Classification & Phylogeny (OCR A Level Biology)

• In the past scientists classified organisms on the basis of shared visible features
• Today scientists aim to classify organisms on the basis of phylogeny
• Phylogeny can be defined as:

The evolutionary history of organisms

• Classifying organisms according to their phylogeny means that species that share a more recent common ancestor are classified together, while species with a more distant common ancestor are classified in separate groups
• Phylogenetic classification often means that historical organism groups need to be changed
  • E.g. grouping organisms on the basis of shared characteristics may result in birds and bats being classified together, but we know that these two organisms are not close evolutionary relatives
• Advances in DNA, RNA and protein sequencing have allowed scientists to classify organisms according to their phylogeny more accurately than using visible characteristics
• Molecular analysis allows scientists to build phylogenetic tree diagrams that show the relationships between organisms

Using molecular evidence in classification

• Three types of sequence data are used to investigate evolutionary relationships
  • DNA
  • mRNA
  • Amino acids (of a protein)
• Sequencing technology can determine the order of DNA bases, mRNA bases and amino acids within an organism's genome
• This technology is especially useful for comparison with an extinct species (using ancient DNA) or when distinguishing between species that are very physically similar
• Scientists will choose specific proteins or sections of the genome for comparison between organisms
  • Looking at multiple proteins or multiple regions of the genome will allow for a more accurate estimate of evolutionary relatedness
  • Note the protein used needs to be present in a wide range of organisms and show sufficient variation between species
  • Cytochrome c is often used as it is an integral protein to respiration (in the electron transport chain) which is used by all eukaryotic organisms
• For all types of sequence data it can be said that the more similar the sequences, the more closely related the species are
• Two groups of organisms with very similar sequences will have separated into separate species more recently than two groups with less similarity in their sequences
• Species that have been separated for longer have had a greater amount of time to accumulate mutations and changes to their DNA,mRNA and amino acid sequences
• Sequence analysis and comparison can be used to create phylogenetic trees that show the evolutionary relationships between species

Example of a phylogenetic tree showing the relationship between primate species. The tree is based on the DNA sequence of the gene that codes for cytochrome c.

DNA Analysis and Comparison

• DNA is extracted from the nuclei of cells taken from an organism
  • DNA can be extracted from blood or skin samples from living organisms or from fossils

• The extracted DNA is processed, analysed and the base sequence is obtained
• The base sequence is compared to that of other organisms to determine evolutionary relationships
  • The more similarities there are in the DNA base sequence, the more closely related (in that the less distant the species separation) members of different species are

• In 2005, the chimpanzee genome was sequenced, and when compared to the human genome it was discovered that humans and chimpanzees share almost 99% of their DNA sequences, making them our closest living relatives
  • In 2012, the sequencing of the bonobo genome also revealed that humans and bonobos share 98% of their genome (with slight differences to the differences seen in chimpanzees)

The DNA base sequences of two closely related species being compared - Species Y is the ancestor of Species X