Historically, biological classification schemes involved arranging organisms into groups (taxa) on the basis of shared characteristics

• Taxa function as a series of hierarchical units  – the more taxa organisms share, the more similar they are considered to be
  

• The taxa used are domain, kingdom, phylum, class, order, family, genus and species (genus + species = scientific name)
  

There are many limitations involved with classifying organisms according to a hierarchy of taxa:

• Taxonomic ranks set an arbitrary limit on the number of groupings by which an organism can be classified
  

• Populations that are genetically diverging may be difficult to distinguish using limited taxonomic divisions (fixed rankings do not reflect the gradation of variation)

• Organisms may share structural similarities due to convergent evolution (i.e. analogous structures) and may not actually be closely related
  

Hierarchy of Taxa