Phylogenetic trees can be constructed by using the parsimony principle to find the simplest explanation for phylogenetic data. Maximum likelihood methods incorporate more explicit mathematical models of evolutionary change to reconstruct evolutionary history.
learning outcomes
You should be able to:
Analyze a phylogenetic tree to identify synapomorphies, homoplasies, and relationships among taxa.
Reconstruct a phylogenetic tree from a data matrix of characters.
The matrix below supplies data for seven land plants and an outgroup: an aquatic plant known as a stonewort. Each trait is scored as either present (+) or absent (–) in each of the plants. Use this data matrix to reconstruct the phylogeny of land plants and answer the questions that follow.
| Trait | ||||||||
|---|---|---|---|---|---|---|---|---|
| Taxon | Protected embryos | True roots | Persistently green sporophyte | Vascular cells | Stomata | Megaphylls (true leaves) |
Seeds | |
| Stonewort (outgroup) | – | – | – | – | – | – | – | |
| Liverwort | + | – | – | – | – | – | – | |
| Pine tree | + | + | + | + | + | + | + | |
| Bracken fern | + | + | + | + | + | + | – | |
| Club moss | + | + | + | + | + | – | – | |
| Sphagnum moss | + | – | – | – | + | – | – | |
| Hornwort | + | – | + | – | + | – | – | |
| Sunflower | + | + | + | + | + | + | + | |
Which two of these taxa are most closely related?
The pine tree and the sunflower.
Plants that produce seeds are known as seed plants. What is the sister group to the seed plants among these taxa?
The fern.
Which two traits evolved along the same branch of your reconstructed phylogeny?
True roots and vascular cells.
Are there any homoplasies in your reconstructed phylogeny?
No, because each trait can be placed along a single branch in the tree.
Why do biologists expend the time and effort necessary to reconstruct phylogenies? Information about the evolutionary relationships among organisms is a useful source of data for scientists investigating a wide variety of biological questions. Next we will describe how phylogenetic trees are used to answer questions about the past, and to predict and compare traits of organisms in the present.