Evolutionary Relationships (AQA A Level Biology): Revision Note

Evolutionary relationships

• 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 have allowed scientists to further investigate the evolutionary relationships between species

Genome sequencing

• 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 that 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

Immunology

• 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 is extracted from various species and injected into separate rabbits

  • Each rabbit produces antibodies specific to that albumin

  • These antibodies are then mixed with different albumin samples, and the resulting precipitate (antigen–antibody complexes) is weighed

• Results andinterpretation:

  • The heavier the precipitate, the greater the similarity between the antibody and albumin

  • For example, antibodies to human albumin form more precipitate with chimpanzee albumin than with rat albumin, reflecting closer evolutionary relatedness