Phylogeny Trees

ninja icon
Key Knowledge:
  • The use and interpretation of phylogenetic trees as evidence for the relatedness between species


Cladistics is a method of classifying organisms into groups of species called clades (from Greek ‘klados' = branch)

  • Each clade consists of an ancestral organism and all of its evolutionary descendants
  • Members of a clade will possess common characteristics as a result of their shared evolutionary lineage


Cladograms

Clades can be organised according to branching diagrams (cladograms) in order to show evolutionary relationships

  • Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution


Cladograms show the probable sequence of divergence and hence demonstrate the likely evolutionary history (phylogeny) of a clade

  • The fewer the number of nodes between two groups the more closely related they are expected to be


phylogeny


Molecular Clock

Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years)

If this rate of change is stable and reliable, scientists can calculate the time of divergence according to the number of differences

  • E.g.  If a gene which mutates at a rate of 1 bp per 100,000 years has 6 bp different, divergence occurred 600,000 years ago


This concept is called the molecular clock and is limited by a number of factors:

  • Different genes or proteins may change at different rates (e.g. haemoglobin mutates more rapidly than cytochrome c)
  • The rate of change for a particular gene may differ between different groups of organisms
  • Over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions


Phylograms

Phylograms are tree diagrams where the branch lengths may differ according to the length of time since speciation

  • In cladograms, the length of each branch is the same – the branches do not infer an evolutionary time scale
  • Unlike cladograms, phylograms do infer the amount of evolutionary time separating two species


Phylogram construction requires comparison of sequences that have a constant rate of mutation (i.e. ‘molecular clocks’)

  • Mitochondrial DNA is a useful source as it is maternally derived, has a known mutation rate and lacks recombination 


phylogram