Why are plant families grouped together into orders in the APG classification system?

In the Angiosperm Phylogeny Group (APG) classification system, plant families are grouped into orders primarily to reflect their phylogenetic relationships, meaning they are arranged together because they are believed to be related by descent from a common ancestor.12

The APG system focuses on the family level as the primary unit around which botanists organize their understanding of plant diversity, with related families clustered into orders to signal their relative position within an evolutionary hierarchy. This approach represents a shift from older, morphology-based systems—such as the Cronquist system—toward a framework driven by robust molecular evidence, specifically the sequencing of DNA from chloroplasts and nuclei.1

By grouping families into orders, the system presumes that the included families are monophyletic, meaning they form a lineage derived from a common ancestor. As new molecular data become available, these groupings are continuously refined to better align with the latest understanding of plant evolution.12