Classification and SpeciationAQA GCSE Biology: Revision notes

The traditional Linnaean classification system organises living things into a hierarchy of groups, from broadest to narrowest:

Kingdom → Phylum → Class → Order → Family → Genus → Species

Organisms are named using the binomial system, which gives each species a two-part scientific name made up of its genus and species (e.g. Homo sapiens).

Classification systems have changed as new evidence has emerged over time:

• Improvements in microscopes revealed internal cell structures that were previously invisible, allowing more accurate grouping of organisms
• A deeper understanding of biochemical processes inside cells led scientists to develop new classification models
• Chemical analysis eventually led to the three-domain system, developed by Carl Woese, which divides all organisms into three domains:
  • Archaea — primitive bacteria, often living in extreme environments
  • Bacteria — true bacteria
  • Eukaryota — protists, fungi, plants and animals

Evolutionary trees are diagrams that show how scientists believe different organisms are related through evolution, and how recently they may have shared a common ancestor.

• For living organisms, evolutionary trees are built using current classification data (e.g. physical and genetic similarities)
• For extinct organisms, evolutionary trees make use of fossil data

Speciation is the process by which a new species forms. It occurs when two populations of the same species become so different in their characteristics (phenotype) that they can no longer interbreed to produce fertile offspring. At this point, the two populations are considered to be two separate species.