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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Angiosperm Phylogeny Group | 2/3 | https://en.wikipedia.org/wiki/Angiosperm_Phylogeny_Group | reference | science, encyclopedia | 2026-05-05T10:21:13.405150+00:00 | kb-cron |
The initial 1998 paper by the APG made angiosperms the first large group of organisms to be systematically re-classified primarily on the basis of genetic characteristics. The paper explained the authors' view that there is a need for a classification system for angiosperms at the level of families, orders and above, but that existing classifications were "outdated". The main reason why existing systems were rejected was because they were not phylogenetic, i.e. not based on strictly monophyletic groups (groups which consist of all descendants of a common ancestor). An ordinal classification of flowering plant families was proposed as a "reference tool of broad utility". The broad approach adopted to defining the limits of orders resulted in the recognition of 40 orders, compared to, for example, 232 in Takhtajan's 1997 classification. In 1998 only a handful of families had been adequately studied, but the primary aim was to obtain a consensus on the naming of higher orders. Such a consensus proved relatively easy to achieve but the resultant tree was highly unresolved. That is, while the relationship of orders was established, their composition was not. Other features of the proposed classification included:
Formal, scientific names are not used above the level of order, named clades being used instead. Thus eudicots and monocots are not given a formal rank on the grounds that "it is not yet clear at which level they should be recognized". A substantial number of taxa whose classification had traditionally been uncertain are given places, although there still remain 25 families of "uncertain position". Alternative classifications are provided for some groups, in which a number of families can either be regarded as separate or can be merged into a single larger family. For example, the Fumariaceae can either be treated as a separate family or as part of Papaveraceae. A major outcome of the classification was the disappearance of the traditional division of the flowering plants into two groups, monocots and dicots. The monocots were recognized as a clade, but the dicots were not, with a number of former dicots being placed in separate groups basal to both monocots and the remaining dicots, the eudicots or 'true dicots'. The overall scheme was relatively simple. This consisted of a grade consisting of isolated taxa (referred to as ANITA), followed by the major angiosperm radiation, clades of monocots, magnolids and eudicots. The last being a large clade with smaller subclades and two main groupings, rosids and asterids, each in turn having two major subclades.
== APG II (2003) ==
As the overall relationship between groups of flowering plants became clearer, the focus shifted to the family level, in particular those families generally accepted as problematic. Again, consensus was achieved relatively easily resulting in an updated classification at the family level. The second paper published by the APG in 2003 presented an update to the original classification of 1998. The authors stated that changes were proposed only when there was "substantial new evidence" which supported them. The classification continued the tradition of seeking broad circumscriptions of taxa, for example trying to place small families containing only one genus in a larger group. The authors stated that they have generally accepted the views of specialists, although noting that specialists "nearly always favour splitting of groups" regarded as too varied in their morphology. APG II continued and indeed extends the use of alternative 'bracketed' taxa allowing the choice of either a large family or a number of smaller ones. For example, the large family Asparagaceae includes seven 'bracketed' families which can either be considered as part of the Asparagaceae or as separate families. Some of the main changes in APG II were:
New orders are proposed, particularly to accommodate the 'basal clades' left as families in the first system. Many of the previously unplaced families are now located within the system. Several major families are re-structured. In 2007, a paper was published giving a linear ordering of the families in APG II, suitable for ordering herbarium specimens, for example.
== APG III (2009) ==
The third paper from the APG updates the system described in the 2003 paper. The broad outline of the system remains unchanged, but the number of previously unplaced families and genera is significantly reduced. This requires the recognition of both new orders and new families compared to the previous classification. The number of orders goes up from 45 to 59; only 10 families are not placed in an order and only two of these (Apodanthaceae and Cynomoriaceae) are left entirely outside the classification. The authors say that they have tried to leave long-recognized families unchanged, while merging families with few genera. They "hope the classification [...] will not need much further change." A major change is that the paper discontinues the use of 'bracketed' families in favour of larger, more inclusive families. As a result, the APG III system contains only 415 families, rather than the 457 of APG II. For example, the agave family (Agavaceae) and the hyacinth family (Hyacinthaceae) are no longer regarded as distinct from the broader asparagus family (Asparagaceae). The authors say that alternative circumscriptions, as in APG I and II, are likely to cause confusion and that major herbaria which are re-arranging their collections in accordance with the APG approach have all agreed to use the more inclusive families. This approach is being increasingly used in collections in herbaria and botanic gardens. In the same volume of the journal, two related papers were published. One gives a linear ordering of the families in APG III; as with the linear ordering published for APG II, this is intended for ordering herbarium specimens, for example. The other paper gives, for the first time, a classification of the families in APG III which uses formal taxonomic ranks; previously only informal clade names were used above the ordinal level.
== APG IV (2016) ==