Classification and biodiversity — Edexcel International A Level Biology
The species concept, three-domain classification, biodiversity and endemism, and measuring biodiversity with species richness, heterozygosity and D.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Classification organises the variety of life into groups based on relationships, using similarities and differences in phenotype and genotype, and is built around the species concept. When new molecular evidence is published, the scientific community evaluates it critically. This led to the three-domain system: Archaea, Bacteria and Eukarya.
Biodiversity is the variety of living organisms, which can be considered at the level of habitats, species and genes. Endemic species are found only in one geographical area. Over time evolution has produced an enormous variety of life, but human activities such as habitat destruction, pollution, climate change and overexploitation now threaten it.
Species richness is the number of different species in a habitat, measured by sampling. Genetic diversity within a species is measured with the heterozygosity index, which is the number of heterozygotes divided by the number of individuals in the population. A higher value means more genetic diversity and a population better able to adapt.
The index of diversity D = N(N−1) ÷ Σn(n−1) takes into account both the number of species and the number of individuals of each, where N is the total number of organisms and n is the number of each species. A higher D means a more diverse habitat. It allows different habitats to be compared.
Exam questions
8 exam-style questions (25 marks), each with its mark scheme.
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The whole of plant structure and function, biodiversity and conservation on one page, so you can see where this subtopic fits.
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Quick check questions
Name the three domains of life.
Show answer
Archaea, Bacteria and Eukarya.
What type of evidence led to the three-domain system?
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Molecular evidence, such as differences in rRNA and other nucleotide or protein sequences.
Define endemism.
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A species being found in only one geographical region and nowhere else.
What is species richness?
Show answer
The number of different species in a habitat.
In D = N(N − 1) ÷ Σn(n − 1), what does n stand for?
Show answer
The total number of organisms of each species.
Frequently asked questions
What is the three-domain system of classification?
The three-domain system places all organisms into Archaea, Bacteria or Eukarya. It is based on molecular evidence, such as differences in ribosomal RNA, membrane lipids and DNA-associated proteins, which showed that archaea differ from bacteria as well as from eukaryotes. It replaced the earlier grouping of all prokaryotes together.
What is the difference between biodiversity and endemism?
Biodiversity is the variety of living organisms in an area, including species, genetic and habitat diversity. Endemism is when a species is found in one geographical region and nowhere else, such as lemurs in Madagascar. Endemic species are vulnerable because of their restricted range.
How is genetic diversity measured?
Genetic diversity within a species can be measured with the heterozygosity index: the number of heterozygous individuals divided by the total number of individuals in the population. The value ranges between 0 and 1, and a higher value means greater genetic diversity.
How do you calculate the index of diversity?
Use D = N(N − 1) ÷ Σn(n − 1), where N is the total number of organisms of all species and n is the number of organisms of each species. Work out N(N − 1), sum n(n − 1) for every species, and divide. A higher D means a more diverse habitat.
Why is the index of diversity better than species richness?
Species richness only counts the number of species. The index of diversity also takes into account how many individuals there are in each species, so it shows whether a habitat is dominated by one species. This gives a better comparison of the biodiversity of different habitats.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026