Notes: Cladistics Puzzle

A Summary of clasdistics studing materials from the lecture - Dinosaur & History of Life.

Posted by Jingyi on September 27, 2021

Before we start…

General Terminology

Systematics: Study and reconstruction of evolutionary relationships

Taxonomy: Theory and practice of describing, naming, and ordering (biological classification) groups of organisms termed taxa (singular taxon)

Phylogeny: Evolutionary tree that represents a hypothesis about patterns of taxa relationships

Linnean Hierarchy

from Google

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Mammalia
  • Order: Carnivora
  • Family: Canidae
  • Genus: Canis
  • Species: Canis lupus

from Google


A method for determining phylogenetic trees – which reflect evolutionary history – based on their shared derived characters.

The relationships are represented graphically in a cladogram.

Cladistic terminology


  • An observable feature or trait of an organism (e.g. number of toes on a hind limb, presence of a plantigrade foot, etc.)
  • An observed difference from which we infer an evolutionary event

Character state

  • Variation of the trait, feature, etc., e.g. five toes in humans, one in horses
  • There needs to be at least two of these states


  • A taxon that is “outside” or less related to the group of taxa that you are studying (i.e. the ingroup).
  • Outgroup characters are considered the most “primitive” character state to which ingroup characters are compared

Character Polarization

  • Derived & primitive state by outgroup comparison


A branching diagram that depicts a hypothesis of evolutionary relationships between groups of organisms (taxa) is called a Cladogram.

Systematics - The goal of systematics is to have classification reflect the evolutionary relationships of species. Cladistics classifies organisms according to the order in time that branches arise along a phylogenetic tree, without considering the degree of divergence (how much difference)

from Google

Maximum parsimony

  • Method for determining preferred phylogenetic tree
  • Principle that the simplest solution is the most likely
  • Most parsimonious phylogenetic tree is one that requires the least evolutionary change to explain some observed character state
  • In practice this means the tree with the fewest character changes is preferred

General Methodology

Make hypothesis of evolutionary relationship

  • Identify characters
  • Score characters
  • Identify ancestral & derived characters (based on character state of outgroup)
  • Produce cladogram


  • Examine character polarity
  • Compare with alternative hypotheses