The Angiosperm Phylogeny Group, or APG, is an informal international group of systematic botanists who collaborate to establish a consensus on the taxonomy of flowering plants (angiosperms) that reflects new knowledge about plant relationships discovered through phylogenetic studies.
As of 2016 [update] , four incremental versions of a classification system have resulted from this collaboration, published in 1998, 2003, 2009 and 2016. 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.
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 confirming or clarifying 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). The impetus came from a major molecular study published in 1993 based on 5000 flowering plants and a photosynthesis gene (rbcL). This produced a number of surprising results in terms of the relationships between groupings of plants, for instance the dicotyledons were not supported as a distinct group. At first there was a reluctance to develop a new system based entirely on a single gene. However, subsequent work continued to support these findings. These research studies involved an unprecedented collaboration between a very large number of scientists. Therefore, rather than naming all the individual contributors a decision was made to adopt the name Angiosperm Phylogeny Group classification, or APG for short. The first publication under this name was in 1998, and attracted considerable media attention. The intention was to provide a widely accepted and more stable point of reference for angiosperm classification.
As of 2016 [update] , three revisions have been published, in 2003 (APG II), in 2009 (APG III) and in 2016 (APG IV), each superseding the previous system. Thirteen researchers have been listed as authors to the three papers, and a further 43 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:
The principles of the APG's approach to classification were set out in the first paper of 1998, and have remained unchanged in subsequent revisions. Briefly, these are:
For a detailed discussion on phylogenetic nomenclature, see Cantino et al. (2007).)
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:
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.
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 7 '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:
In 2007, a paper was published giving a linear ordering of the families in APG II, suitable for ordering herbarium specimens, for example.
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.
In the development of a fourth version there was some controversy over the methodology,and the development of a consensus proved more difficult than in previous iterations. In particular Peter Stevens questioned the validity of discussions regarding family delimitation in the absence of changes of phylogenetic relationships.
Further progress was made by the use of large banks of genes, including those of plastid, mitochondrial and nuclear ribosomal origin, such as that of Douglas Soltis and colleagues (2011).The fourth version was finally published in 2016. It arose from an international conference hosted at the Royal Botanical Gardens in September 2015 and also an online survey of botanists and other users. The broad outline of the system remains unchanged but several new orders are included (Boraginales, Dilleniales, Icacinales, Metteniusiales and Vahliales), some new families are recognised (Kewaceae, Macarthuriaceae, Maundiaceae, Mazaceae, Microteaceae, Nyssaceae, Peraceae, Petenaeaceae and Petiveriaceae) and some previously recognised families are lumped (Aristolochiaceae now includes Lactoridaceae and Hydnoraceae; Restionaceae now re-includes Anarthriaceae and Centrolepidaceae; and Buxaceae now includes Haptanthaceae). Due to nomenclatural issues, the family name Asphodelaceae is used instead of Xanthorrhoeaceae, and Francoaceae is used instead of Melianthaceae (and now also includes Vivianiaceae). This brings the total number of orders and families recognized in the APG system to 64 and 416, respectively. Two additional informal major clades, superrosids and superasterids, that each comprise the additional orders that are included in the larger clades dominated by the rosids and asterid s are also included. APG IV also uses the linear approach (LAPG) as advocated by Haston et al. (2009) In a supplemental file Byng et al. provide an alphabetical list of families by orders.
Peter Stevens, one of the authors of all four of the APG papers, maintains a web site, the Angiosperm Phylogeny Website (APWeb), hosted by the Missouri Botanical Garden, which has been regularly updated since 2001, and is a useful source for the latest research in angiosperm phylogeny which follows the APG approach.Other sources include the Angiosperm Phylogeny Poster and The Flowering Plants Handbook.
