Molecular Evidence

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Key Knowledge:
  • Evidence of relatedness between species: molecular homology – DNA and amino acid sequences


All organisms use DNA and RNA as genetic material and the genetic code by which proteins are synthesised is (almost) universal

  • This shared molecular heritage means that base and amino acid sequences can be compared to ascertain levels of relatedness


Molecular Homology

Over the course of millions of years, mutations will accumulate within any given segment of DNA

  • The number of differences between comparable base sequences demonstrates the degree of evolutionary divergence
  • A greater number of differences between comparable base sequences suggests more time has past since two species diverged
  • Hence, the more similar the base sequences of two species are, the more closely related the two species are expected to be


When comparing molecular sequences, scientists may use non-coding DNA, gene sequences or amino acid sequences

  • Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences
  • Gene sequences mutate at a slower rate, as changes to base sequence may potentially affect protein structure and function
  • Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy


Amino acid sequences are typically used to compare distantly related organisms (i.e. different higher order taxa), while DNA or RNA base sequences are often used to compare more closely related organisms (e.g. different lower order taxa – such as species)


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Mitochondrial DNA

Mitochondrial DNA (mtDNA) is an important tool for tracing evolutionary relationships within a species

Mitochondrial DNA offers several benefits over nuclear DNA when determining evolutionary pathways, including:

  • Maternal inheritance – mtDNA is inherited from the mother only, providing a much more direct genetic lineage
  • No recombination – As mtDNA is passed from the mother, no recombination occurs, maintaining sequence fidelity
  • Stable mutation rate – Mitochondria produce reactive oxygen species, which cause sequences to mutate at a higher rate
  • High copy number – As every cell has mitochondria, large amounts of mtDNA can be gathered for sequencing


Mitochondrial DNA is ideal for comparing organisms within a species or those who have diverged recently (< 20 million years)

  • Evidence from mtDNA has been used to divide the human population into distinct haplogroups
  • Humans can trace their origins back to one of seven super-haplogroups originating from an original 'Mitochondrial Eve’


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