Phylogenetics, Homology, and Tree Thinking
Reconstructing Evolutionary Relationships
Phylogenetic trees
Phylogenetic trees are hypotheses about evolutionary relationships. Branching patterns represent common ancestry, while branch lengths may represent time or amount of change depending on the analysis.
Homology versus analogy
Homologous traits are similar because of common ancestry, whereas analogous traits are similar because of convergent evolution. Correctly distinguishing these is essential for building reliable trees.
Tree vocabulary
Which term describes a group consisting of an ancestor and all of its descendants?
That is the definition of a clade, also called a monophyletic group.
Correct answer: Clade
Why can analogous traits mislead phylogenetic inference?
Tree building requires traits that reflect inheritance, not just appearance.
Correct answer: Because similarity caused by convergent evolution may be mistaken for shared ancestry.
Sources of phylogenetic data
Morphological characters
- Useful for fossils and visible structures
- Can be affected by convergence
DNA and protein sequences
- Provide many characters
- Especially powerful for recent divergences
Parsimony and model-based methods
Some methods prefer the tree requiring the fewest changes, while others use explicit models of sequence evolution. Different methods can yield different trees, so confidence should be assessed with resampling or posterior probabilities.
What is an outgroup used for?
An outgroup helps determine the direction of character change and root the phylogeny.
Correct answer: To define the root of a tree
Reading a phylogenetic tree
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1
Find the root or outgroup if present.
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2
Identify sister taxa and clades.
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3
Distinguish shared ancestry from convergent similarity.
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4
Check whether branch lengths have a time meaning or only a change meaning.