AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/7/7-9)
Unit 7 · Topic 7.9
7.9 Phylogeny
Cladograms and phylogenetic trees are diagrams of hypothesized evolutionary relationships. They're built from shared derived characters and, more reliably, from DNA and protein sequences; each branch point (node) is a common ancestor, the outgroup is the least related lineage, and only phylogenetic trees show time or amount of change.
Key terms
- cladogram
- phylogenetic tree
- node
- shared derived character
- outgroup
- molecular clock
Trees as testable hypotheses
A phylogeny is the evolutionary history of a group. Scientists represent it with branching diagrams. Each one is a hypothesis about how groups are related, and it's revised when new evidence comes in. Trees also illustrate speciation: every split in a tree is a point where one lineage became two.
Two kinds of diagrams:
- A cladogram shows only the branching pattern: who shares a more recent common ancestor with whom. Branch lengths mean nothing.
- A phylogenetic tree also shows the amount of change or time, with branch lengths calibrated by fossils or a molecular clock.
Reading a tree
The tips (ends of branches) are present-day or extinct groups. A node, where branches split, represents the most recent common ancestor of everything beyond it. To judge how closely two groups are related, trace back from each tip to the node where their lineages meet. The more recent that shared node, the more closely related they are. Don't judge by how close tips are on the page.
Branches can rotate around a node without changing the tree, like a hanging mobile. Two trees that look different can show identical relationships. A clade is an ancestor and all of its descendants, which you could cut off from the tree with a single snip.
The outgroup is the lineage least closely related to the rest of the organisms in the tree. It branches off first and gives a reference point for deciding which traits are ancestral and which are new.
Building a tree from traits
Traits gained or lost during evolution are used to group organisms. A shared derived character is a trait that evolved in a common ancestor and is shared by its descendants but not by more distant groups. Hair is a shared derived character of mammals. Traits that all the groups share, such as a backbone among vertebrates, don't help sort the groups.
Place traits on the tree in the order they evolved: a trait shared by more groups evolved earlier and sits closer to the root. A good tree usually needs the fewest total trait changes.
Molecular data and the molecular clock
DNA and protein sequences provide far more characters than anatomy and are less likely to be misled by convergent evolution, so molecular data are generally more accurate and reliable for building trees. The more sequence differences between two species, the longer ago they likely shared an ancestor.
Some genes accumulate changes at a fairly steady rate. A molecular clock uses that rate, calibrated with fossils of known age, to estimate when lineages split. The clock is an estimate; rates can vary between genes and lineages.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Building a cladogram from a trait table
Five species are scored for four traits (1 = present, 0 = absent). Species P: 0, 0, 0, 0. Species Q: 1, 0, 0, 0. Species R: 1, 1, 0, 0. Species S: 1, 1, 1, 0. Species T: 1, 1, 1, 1. Describe the cladogram and identify the outgroup.
Show the solutionHide the solution
- Step 1: Outgroup: P has none of the derived traits, so it branches off first.
- Step 2: Trait 1 is shared by Q, R, S and T, so it evolved in their common ancestor, on the branch just after P splits off.
- Step 3: Trait 2 is shared by R, S and T, so it evolved after Q branched off.
- Step 4: Trait 3 is shared by S and T, so it evolved after R branched off. Trait 4 is only in T, so it evolved on T's own branch after S and T split.
- Step 5: Order of branching: P, then Q, then R, then S and T as the most closely related pair.
Answer: A ladder-shaped cladogram: P (outgroup) branches first, then Q, then R, with S and T as sister species. Traits 1, 2 and 3 mark successive nodes, and trait 4 is unique to T.
- Example 2
Molecular clock estimate
Two related beetle species differ at 24 positions in a gene. Fossil calibration shows that differences between two lineages in this gene accumulate at about 2 per million years. Estimate when the species shared a common ancestor.
Show the solutionHide the solution
- Step 1: The rate given is for differences between two lineages, so no extra adjustment is needed.
- Step 2: Time = number of differences ÷ rate = 24 ÷ 2 per million years = 12 million years.
- Step 3: This is an estimate; mutation rates can vary, so scientists cross-check with fossils and other genes.
Answer: About 12 million years ago.
Common mistakes
- Reading relatedness from how close the tips are on the page, or from left-to-right order. Trace back to the most recent shared node.
- Thinking one living species on a tree is the ancestor of another. Living tips are relatives that share ancestors, not ancestors of each other.
- Assuming branch lengths on a cladogram show time. Only phylogenetic trees are scaled to time or amount of change.
- Using a trait shared by every group in the tree to group organisms. Only shared derived characters sort groups.
On the exam
- Expect to construct or complete a cladogram from a trait or sequence table and to justify the placement of one group using a shared derived character.
- Be ready to explain why a tree might be revised: new fossils or molecular data can support different relationships.
Connected topics
Videos
Check yourself
4 questions on 7.9 Phylogeny. Pick an answer to see if you got it, and why.
| Animal | Vertebral column | Hinged jaws | Four limbs | Amniotic egg | Hair |
|---|---|---|---|---|---|
| Lancelet | No | No | No | No | No |
| Shark | Yes | Yes | No | No | No |
| Frog | Yes | Yes | Yes | No | No |
| Lizard | Yes | Yes | Yes | Yes | No |
| Mouse | Yes | Yes | Yes | Yes | Yes |
Character data for five animals
Which animal is the best choice for the outgroup in a cladogram of these five animals?
Based on the characters in the table, which pair of animals shares the most recent common ancestor?
In a cladogram of three living groups, the lizard lineage branches off first. The remaining lineage then splits into a crocodile branch and a bird branch. Which statement is supported by this cladogram?
A phylogenetic tree has branch lengths scaled to time using fossil dates, while a cladogram of the same species shows only the branching order. Which question can be answered with the phylogenetic tree but not with the cladogram?
0 of 4 answered