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How do biologists show how species are related, and how do you read a phylogenetic tree?

Explain how common ancestry is represented by phylogenetic trees and cladograms, and interpret these diagrams using shared characteristics and molecular data to infer relationships (MA STE HS-LS4-1, patterns).

A standard-level answer on common ancestry and phylogeny for the Massachusetts High School Biology MCAS: how phylogenetic trees and cladograms represent evolutionary relationships, and how to read them using shared characteristics and molecular data under HS-LS4.

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  1. What this topic is asking
  2. What a phylogenetic tree shows
  3. How to read a tree
  4. How trees are built
  5. Try this

What this topic is asking

The Massachusetts STE framework (HS-LS4-1) asks you to use evidence to show common ancestry, and a key way biologists represent that is with phylogenetic trees and cladograms. On the High School Biology MCAS, this is almost always tested by giving you a tree or cladogram and asking which species are most closely related, which diverged earliest, or how a shared characteristic groups species. The crosscutting concept is patterns: relationships are read from the branching pattern of the diagram.

What a phylogenetic tree shows

The whole idea rests on common ancestry: today's species descended from shared ancestors, and a tree maps that descent. Reading a tree is like reading a family tree, except the branch points are ancestral species rather than grandparents. Because all life shares a common origin, every species fits somewhere on one great tree of life, an idea supported by the evidence for evolution.

How to read a tree

The MCAS asks the same kinds of questions, so learn the reading rules:

  • Branch points are common ancestors. Where two lines meet as you trace back, they share a common ancestor at that point.
  • More recent shared ancestor means more closely related. Two species that meet at a branch point near the tips are closely related; they diverged recently.
  • Earlier branching means more distantly related. A species that branches off near the base of the tree diverged a long time ago and is the most distantly related to the others.
  • Order along the tips is not the relationship. What matters is the branching pattern (which species share branch points), not how close two species happen to sit on the page.

So to find the most closely related pair, look for the two species that share the most recent common ancestor; to find the most distantly related, look for the species that branches off earliest.

How trees are built

Trees are not guesses; they are built from evidence:

  • Shared characteristics. Species that share features (especially homologous structures) are grouped together, because shared features suggest a common ancestor. The more characteristics two species share, the more closely related they tend to be.
  • Molecular data. DNA and protein comparisons give quantitative evidence: fewer molecular differences mean a more recent common ancestor (see evidence for evolution). Molecular data has become the main tool for building modern trees because it is objective and detailed.

When the diagram is a cladogram, the groupings are based directly on shared derived characteristics, and each branch marks the appearance of a new shared feature.

Try this

Q1. State what a branch point on a phylogenetic tree represents. [1]

  • Cue. A common ancestor from which the branching lines diverged.

Q2. Explain how you decide which two species on a tree are most closely related. [2]

  • Cue. The two species that share the most recent common ancestor (the branch point closest to the tips) are the most closely related.

Exam-style practice questions

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

HS Biology MCAS (style)3 marksA phylogenetic tree shows four species (W, X, Y, Z). W and X share a recent common ancestor, while Z branches off near the base of the tree. (a) Which two species are most closely related? (b) Which species is most distantly related to the others? (c) Explain how the tree shows relationships.
Show worked answer →

A 3-point item on patterns with the practice of analyzing models.

(a) 1 point: W and X (they share the most recent common ancestor).
(b) 1 point: Z (it branches off earliest, so it diverged longest ago).
(c) 1 point: each branch point represents a common ancestor; species that share a more recent branch point are more closely related, while species whose shared ancestor is further back are more distantly related. Markers reward reading branch points as common ancestors.

HS Biology MCAS (style)2 marksA cladogram is built using shared characteristics. Explain how a shared characteristic is used to group species, and what a shared characteristic suggests about ancestry.
Show worked answer →

A 2-point item on patterns.

1 point: species that share a characteristic are grouped together (branch from a common point), because the characteristic is used as evidence of relationship.
1 point: a shared characteristic suggests the species inherited it from a common ancestor, so the more characteristics two species share, the more closely related they tend to be. Markers reward linking shared characteristics to common ancestry.

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