7.9 Phylogeny
- Syllabus
- 2025
- Topic
- 7.9
- Level
- —
Learning objectives
7.9A—Describe the types of evidence that can be used to infer an evolutionary relationshipDescribe the types of evidence that can be used to infer an evolutionary relationship.• Phylogenetic trees and cladograms show hypothetical evolutionary relationships among lineages that can be tested.• Phylogenetic trees show the amount of change over time calibrated by fossils or a molecular clock, whereas cladograms do not show time scale or the evolutionary difference between groups.• T raits that are either gained or lost during evolution can be used to construct phylogenetic trees and cladograms. The out-group represents the lineage that is least closely related to the remainder of the organisms in the phylogenetic tree or cladogram.- i. Shared derived characters can be present in more than one lineage and indicat e common ancestry. These are informative for the construction of phylogenetic trees and cladograms.- ii. Molecular data typically provide more accurat e and reliable evidence than morphological traits in the construction of phylogenetic trees or cladograms.7.9B—Explain how phylogenetic trees and cladograms can be used to infer evolutionary relatednessExplain how phylogenetic trees and cladograms can be used to infer evolutionary relatedness.• Phylogenetic trees and cladograms can be used to illustrate speciation that has occurred. The nodes on a tree represent the most recent common ancestor of any two groups or lineages.• Phylogenetic trees and cladograms can be constructed from morphological similarities of living or fossil species and from DNA and protein sequence similarities.• Phylogenetic trees and cladograms represent hypotheses that are constantly being revised based on evidence.
Evidence Used to Build Phylogenetic Hypotheses
Phylogenetic trees and cladograms are testable hypotheses about evolutionary relationships among lineages. They are constructed by comparing informative character changes and molecular or morphological evidence.
| Evidence or tool | How it informs the hypothesis |
|---|---|
| Shared derived character | A gained or lost trait shared by multiple lineages supports common ancestry |
| Outgroup | The least closely related comparison lineage helps identify which character states are derived |
| Morphological traits | Compare structures in living or fossil organisms |
| DNA or protein sequences | Compare molecular similarity; these data are typically more accurate and reliable than morphology alone |
| Fossils or molecular clock | Calibrate amount of change over time in a phylogenetic tree |
| Model | What it shows | What it does not necessarily show |
|---|---|---|
| Phylogenetic tree | Hypothesized relationships and an amount of change over time when calibrated | Certainty that the hypothesis is final |
| Cladogram | Branching order based on shared derived characters | Time scale or amount of evolutionary difference between groups |
A shared trait is informative only when its evolutionary origin is interpreted correctly. Overall appearance can mislead, which is why molecular evidence and multiple independent characters are valuable.
Reading Relatedness from Branching Patterns
Evolutionary relatedness is inferred from branching ancestry. A node represents the most recent common ancestor of the lineages that descend from that branch point.
Choose two lineages → trace each branch backward → find their first shared node → compare that node with the shared nodes of other pairs. The pair whose common node is more recent is inferred to be more closely related.
If lineages A and B meet at one recent node, while lineage C joins their branch at an earlier node, A and B are more closely related to each other than either is to C. Moving or rotating the tips around a node does not change that ancestry.
Branching hypotheses can be constructed from morphology in living or fossil species and from DNA or protein sequence similarities. The branches can illustrate speciation events that produced distinct lineages.
Tip order, horizontal closeness, or one lineage's position at the top does not determine relatedness. Trees and cladograms are hypotheses, so new evidence can change the preferred branching pattern.