Classification and Cladistics: notes and practice questions
- This topic explores the classification of organisms, contrasting traditional taxonomy with cladistics for evolutionary relationships.
- Classification helps organize immense species diversity; traditional ranks are often arbitrary.
- Clades group organisms by common ancestry and shared characteristics, evidenced by gene or amino acid sequences.
- The molecular clock estimates divergence times based on accumulated sequence differences.
- Cladograms depict evolutionary relationships, with nodes representing common ancestors.
- Parsimony analysis aids in selecting the most probable cladogram.
- All life is classified into three domains based on rRNA sequences.
How it is examined
Cladogram reading is the workhorse: deduce which two species are most closely related, identify the node representing a common ancestor, outline what a clade is. It suits Paper 1B data items and HL Paper 2. Students routinely lose marks by reading proximity along the top of a cladogram instead of the position of the node.
There is no SL content in A3.2. The whole subtopic is HL.
Do not set this topic for SL students.
Guiding questions
- What tools are used to classify organisms into taxonomic groups?
- How do cladistic methods differ from traditional taxonomic methods?
Linking questions
- How can similarities between distantly related organisms be explained?
- What are some examples of ideas over which biologists disagree?
Practice questions
3 questions · 1 easy · 2 mediumQuestion 1
EasyPaper 2 · calculator5 marks(a) State the biological term for all the interacting organisms in an ecosystem.
(b) State the biological term for the splitting of a pre-existing species into two or more new species.
(c) State the biological term for a group of organisms that have evolved from a common ancestor.
(d) State the biological term for the accumulation of carbon compounds in biomass by autotrophs.
(e) State the biological term for the maximum population size an environment can sustain.
Think about the level of ecological organization that includes multiple different species living together, but does not include the abiotic factors.
What is the evolutionary process that increases biodiversity by creating new distinct lineages?
This term is used in phylogenetic classification to describe a complete branch on an evolutionary tree.
This represents the energy captured by plants that is converted into organic matter.
This limit is determined by the availability of resources such as food, water, and space in the environment.
Question 2
MediumPaper 2 · calculator10 marks(a.i) Based on differences in ribosomal RNA sequences, all living organisms are now classified into three broad groups above the kingdom level.
State the term used for this highest rank in modern taxonomy.
(a.ii) Eukaryota is one of these three groups. Identify the other two groups.
(b) Traditional taxonomy relies on a hierarchy of ranks to classify organisms. State the seven traditional taxonomic ranks used to classify eukaryotic organisms, in descending order from the most inclusive to the least inclusive.
Think of the rank that includes Archaea, Bacteria, and Eukaryota.
Both of the other groups consist entirely of prokaryotic organisms.
Start with Kingdom and end with the two parts of a binomial name.
Question 3
MediumPaper 2 · calculator5 marks(a) Explain how the molecular clock is used to estimate the time of divergence between species.
Think about how changes in DNA or protein sequences accumulate over time and how we can use the rate of these changes, along with fossil evidence, to calculate when two species shared a common ancestor.
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Where marks are lost
- Command terms are consistently under-read. Students answer describe when the question said explain, so they give an account with no reasons and cap at half marks. The reverse also happens on outline, where a student writes an essay for a 2-mark summary and runs out of time.
