The question must refer to the modern classification system in biology.
It started with the work of Linnaeus - so its early version can be called a Linnaean classification - but right now it has been expanded and changed to fit the evolutionary relationships between the species - which is the best description, i.e. the answer is "it is based on the evolutionary relationship of the species"
The
modern classification system is mainly based on common ancestry.
Therefore it is based on evolutionary relationships. A common
ancestor refers to the most recent ancestral species from which other
different species evolved.
