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Wize High School Grade 11 Biology Textbook > Biodiversity & Ecosystem Health

Classifying Organisms: Cladistics

Classifying Organisms: TaxonomyPrevious Section
Ecosystems in the BiosphereNext Section
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Phylogenetic Trees

Phylogenetic trees are the best way to present how all life forms on earth are related to each other.

Phylogenetic Trees: branching diagrams that show the evolutionary history of organisms
  • Phylogeny: evolutionary history of a species or group
  • vs. Taxonomy: groups organisms based on similarities
  • Show evolutionary relationships between taxa
  • Represent hypotheses about evolutionary history
  • Build trees using:
  • Morphology
  • Behavior
  • DNA
  • Biochemistry

Watch Out!
More recently evolved does not mean more advanced!



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  • Taxon (Taxa): the taxonomic unit at any level Example Animalia is the Kingdom taxon
  • Sister taxa: Two or more taxa next to each other on the tree (i.e. closely related) Example: Taxon B and Taxon C are sister taxa
  • Clade: A group of taxa that share a common ancestor Example: Taxon A, B, C form a clade
  • In group: a group of interest usually the focus of a study
  • Out group: one or more taxa that is distantly related to the In group
  • Character: a specific characteristic Example: a wing or a tail
  • State: a particular property of a character Example: the shape of the wing, or the color of the tail

Wize Tip
You can rotate any part of the tree and it doesn't change any relationships

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Similar Traits Within & Between Phylogenetic Groups


  • Analogous traits: appear similar but actually evolved independently
  • Convergent evolution: natural selection produces a similar trait from different starting points Example: bat and bird wings
  • Homologous traits: similar due to true shared ancestry
  • Descended from the same trait in a common ancestor Example: dolphin and whale flippers



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Dichotomous Keys

Dichotomous Keys are a two-part system used to classify living things.
  • The system requires there to ONLY be 2 choices at each node to take you to the next branch
  • Each branch includes fewer and fewer organisms until each organism has been classified

Wize Tip
Dichotomous can be broken down into "Di" = two questions and "key" = system.

Dichotomous Key = Two Question System


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Below is an example of using a dichotomous key to classify different species of tree leaves.

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Example: Using Dichotomous Keys

Create a Dichotomous Key to accurately classify each of the big cats in the photo bank.


See video for solution












Practice: Phylogenetic Trees

Which of the following could NOT be used to create a phylogenetic tree?
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Example: Using Trait Tables to Build Phylogenetic Trees

Fill in the following cladistics table with an X for a trait the organism has and leave blank if the organism does not have that trait. Then, using the filled in cladistics table build a phylogenetic tree.


Build the tree








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Example: Phylogenetic Trees

Which of the following phylogenetic trees is more likely correct? Use the following traits to help you: Wings, Fur, Gills, Live Births (Amnion)


Traits to use: Fur, Wings, amnion, and gills



Tree on the left has 5 steps, and tree on the right has 6. According to the parsimony principle
the tree on the left is simpler than the right so is likely correct.


Practice: Using Trait Tables to Build Phylogenetic Trees

Which of the following phylogenies best represents the below trait table:

Practice: Cladistics

Which of the following statements are TRUE regarding the relationship between species I, II and III given the provided table?