Classification and phylogenyEdexcel A-Level Biology A: Revision notes
Section 1
Why we classify
Classification organises the great variety of life into groups, based on the relationships between organisms. Related organisms share a common ancestor, and the more recently they shared it, the more closely related they are. The study of these evolutionary relationships is called phylogeny.
Organisms are grouped in a hierarchy of taxa: domain, kingdom, phylum, class, order, family, genus, species. Each species has a two-part scientific name, the binomial (genus then species, for example Homo sapiens).
Section 2
The species concept
Classification is built around the idea of the species: a group of organisms that can interbreed to produce fertile offspring.
This definition has limits:
- it cannot be applied to organisms that reproduce asexually, such as bacteria
- it cannot be tested on extinct organisms known only from fossils
- some different species can interbreed to produce hybrids
- populations that live apart cannot easily be tested
For these cases, differences in phenotype and genotype are used instead.
Two organisms that look alike are not necessarily the same species, and organisms that look different may be the same species. Always refer to interbreeding and fertile offspring.
Section 3
Phenotypic evidence
Phenotypic evidence is based on observable characteristics:
- anatomy and morphology, such as body structure and the bones of limbs
- behaviour, such as courtship calls
- biochemistry, such as the molecules in the cell wall or membrane
- cell structure seen under the microscope
Similar phenotypes suggest relationship, but they can be misleading. Unrelated organisms in similar environments may evolve similar features (convergent evolution), and the phenotype can also be affected by the environment.
Section 4
Genotypic evidence
Genotypic evidence compares the genetic information of organisms:
- the base sequences of DNA or RNA, for example the gene for ribosomal RNA
- the amino acid sequences of proteins, such as cytochrome c, which are coded for by DNA
Mutations build up over time, so the more differences there are in the sequences, the longer ago the organisms shared a common ancestor. A phylogenetic tree shows the relationships worked out in this way, with the branch points showing common ancestors.
Sequence data are objective, can be compared between very different organisms and are not affected by the environment, so they are more reliable than phenotype alone.
Link the steps: more differences in the DNA, so more mutations, so a more distant common ancestor.
Section 5
Critical evaluation by the scientific community
New data about organisms may mean that the classification has to change. Before this happens, other scientists critically evaluate the data:
- results are checked by experts through peer review before publication
- other scientists repeat the work and look for independent evidence
- they check that the conclusions are supported by the data and are free from bias or error
If the evidence is accepted by the scientific community, the classification is revised. If not, the claim is rejected or revised. This ensures that classification is based on reliable evidence.
Section 6
The three domains of life
Until the 1970s organisms were classified into five kingdoms, with all prokaryotes in one kingdom. In 1977 Carl Woese compared the base sequences of ribosomal RNA from many organisms. He found that one group of prokaryotes, the Archaea, was very different from the true bacteria.
Later work supported this, for example the different lipids in archaeal cell membranes. The three-domain system was proposed:
- Bacteria
- Archaea
- Eukaryota
The domains are based on molecular phylogeny, and have been widely accepted after evaluation by the scientific community.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Classification and phylogeny
- A taxonomist is studying two populations of lizards that live in separate valleys. The lizards look very similar and she is unsure whether they are one species or two. She has tissue samples from both populations.Describe how DNA could be used to help decide whether the two populations of lizard are the same species.2 marks
- Cytochrome c is a protein used in respiration and is found in all aerobic organisms. A biologist compares the amino acid sequence of human cytochrome c with that of other species. The numbers of differences from the human sequence are: rhesus monkey 1, horse 12 and tuna 21.Explain how the data suggest that humans are more closely related to horses than to tuna.2 marks
- Until the 1970s living things were classified into five kingdoms, and all prokaryotes were placed in one kingdom because of their similar cell structure. In 1977 Carl Woese and colleagues compared the base sequences of ribosomal RNA from many organisms. They found that some prokaryotes, later called Archaea, had ribosomal RNA sequences very different from those of both bacteria and eukaryotes. Later, other scientists found that archaeal cell membranes contain different lipids from bacterial membranes. In 1990 a classification with three domains, Bacteria, Archaea and Eukaryota, was proposed.Explain how the comparison of ribosomal RNA sequences led to a new classification of living organisms.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).