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How do cladograms and phylogenetic trees show evolutionary relationships?

Use models such as cladograms and phylogenetic trees to illustrate and interpret evolutionary relationships among organisms (North Carolina Standard Course of Study, Biology, LS.Bio.10).

A standard-level answer on phylogeny for the North Carolina Biology EOC: how to read a cladogram or phylogenetic tree, what branch points and shared traits mean, and how molecular data builds these trees.

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  1. What this topic is asking
  2. What a cladogram shows
  3. Reading relatedness
  4. Shared traits and building trees
  5. Try this

What this topic is asking

North Carolina LS.Bio.10 asks you to use cladograms and phylogenetic trees to illustrate and interpret evolutionary relationships. For the Biology EOC you need to read a tree: identify common ancestors (branch points), tell which species are more closely related, and understand that shared traits at branch points mark where features arose. You also need to know that molecular data builds these trees. Items are usually diagram-based.

What a cladogram shows

Reading a tree is about following the branches. Where two lines meet at a branch point (a node), that point represents the common ancestor they share. The pattern of branching tells you who is related to whom and how recently. Crucially, a cladogram is a model of descent from common ancestors, not a ladder from "simple" to "advanced"; organisms at the tips are all equally modern.

Reading relatedness

This is the single most common cladogram item: given a tree, decide which two organisms are most closely related (answer: the pair sharing the most recent branch point).

Shared traits and building trees

Cladograms also show where traits arose. A trait marked at a branch point is a shared derived characteristic: a feature that appeared in the common ancestor at that node and is therefore present in all the organisms that branch off after it. For example, if "backbone" is placed at a node, every organism above that node has a backbone, and the trait helps define that group.

Today these trees are built mainly from molecular data: scientists compare the DNA and protein sequences of organisms, and the more similar the sequences, the more closely related the organisms (and the more recent their common ancestor). Anatomical features are also used. This connects the topic to the molecular evidence for evolution and to the genetic code from the molecular genetics module.

Try this

Q1. On a cladogram, how do you tell which two species are most closely related? [2]

  • Cue. They share the most recent (closest) common ancestor, the closest branch point on the tree.

Q2. State what scientists mainly use to build phylogenetic trees today. [1]

  • Cue. Molecular data: comparisons of DNA and protein sequences (supported by anatomy).

Exam-style practice questions

Practice questions written in the style of NCDPI exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

NC Biology EOC (style)1 marksOn a cladogram, two species that branch from the most recent common ancestor are: (A) the least related on the diagram. (B) the most closely related shown. (C) always the same species. (D) unrelated.
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A 1-point item on reading a cladogram.

The correct answer is B. Two species sharing the most recent (closest) common branch point are the most closely related shown, because they shared an ancestor most recently. The further back you must go to find a shared branch point, the more distant the relationship.

Closer (more recent) branch point equals more closely related.

NC Biology EOC (style)2 marksA cladogram places a new trait (such as having a backbone) at a branch point. (a) Explain what placing a trait at a branch point means. (b) State what scientists use to build these trees today.
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A 2-point item on interpreting and building cladograms.

(a) 1 point: a trait placed at a branch point is a shared derived characteristic that appeared in the common ancestor at that point and is found in all the organisms branching off after it.
(b) 1 point: molecular data (DNA and protein sequence comparisons), along with anatomical features, are used to build the trees.

Markers reward explaining a branch-point trait as shared by all later branches and naming molecular data.

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