All revision notes topics

Classification and the three-domain systemEdexcel International A Level Biology: Revision notes

Section 1

The species concept

Classification arranges organisms into groups according to their similarities. The basic unit is the species: a group of organisms with similar characteristics that can interbreed to produce fertile offspring.

Limitations:

  • it cannot be applied to organisms that reproduce asexually, such as bacteria, because they do not interbreed
  • it cannot be tested on fossils
  • some different species can interbreed (hybrids), and hybrids are often sterile, for example a mule
  • organisms in distant places cannot be tested for breeding

So other evidence is also used.

Key termsspeciesinterbreedhybrid

Section 2

Phenotypic and genotypic similarity

Organisms are grouped by their similarities.

  • Phenotypic similarity is based on observable features, such as structure, appearance and behaviour. It can mislead, because similar features can evolve in unrelated organisms (convergent evolution), and individuals vary.
  • Genotypic similarity is based on the DNA (or RNA) base sequences or the amino acid sequences of proteins. Closely related organisms have fewer differences because they share a more recent common ancestor.

Molecular evidence is objective and quantitative, and it can be applied to organisms with few observable features.

Key termsphenotypegenotypeconvergent evolution
Exam tip

When asked why molecular evidence is better, say it is objective, quantitative and usable where appearance is similar.

Section 3

Molecular evidence leads to new groupings

As techniques developed, scientists could compare DNA base sequences, ribosomal RNA sequences and amino acid sequences of proteins. Ribosomal RNA is found in all living organisms and changes slowly, so it is useful for comparing distantly related organisms.

In 1977 Carl Woese compared rRNA sequences. He found that some prokaryotes, previously grouped with the bacteria, were as different from the bacteria as from the eukaryotes. This led to the three-domain system.

Key termsribosomal RNAmolecular evidence

Section 4

The three domains

All living organisms are placed in one of three domains.

  • Bacteria: prokaryotes with peptidoglycan in their cell walls, circular DNA, no membrane-bound nucleus and 70S ribosomes.
  • Archaea: prokaryotes with no peptidoglycan in their walls, circular DNA, no nucleus, and different membrane lipids. They include many that live in extreme conditions such as hot springs. Their rRNA sequences differ from both bacteria and eukaryotes.
  • Eukarya: eukaryotes with a membrane-bound nucleus with linear DNA, membrane-bound organelles and 80S ribosomes. They include protoctists, fungi, plants and animals.
Key termsArchaeaBacteriaEukaryapeptidoglycan
Common mistake

Archaea are prokaryotes but they are not bacteria. They are a separate domain.

Section 5

Evaluating new data

New evidence can lead to new groupings, but it must be critically evaluated.

  • The work is published and peer reviewed by other scientists, who check the methods and conclusions.
  • Other scientists repeat the work and look for further evidence, such as different genes or proteins, to see whether the results are reproducible.
  • The new data are accepted if they are supported and explain more than the earlier system. If not, the system is not changed.

Classification is therefore revised as scientific knowledge changes. The move from two and five kingdoms to three domains is an example.

Key termspeer reviewreproducible

Must Know

  • A species can interbreed to produce fertile offspring. This cannot be used for asexual organisms or fossils.
  • Phenotypic evidence uses observable features. Genotypic evidence uses DNA, RNA or protein sequences.
  • Molecular evidence is objective and quantitative, and fewer differences mean a closer relationship.
  • Woese compared rRNA sequences and showed that the Archaea are a separate domain.
  • Bacteria have peptidoglycan walls, Archaea do not, and Eukarya have a nucleus.
  • New data are evaluated by peer review and repetition before groupings are changed.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Classification and the three-domain system

  1. Two populations of small beetles live on different islands. They look very similar, but the males of one population make a different courtship sound. Biologists want to decide whether the two populations are the same species or different species.
    Beetles from the two islands are brought together, and they mate and produce offspring, but all the offspring are sterile. Explain what this suggests about whether the beetles are the same species.2 marks
  2. Biologists have discovered three new single-celled organisms, P, Q and R, in a hot spring. None of them has a nucleus. The biologists compare the base sequences of the genes for ribosomal RNA (rRNA) in the three organisms. The sequences of P and Q are similar to each other, but both differ greatly from the sequence of R.
    Suggest why using molecular evidence is more reliable than using observable features alone for classifying organisms such as P, Q and R.2 marks
  3. Before 1977 living organisms were classified into kingdoms using observable features. In 1977 Carl Woese and his colleagues compared the sequences of ribosomal RNA from many organisms, and the results led to a new system in which all living organisms are placed in one of three domains.
    State the three domains, and give two features of cells in the domain Eukarya that distinguish them from cells in the other two domains.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).