Interpreting evolutionary trees and relatedness
Inheritance, variation and evolution • Classification of living organisms
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Key concepts
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Basic structure of an evolutionary tree
An evolutionary tree uses lines and nodes to represent relationships. Lines represent lineages, and nodes represent common ancestors from which two or more lineages diverge. Because a node represents an ancestral population, organisms that descend from the same node share inherited traits from that ancestor. Trees display relatedness by branching order; when two species share a node closer to the tips, they have a more recent common ancestor and are therefore more closely related. Textbook figures show vertebrate and primate trees where branch order and nodes communicate relatedness clearly .
Common ancestor and nodes
A node marks a speciation event or hypothetical ancestor, and a node closer to the tips indicates a more recent shared ancestor. Because shared derived traits originate in common ancestors, the presence of similar derived traits implies closer relatedness. Limitations arise when nodes are unresolved or when the fossil or molecular record is incomplete; unresolved nodes reduce confidence in the exact order of divergence and therefore weaken claims about which species are most closely related.
Branch length and what it can represent
Branch length may represent amount of evolutionary change or time, depending on how the tree is drawn. When branch length equals genetic change, longer branches indicate more accumulated differences; when length equals time, longer branches indicate longer periods since divergence. Misinterpretation occurs if the diagram does not label branch length meaning. Textbook captions often state whether length shows time or genetic difference; readers must check labels before comparing branch lengths.
Reading relatedness versus similarity
Relatedness depends on shared ancestry, not on superficial similarity. Convergent evolution causes unrelated lineages to evolve similar traits for similar functions, which can mislead interpretations based solely on appearance. Because molecular data reduce errors from convergence, trees based on DNA or RNA sequences often reorder relationships compared with morphology-based trees; educational resources highlight trees based on DNA similarities to show this effect .
Evidence and limitations in constructing trees
Fossils, anatomical traits and molecular sequences provide evidence for tree construction. Because DNA data provide many characters, molecular trees often increase resolution and reveal unexpected relationships. Examples in classification chapters show evolutionary trees drawn from DNA similarities and encourage extraction of information from diagrams . Limiting factors include incomplete fossil records, horizontal gene transfer in microorganisms, and sampling bias; these factors create uncertainty in inferred trees and require careful statement of assumptions and confidence.
Key notes
Important points to keep in mind