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Phylogeny (AP Biology Topic 7.9)

  • Study of evolutionary relationships among species
  • Phylogenetic trees and cladograms
  • Common ancestry and shared traits
  • Evidence from DNA, proteins, and morphology

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What is Phylogeny?

Phylogeny: the study of evolutionary relationships between organisms

  • Shows how species descend from common ancestors
  • Often represented using tree-like diagrams
  • Helps scientists understand evolutionary history

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Phylogenetic Trees

Diagram showing evolutionary relationships

  • Based on scientific evidence
  • Represents hypotheses about ancestry
  • Branch points represent common ancestors
  • Closer branch points = more closely related species

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Reading a Phylogenetic Tree

Tips for interpretation:

  • Each branch point (node) represents a common ancestor
  • Species sharing a recent node are closely related
  • Moving left generally represents going back in time
  • The base represents the oldest common ancestor

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Example: Dogs and Wolves

  • Domestic dogs share a recent common ancestor with gray wolves
  • Dogs and wolves are more closely related than dogs and foxes
  • Evidence comes from DNA comparisons and evolutionary history

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  • Branch points on a tree represent common ancestors (where speciation has occurred)

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what does each node show?

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Outgroup

Outgroup: the lineage least closely related to others in the tree

  • Used as a reference point when building phylogenetic trees
  • Helps scientists determine evolutionary direction

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Evidence Used to Build Trees

1. Morphological evidence (body structures)

2. Molecular evidence (DNA and protein sequences)

3. Fossil record

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**Molecular evidence is often the most reliable

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Phylogenetic trees are hypotheses and will be updated if new evidence suggests changes

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Phylogenetic trees show the amount of change that occurred over time, farther to the left represents longer time ago

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Cladograms

A type of phylogenetic diagram

  • Shows relationships based on shared derived characteristics
  • Does NOT represent time
  • Organisms are grouped by traits they share

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Shared Derived Characteristics

Derived Characteristics are traits that appear in more recent ancestors

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Example traits:

  • Vertebrae
  • Bony skeleton
  • Four limbs
  • Amniotic egg
  • Organisms sharing more

traits are more closely related

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Key Vocabulary

Phylogeny- study of evolutionary relationships

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Phylogenetic tree- diagram of evolutionary history

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Cladogram- diagram based on shared traits

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Node- branch point representing a common ancestor

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Outgroup- least related lineage

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Recap

Phylogeny is the study of evolutionary relationships; phylogenetic trees represent revisable hypotheses about evolutionary relationships

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Trees are built upon anatomical/morphological similarities, but mostly on DNA/protein sequence homology

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Species with recent common ancestors are closely related. Phylogenetic trees show estimated time of divergence, cladograms do not

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Cladograms use shared characteristics to show relationships

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Trees can change when new evidence is discovered