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•  Analysis of cladograms to deduce evolutionary relationships

Constructed cladograms all typically share certain key features:

  • Root – The initial ancestor common to all organisms within the cladogram (incoming line shows it originates from a larger clade)
  • Nodes – Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter taxa
  • Outgroup – The most distantly related species in the cladogram which functions as a point of comparison and reference group
  • Clades – A common ancestor and all of its descendants (i.e. a node and all of its connected branches)

Key Features of a Cladogram

cladogram features

Constructing Cladograms

Cladograms can be constructed based on either a comparison of morphological (structural) features or molecular evidence

  • Historically, structural features were used to construct cladograms, but molecular evidence is now more commonly used

1.  Using Structural Evidence

Step 1:  Organise selected organisms according to defined characteristics

  • Use characteristics that are developmentally fixed (i.e. innate) and not influenced by environmental pressures
cladogram table

Step 2:  Sequentially order organisms according to shared characteristics to construct a cladogram

  • Grouping of organisms may be facilitated by constructing a Venn diagram prior to developing a cladogram
  • Each characteristic will be represented by a node, with more common characteristics representing earlier nodes
  • The species with the least number of characteristics in common will represent the outgroup (establishes baseline properties)

cladogram construction

2.  Using Molecular Evidence

Step 1:  Select a gene or protein common to a range of selected organisms

  • Examples of molecules which are ubiquitously found in many animals include haemoglobin and cytochrome c

Step 2:  Copy the molecular sequence (DNA or amino acid) for each of the selected organisms

  • Use online databases such as Genbank or Ensembl to identify relevant DNA or amino acid sequences
  • Sequences can be collated in a Word document and then saved as a document in plain text format (.txt) 
  • Before each sequence, designate a species name preceded by a forward arrow (e.g. '>Human’ or ‘>Chimpanzee’)

Step 3:  Run a multiple alignment to compare molecular sequences (DNA or amino acid)

  • Multiple alignment software compares DNA or protein sequences for similarities and differences
  • Closely related species are expected to have a higher degree of similarity in their molecular sequence
  • Clustal Omega is a free online tool that will align multiple DNA or amino acid sequences for comparison

Step 4:  Generate a phylogeny tree (cladogram) from multiple alignment data

  • Clustal Omega can generate branched phylograms after a sequence alignment is completed (select ‘Phylogenetic Tree’)
  • Below is a plain text file that can be uploaded to compare amino acid sequences from different species:
    • HBA  –  Haemoglobin alpha chain (amino acid sequence) from various species

Multiple Alignment of a Protein Sequence from Various Species

multiple alignment