AQA A Level Biology

Revision Notes

4.5.4 Evolutionary Relationships

Appreciating How Evolutionary Relationships Were Clarified

  • In the past scientists encountered many difficulties when trying to determine the evolutionary relationships of species
  • 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 genome sequencing and immunology has allowed scientists to further investigate the evolutionary relationships between species

Genome Sequencing

  • 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 family trees that show the evolutionary relationships between species

Classification & Phylogeny, downloadable AS & A Level Biology revision notesThe DNA base sequences of two closely related species being compared – Species Y is the ancestor of Species X.


  • 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. It is often easier to find and isolate proteins from cells.

Genes for the same protein may have slightly different base sequences in different species as different species may contain differences in their introns which are not translated into differences in the protein molecules.  Genes can also be found on different chromosomes in different species.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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