Classification and the species conceptEdexcel A-Level Biology B: Revision notes
Section 1
The classification hierarchy
Organisms are classified in a hierarchy of groups, each nested within the one above:
Domain, kingdom, phylum, class, order, family, genus, species
Organisms in the same group share features and, usually, a recent common ancestor. Each species has a two-part scientific name (binomial) of genus then species, for example Homo sapiens.
A memory aid for the order: Dear King Philip Came Over For Good Soup. Domain is the highest rank and species the lowest.
Section 2
The species concept and its limitations
A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring. The definition has limitations:
- Asexual organisms (many bacteria) do not interbreed
- Fossils: breeding cannot be tested
- Hybrids: some species interbreed, such as lions and tigers in captivity, and a horse and donkey produce an infertile mule
- Isolated populations cannot be tested for interbreeding
- Different species can look similar, and members of one species can look different, so appearance is unreliable
For these reasons it is often difficult to assign organisms to a species, or to identify a new species.
Section 3
Gel electrophoresis
Gel electrophoresis separates DNA fragments (or proteins) by size.
- DNA is cut with restriction enzymes
- Fragments are loaded into wells in an agarose gel and an electric field is applied
- DNA is negatively charged (phosphate groups), so it moves towards the positive electrode
- Smaller fragments move faster and further, giving a banding pattern
Comparing the patterns shows which species are most similar: the more bands shared, the more closely related. It is quick and cheap, but gives only limited information.
Section 4
DNA sequencing and bioinformatics
DNA sequencing determines the exact order of bases in a gene, such as a gene for ribosomal RNA or cytochrome c. Bioinformatics uses computers to store, compare and analyse sequences from many species, calculating percentage similarity.
The more differences in the sequences, the longer ago the species shared a common ancestor. Sequencing is more precise than gel electrophoresis, and it can be used with fossils or microbes where interbreeding cannot be tested.
Section 5
Validating new evidence
New claims, such as a new species or a new classification, are checked by the scientific community:
- Results are published in scientific journals
- Peer review: independent experts check the methods and conclusions before publication
- Other scientists repeat the work or test it with other evidence
- Results are discussed at scientific conferences
This shows why the theory of evolution is accepted. Many independent lines of evidence, including DNA sequences, support it and have been validated in this way.
Section 6
Three domains or five kingdoms?
The five-kingdom model (Prokaryotae, Protoctista, Fungi, Plantae, Animalia) places all prokaryotes in one kingdom.
The three-domain model (Archaea, Bacteria, Eukarya) came from Carl Woese, who compared ribosomal RNA sequences. Archaea differ from bacteria as much as they differ from eukaryotes. Other evidence includes different membrane lipids, no peptidoglycan in the cell walls of archaea, and RNA polymerase and ribosomes in archaea that resemble those of eukaryotes.
The model was accepted after the evidence was published, peer reviewed and confirmed by other scientists.
Archaea are not bacteria. In the three-domain model they form a separate domain, and are in some ways closer to eukaryotes.
Must know
- Hierarchy: domain, kingdom, phylum, class, order, family, genus, species
- Species concept: interbreed to produce fertile offspring, with limitations (asexual, fossils, hybrids)
- Gel electrophoresis: fragments separated by size; more shared bands, more closely related
- DNA sequencing and bioinformatics compare base sequences
- Journals, peer review and conferences validate evidence
- Three domains (Archaea, Bacteria, Eukarya) vs five kingdoms; rRNA evidence
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Classification and the species concept
- Researchers collected several specimens of an unfamiliar beetle from the canopy of a rainforest. All the specimens are female, no males have ever been found, and the beetles are thought to reproduce without mating.Suggest how the researchers could use DNA to decide whether the beetle is a new species.2 marks
- Gel electrophoresis was used to compare DNA from four related species of finch, A to D. DNA from each species was cut with the same restriction enzyme and the fragments were separated on an agarose gel. Species A and B shared five of their six bands, species A and C shared three bands, and species A and D shared only one band.Explain why the smaller DNA fragments are found further from the wells at the end of the run.2 marks
- Until the 1970s, living organisms were classified into five kingdoms: Prokaryotae, Protoctista, Fungi, Plantae and Animalia. In 1977 Carl Woese compared the base sequences of ribosomal RNA from many organisms. He found that the prokaryotes fell into two groups, bacteria and archaea, which differed from each other as much as each differs from the eukaryotes. Archaea also differ from bacteria in the lipids of their membranes and in their RNA polymerase.Explain how Woese's evidence supports the three-domain model rather than the five-kingdom model.3 marks
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).