The Angiosperm Phylogeny Group, or APG, refers to an informal international group of systematic botanists who came together to try to establish a consensus on the taxonomy of flowering plants (angiosperms) that would reflect new knowledge about plant relationships discovered through phylogenetic studies.
, three incremental versions of a classification system have resulted from this collaboration published in 1998, 2003 and 2009. An important motivation for the group was, what they considered, deficiencies in prior angiosperm classifications since they were not based on monophyletic groups (i.e. groups that include all the descendants of a common ancestor).
APG publications are increasingly influential, with a number of major herbaria changing the arrangement of their collections to match the latest APG system.
==Angiosperm classification and the APG==
In the past, classification systems were typically produced by an individual botanist or by a small group. The result was a large number of systems (see List of systems of plant taxonomy). Different systems and their updates were generally favoured in different countries. Examples are the Engler system in continental Europe, the Bentham & Hooker system in Britain (particularly influential because it was used by Kew), the Takhtajan system in the former Soviet Union and countries within its sphere of influence and the Cronquist system in the United States.
Before the availability of genetic evidence, the classification of angiosperms (also known as ''flowering plants'', ''Angiospermae'', ''Anthophyta'' or ''Magnoliophyta'') was based on their morphology (particularly of their flower) and biochemistry (the kinds of chemical compounds in the plant).
After the 1980s, detailed genetic evidence analysed by phylogenetic methods became available and while confirmed or clarified some relationships in existing classification systems, it radically changed others. This genetic evidence created a rapid increase in knowledge that led to many proposed changes; stability was "rudely shattered". This posed problems for all users of classification systems (including encyclopaedists).
In the late 1990s, an informal group of researchers from major institutions worldwide came together under the title of the 'Angiosperm Phylogeny Group' or APG. Their intention was to provide a widely accepted and more stable point of reference for angiosperm classification. Their first attempt at a new system was published in 1998 (the APG system). , two revisions have been published, in 2003 (APG II) and in 2009 (APG III), each superseding the previous system. Eight researchers have been listed as authors to the three papers, and a further 33 as contributors (see Members of the APG below).
A classification presents a view at a particular point in time, based on a particular state of research. Independent researchers, including members of the APG, continue to publish their own views on areas of angiosperm taxonomy. Classifications change, however inconvenient this is to users. However, the APG publications are increasingly regarded as an authoritative point of reference and the following are some examples of the influence of the APG system:
* A significant number of major herbaria, including Kew, are changing the order of their collections in accordance with APG.
* The influential World Checklist of Selected Plant Families (also from Kew) is being updated to the APG III system.
* In the USA in 2006, a photographic survey of the plants of the USA and Canada is organized according to the APG II system.
* In the UK, the 2010 edition of the standard flora of the British Isles (by Stace) is based on the APG III system. The previous editions were based on the Cronquist system.
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