Classification Processes – Organisms

Classification by Molecular Sequences

Organisms can also be classified based on their molecular sequences (molecular phylogeny). This is the branch of phylogeny that analyses genetic, hereditary molecular differences, predominately in DNA sequences, to gain information on an organism’s evolutionary relationships. This is also called cladistics.

  • All living organisms on Earth are thought to have one common ancestor.
  • If two species have a similar set of proteins, chromosomes or DNA sequences, it provides evidence that they share a recent common ancestor.
  • Please note that ‘recent’ in evolutionary terms can be hundreds of thousands of years!
  • We can compare DNA sequences of different species and look at the similarities between the bases.
  • The greater the difference in DNA sequences, more time is presumed to have passed since they shared a common ancestor.

A molecular clock uses constant rates of evolution in some genes and conserved sequences to estimate the absolute time of evolutionary change. The number of changes is assumed to be proportional  to the time since they last shared a common ancestor. DNA, amino acids and mtDNA can all be used.

Mitochondrial DNA (mtDNA)

Remember, mitochondria contain their own DNA. This also mutates just like nuclear DNA. However, it cannot repair itself like nuclear DNA, it will have a higher rate of mutation. Nevertheless, an advantage is that it is very easy to obtain.

What is mitochondrial DNA (mtDNA) and how has it been used to provide evidence for evolutionary relationships between species?

Mitochondrial DNA (mtDNA) is DNA located in the mitochondria of cells. mtDNA mutates more often than nuclear DNA, and the amount of mutation of mtDNA is roughly proportional to the amount of time that has passed. Similarities and differences in the mtDNA of species can thus show evolutionary relationships.

Amino acid sequences

Amino acids form proteins, and these mutate at different rates. As species diverge, there will be differences in the amino acid sequence due to an accumulation of mutations.

Describe how the amino acids can be used to determine the degree of similarity between species.

By comparing the specific type and sequence of amino acids in similar proteins from different species, the degree of similarity can be established. The greater the similarity between the amino acid sequences of two organisms, the closer their relationship. Conversely, the greater the differences, the more distant the relationship.

One protein that is commonly studied in attempting to determine the relatedness of species is cytochrome-c. This is a protein that is used in the electron transport chain of cellular respiration. It has changed very little over millions of years of evolution so the more similarity there is between the cytochrome-c from different species, the more recently the species have evolved from a common ancestor. The table below shows the molecular homology of cytochrome-c between different species.

HumanGlnProTyrSerThrAlaLysAsnLysIleGlyGluAspThrLeuMetGluLysAlaThrAsnGlu
ChickenGlnGluPheSerThrAspLysAsnLysThrGlyGluAspThrLeuMetGluLysAlaThrSerLys
HorseGlnPropheThrThrAlaLysAsnLysThrLysGluGluThrLeuMetGluLysAlaThrAsnGlu
FrogGlnAlaPheSerThrAspLysAsnLysThrGlyGlyAspThrLeuMetGluSerAlaCysSerLys
SharkGlnGlnPheSerThrAspLysSerLysThrGlnGlnGluThrLeuArgIleLysThrAlaAlaSer
MonkeyGlnProTyrSerThrAlaLysAsnLysThrGlyGluAspThrLeuMetGluLysAlaThrAsnGlu
RabbitGlnValPheSerThrAspLysAsnLysThrGlyGluAspThrLeuMetGluLysAlaThrAsnTh

Using the table, organise the species from most closely to least related to humans.

Monkey (1) – Rabbit (4) – Horse (5) – Chicken (6) – Frog (8) – Shark (14). (Number of amino acid differences shown in brackets)