Classification & phylogeny
- 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
Examiner Tip
You may be wondering why you would use amino acids when you could look at DNA or mRNA. This is because it is often easier to find and isolate proteins from cells and as a result protein sequencing was the method traditionally used.In some cases, however, amino acid sequences may be exactly the same between different species even if there are differences in the corresponding DNA sequences. This is because genes for the same protein may have slightly different base sequences in different species due to differences in their introns which are not translated into differences in the protein molecules. In addition, the genetic code is a degenerate code, meaning that more than one codon may code for the same amino acid.As a result, DNA sequencing has largely replaced protein sequencing in taxonomy and the creation of phylogenetic trees.
