Classification (OCR Gateway GCSE Biology: Combined Science)

• Classification can be done based on the similarities and differences between organisms
• Over time, and with development in science, methods used to classify organism have become more sophisticated
• There are two classification systems used by scientists
  • Artificial classification
  • Natural classification

Artificial classification

• The earliest methods of classification used comparisons of observable and non-evolutionary features to group organisms
• Observable characteristics include things like whether they give birth to live young or lay eggs etc
• Basic ecological keys still use artificial classification to group organisms in a way that allows identification
• The less scientific nature of these methods means they are not considered to be the best way of classifying organisms

Natural classification

• Natural classification is based on evolutionary relationships between organisms
• Organisms are categorised using information about common ancestors and common structural features
• This system of classification was developed by Swedish naturalist Linnaeus, in a way that allows the subdivision of living organisms into smaller and more specialised groups
• The species in these groups have more and more features in common the more subdivided they get
• He named organisms in Latin using the binomial system where the scientific name of an organism is made up of two parts starting with the genus (always given a capital letter) and followed by the species (starting with a lower case letter)
• When typed binomial names are always in italics (which indicates they are Latin) e.g. Homo sapiens
• The sequence of classification by this natural classification system is: Kingdom, Phylum, Class, Order, Family, Genus, Species

Linnaeus’s system of classification

Advances in classification

• Originally, organisms were classified using morphology (the overall form and shape of the organism, e.g. whether it had wings or legs) and anatomy (the detailed body structure as determined by dissection)
• As technology advanced, microscopes, knowledge of biochemistry and eventually DNA sequencing allowed us to classify organisms using a more scientific approach
• Studies of DNA sequences of different species show that the more similar the base sequences in the DNA of two species, the more closely related those two species are (and the more recent in time their common ancestor is)
  • Classification based on this information is called molecular phylogeny
• This means that the base sequences in a mammal’s DNA are more closely related to all other mammals than to any other vertebrate groups

DNA sequences can show how closely related species are to each other

• The partial DNA sequences (of a gene) above show that Brachinus armiger and Brachinus hirsutus are more closely related than any other species in the list as their DNA sequences are identical except for the last but one base (B.armiger has a T in that position whereas B.hirsutus has an A)
• As DNA base sequences are used to code for amino acid sequences in proteins, the similarities in amino acid sequences can also be used to determine how closely related organisms are