OCR AS Biology

Revision Notes

4.3.5 Classification & Phylogeny

Classification & Phylogeny

  • Traditional biological classification systems grouped organisms based on the features that they shared
    • This is known as homology (features are homologous if they are shared by organisms that have evolved from a common ancestor)
  • In the past, scientists have encountered many difficulties when trying to determine the evolutionary relationships of species based on this method
    • Using the physical features of species (such as colour/shape/size) has many limitations and can often lead to the wrong classification of species
  • Advances in DNA, RNA and protein sequencing, as well as immunology, has allowed scientists to further investigate the evolutionary relationships between species
  • This has allowed us to understand the true phylogeny of taxa and to correctly group them together and show how they are evolutionarily related to one another
    • Phylogeny is the term used to describe the evolutionary history of organisms
    • Phylogenetic trees are diagrams that show the evolutionary relationships between different taxa

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

Primate species tree, downloadable AS & A Level Biology revision notes

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)

Comparisons of DNA sequences, downloadable AS & A Level Biology revision notes

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

Immunology: using antibodies in classification

  • The proteins of organisms can also be compared using immunological techniques
  • The protein albumin is found in many species and is commonly used for these experiments
  • Method:
    • Pure albumin samples are extracted from blood samples taken from multiple species
    • Each pure albumin sample is injected into a different rabbit
    • Each rabbit produces antibodies for that specific type of albumen
    • The different antibodies are extracted from the different rabbits and are then mixed with the different albumin samples
    • The precipitate (antibody-antigen complexes) resulting from each mixed sample is weighed
  • Results and Interpretation:
    • The greater the weight of the precipitate, the greater the degree of complementarity between the antibody and albumin
    • For example, antibodies produced against human albumin will produce a larger amount of precipitate when exposed to chimpanzee albumin than when exposed to rat albumin because humans are more closely related to chimpanzees

Evolutionary Relationships (1)_1, downloadable AS & A Level Biology revision notesEvolutionary Relationships (2)_2, downloadable AS & A Level Biology revision notesEvolutionary Relationships (3)_2, downloadable AS & A Level Biology revision notes

An image showing the use of albumin and antibody production in comparing the relationship between different species.

Exam 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.

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