The APG Classification

Angiosperms were the first large group of organisms to be systematically reclassified primarily on the basis of genetic characteristics, according to the APG's initial paper published in 1998. The authors' viewpoint was that there is a need for a classification system for angiosperms at the family, order, and higher levels, but that existing classifications were "outdated." Existing systems were rejected primarily because they were not phylogenetic, that is, they were not based on strictly monophyletic groups (groups which consist of all descendants of a common ancestor).

As a "reference tool of broad utility," an ordinal classification of flowering plant families was developed. The broad technique used to define order limits resulted in the acknowledgment of 40 orders, compared to 232 in Takhtajan's 1997 classification. In 1998, just a few families had been thoroughly investigated, but the primary goal was to reach an agreement on the nomenclature of higher orders. Although such agreement was very easy to get, the resulting tree was severely unresolved. That is, while the order's link was defined, its composition was not.

Other features of APG I classification

Above the level of order, scientific names are not given; instead, named clades are utilised. As a result, eudicots and monocots are not granted a formal rank since "it is not yet obvious at which level they should be acknowledged."

A significant number of taxa whose categorization had previously been ambiguous have been assigned locations, while there are still 25 families with "uncertain position."

For some groupings, alternative classifications are provided, in which a number of families might be considered independent or united into a single bigger family. For example, the Fumariaceae might be classified as a distinct family or as a subfamily of the Papaveraceae.

The classification has a significant impact on the removal of the conventional split of flowering plants into two groups, monocots and dicots. The monocots were recognised as a clade, but the dicots were not, with several former dicots classified in different groupings basal to both the monocots and the remaining dicots, the eudicots or 'real dicots.'

The main plan was straightforward. This included a grade of solitary taxa, followed by the major angiosperm radiation, which included clades of monocots, magnolids, and eudicots. The latter is a huge clade with smaller subclades and two major groupings, rosids and asterids, each of which has two major subclades.