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4.5.2Genetics, Variation and Interdependence

Phylogenetic Classification — AQA A Level Biology 7402 Revision Notes & Topical Questions

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4.5.2 Phylogenetic Classification — AQA A Level Biology 7402 notes: Describe how organisms are classified into groups by their evolutionary relationships and explain the idea of a phylogenetic tree.Taxonomy is the science of naming and classifying living things.Classification pla…

  • artificial
  • branch point (node)
  • common ancestor
  • base sequence of DNA
  • amino acid sequence of proteins
  • convergent evolution

Section 1 of 9Classification and taxonomy

Taxonomy is the science of naming and classifying living things.

Classification and taxonomy
Classification places organisms into groups that make it easier to identify them, to predict their features and to study how they are related.
There are two broad approaches.
Artificial
An artificial system sorts organisms by a few easily seen features (for example, flowers that are red).
Natural
A natural system groups them by many shared features, and a phylogenetic system goes further and groups them according to their evolutionary history.

Section 2 of 9What phylogeny means

Phylogeny is the evolutionary history of a group of organisms: the order in which lines of descent split from common ancestors.

What phylogeny means
Phylogenetic classification
A phylogenetic classification arranges species into groups, called taxa, that are based on their evolutionary origins and relationships.
Clade
Each group contains an ancestor and all its descendants (a clade), so the members of a group are related by descent and not just by looking alike.

Charles Darwin's idea of descent with modification means that all living things share ancestors if we go far enough back. The more recent the common ancestor of two species, the more closely related they are, and the more features (and the more similar DNA) they are expected to share.

Section 3 of 9Phylogenetic trees

A phylogenetic tree is a branching diagram that shows these relationships.

Branch point (node)
Each branch point (node) represents a common ancestor from which two or more lines of descent split.
Tips
The tips of the branches are the living species (or fossil species) being compared.
Species whose branches split from the same recent node are more closely related than species whose most recent common ancestor lies further back.
In some trees, the length of a branch is drawn in proportion to the amount of change or the time passed.

When reading a tree, trace each pair of species back to the point where their lines join. The pair that joins most recently is the most closely related, whatever the order in which the tips are drawn. The left to right order of the tips is not meaningful, since branches can be swivelled at a node.

Section 4 of 9Evidence used to build classifications

Early taxonomists used appearance and anatomy.

1
Base sequence of DNA
Today the evidence includes the base sequence of DNA, the amino acid sequence of proteins, immunological tests, comparative anatomy, embryology and fossil evidence (see the lesson on evolutionary relationships).
2
Convergent evolution
Molecular evidence is powerful because it can be compared in numbers,
  • and it avoids confusion caused by convergent evolution,
  • where unrelated organisms in a similar environment evolve similar structures (for example, wings in birds and bats,
  • or streamlined bodies in sharks and dolphins).

New evidence can change the classification. For example, comparison of ribosomal RNA led to the placing of prokaryotes into two domains, Bacteria and Archaea, rather than a single group.

Section 5 of 9Advantages of phylogenetic classification

It is based on real evolutionary relationships, so groups are not arbitrary.
Hierarchical
It is hierarchical: groups fit inside larger groups with no overlap (see the next lesson).
It lets scientists predict features. If a chemical is useful in one species, a close relative may contain a similar one.
It can be tested and revised when new data appear, so it is a scientific, testable system.

Section 6 of 9Using phylogeny: worked reasoning

Suppose a tree shows five species.

1
Species 1 and 2 split from a common ancestor X, and species 3 and 4 split from ancestor Y.
2
X and Y share a more distant ancestor Z, and species 5 branched from the lineage before Z.
3
Then 1 and 2 are most closely related to each other; 1 and 3 are less closely related,
  • since their most recent common ancestor is Z;
  • and species 5 is the most distantly related to the other four.
4
No species on the tree is an ancestor of another: all of them are present-day tips.

It helps to remember that phylogenetic classification is tentative. It is a hypothesis about history that is supported by the available evidence and is revised if new sequences or fossils are discovered. This is why the same tree may be redrawn when more genes are compared, and it is an example of how scientific knowledge develops.

In exams, you may be asked to say how the classification of a species changed after DNA analysis, or to explain why a system based on evolutionary relationships is more useful than one based on a single visible feature, such as the colour of the flowers.

Section 7 of 9Exam-style questions

  1. What is meant by a phylogenetic classification? [2]2 marks

  2. A phylogenetic tree shows that species P and Q share a common ancestor 4 million years ago, and that species R joins them at a common ancestor 12 million years ago. State which two species are the most closely related and explain your answer. [2]2 marks

  3. Explain why two species with similar-looking body structures are not necessarily closely related in a phylogenetic classification. [2]2 marks

Section 8 of 9Key terms

TermMeaning
TaxonomyThe science of identifying, naming and classifying organisms.
PhylogenyThe evolutionary history and relationships of a group of organisms.
Taxon (plural taxa)A group of organisms at any level of classification.
CladeA group made up of a common ancestor and all of its descendants.
Phylogenetic treeA branching diagram showing evolutionary relationships between species.
Common ancestorA species from which two or more different species have evolved.
Convergent evolutionThe independent evolution of similar features in unrelated organisms.

Section 9 of 9Exam technique

1
Believing the order of the tips on a tree shows relatedness. Only the branching points (common ancestors) matter.
2
Saying that humans evolved from chimpanzees. The two species share a common ancestor, which was neither.
3
Mixing up phylogeny (evolutionary history) with taxonomy (naming and grouping).
4
Thinking classification only uses visible features. Modern phylogeny relies heavily on DNA and protein evidence.

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