technique to its purpose: electrophoresis for banding patterns, sequencing and bioinformatics for base comparisons.
- 💡For validation questions, name the mechanism (peer review, journal, conference) and say what it does to the evidence.
- 💡Use the sequence variation, selection, reproduction, inheritance when explaining natural selection.
- 💡Name the type of adaptation (physiological, behavioural or anatomical) explicitly in niche questions.
- 💡For speciation, state the isolating mechanism and explain how it stops gene flow.
- 💡For pathogen questions, link resistance to selection and the need for new medicines.
- 💡When calculating D, work out N(N - 1) and Σn(n - 1) separately before dividing, and keep the arithmetic clear so the method can be followed.
- 💡Link each conservation example to whether it is ex-situ or in-situ, then state one issue with that method to reach the higher tariff.
- 💡For ecosystem services, name a specific service and explain who benefits economically, rather than writing only that biodiversity is useful.
Common Mistakes
- Listing the hierarchy in the wrong order, for example placing family above order; correct order is domain, kingdom, phylum, class, order, family, genus, species.
- Assuming all organisms fit the interbreeding definition of a species; asexual organisms and fertile hybrids show the definition is limited.
- Confusing gel electrophoresis with DNA sequencing; electrophoresis separates fragments by size and charge, while sequencing reads base order.
- Believing the three-domain model replaced the five-kingdom model without evidence; molecular data and scientific community validation support it.
- Saying organisms adapt during their lifetime; variation already exists and selection acts on it across generations.
- Confusing allopatric and sympatric speciation; allopatric involves a geographical barrier, sympatric does not.
- Treating reproductive isolation as the same as geographical isolation; reproductive isolation is the key requirement for speciation.
- Believing medicines create resistance; resistance arises from existing variation and is selected by medicine use.
- Confusing N with n in the index formula: N is the total number of individuals of all species, while n is the number of individuals of a single species.
- Assuming a high index of diversity means a habitat contains only one dominant species; in fact a high D reflects many species with relatively even numbers.
- Treating genetic diversity as the number of species in a habitat; genetic diversity is instead the variety of alleles in the gene pool of a population.
- Describing ex-situ conservation as protection of a habitat in its natural location; ex-situ conservation takes place away from the natural habitat, as in zoos and seed banks.