|Name||APG I||APG II||APG III||APG IV||Institutional affiliation|
|Birgitta Bremer||c||a||a||Swedish Academy of Sciences|
|Kåre Bremer||a||a||a||Uppsala University; Stockholm University|
|James W. Byng||a||Plant Gateway; University of Aberdeen|
|Mark Wayne Chase||a||a||a||a||Royal Botanic Gardens, Kew|
|Maarten J.M. Christenhusz||a||Plant Gateway; Royal Botanic Gardens, Kew|
|Michael F. Fay||c||c||a||a||Royal Botanic Gardens, Kew|
|Walter S. Judd||a||University of Florida|
|David J. Mabberley||a||University of Oxford; Universiteit Leiden; Naturalis Biodiversity Center; Macquarie University; National Herbarium of New South Wales|
|James L. Reveal||a||a||University of Maryland; Cornell University|
|Alexander N. Sennikov||a||Finnish Museum of Natural History; Komarov Botanical Institute|
|Douglas E. Soltis||c||a||a||a||University of Florida|
|Pamela S. Soltis||c||a||a||a||Florida Museum of Natural History|
|Peter F. Stevens||a||a||a||a||Harvard University Herbaria; University of Missouri-St. Louis and Missouri Botanical Garden|
a = listed as an author; c = listed as a contributor
|Name||APG I||APG II||APG III||APG IV|
|Arne A. Anderberg||c||c||c|
|Barbara G. Briggs||c||c|
|John C. Clark||c|
|Peter K. Endress||c|
|Mats H.G. Gustafson||c|
|Sara B. Hoot||c|
|Walter S. Judd||c||c|
|Elizabeth A. Kellogg||c|
|Kathleen A. Kron||c||c|
|Donald H. Les||c|
|Michael J. Moore||c||c|
|Cynthia M. Morton||c|
|Daniel L. Nickrent||c||c|
|Richard G. Olmstead||c||c||c||c|
|J. Chris Pires||c|
|Robert A. Price||c|
|Christopher J. Quinn||c|
|James E. Rodman||c||c|
|Paula J. Rudall||c||c||c|
|Kenneth J. Sytsma||c||c||c|
|David C. Tank||c||c|
|Michelle van der Bank||c|
|Jenny Q.-Y. Xiang||c||c|
c = listed as a contributor
The Alismatales (alismatids) are an order of flowering plants including about 4500 species. Plants assigned to this order are mostly tropical or aquatic. Some grow in fresh water, some in marine habitats.
The Brassicales are an order of flowering plants, belonging to the eurosids II group of dicotyledons under the APG II system. One character common to many members of the order is the production of glucosinolate compounds. Most systems of classification have included this order, although sometimes under the name Capparales.
The Dioscoreales are an order of monocotyledonous flowering plants in modern classification systems, such as the Angiosperm Phylogeny Group and the Angiosperm Phylogeny Web. Within the monocots Dioscoreales are grouped in the lilioid monocots where they are in a sister group relationship with the Pandanales. Of necessity the Dioscoreales contain the family Dioscoreaceae which includes the yam (Dioscorea) that is used as an important food source in many regions around the globe. Older systems tended to place all lilioid monocots with reticulate veined leaves in Dioscoreales. As currently circumscribed by phylogenetic analysis using combined morphology and molecular methods, Dioscreales contains many reticulate veined vines in Dioscoraceae, it also includes the myco-heterotrophic Burmanniaceae and the autotrophic Nartheciaceae. The order consists of three families, 22 genera and about 850 species.
The Malpighiales comprise one of the largest orders of flowering plants, containing about 16,000 species, about 7.8% of the eudicots. The order is very diverse, containing plants as different as the willow, violet, poinsettia, and coca plant, and are hard to recognize except with molecular phylogenetic evidence. It is not part of any of the classification systems based only on plant morphology. Molecular clock calculations estimate the origin of stem group Malpighiales at around 100 million years ago (Mya) and the origin of crown group Malpighiales at about 90 Mya.
The Magnoliales are an order of flowering plants.
Sapindales is an order of flowering plants. Well-known members of Sapindales include citrus; maples, horse-chestnuts, lychees and rambutans; mangos and cashews; frankincense and myrrh; mahogany and neem.
Canellales is the botanical name for an order of flowering plants, one of the four orders of the magnoliids. It is recognized by the most recent classification of flowering plants, the APG IV system. It is defined to contain two families: Canellaceae and Winteraceae, which comprise 136 species of fragrant trees and shrubs. The Canellaceae are found in tropical America and Africa, and the Winteraceae are part of the Antarctic flora. Although the order was defined based on phylogenetic studies, a number of possible synapomorphies have been suggested, relating to the pollen tube, the seeds, the thickness of the integument, and other aspects of the morphology.
The Cornales are an order of flowering plants, basal among the asterids, containing about 600 species. Plants within the Cornales usually have four-parted flowers, drupaceous fruits, and inferior gynoecia topped with disc-shaped nectaries.
The Nymphaeales are an order of flowering plants, consisting of three families of aquatic plants, the Hydatellaceae, the Cabombaceae, and the Nymphaeaceae. It is one of the three orders of basal angiosperms, an early-diverging grade of flowering plants. At least 10 morphological characters unite the Nymphaeales. Molecular synapomorphies are also known.
Under the International Code of Nomenclature for algae, fungi, and plants (ICN), Rosidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used; the only requirement being that it includes the family Rosaceae.
Burmanniaceae is a family of flowering plants, consisting of 99 species of herbaceous plants in eight genera.
Asphodelaceae is a family of flowering plants in the order Asparagales. Such a family has been recognized by most taxonomists, but the circumscription has varied widely. In its current circumscription in the APG IV system, it includes about 40 genera and 900 known species. The type genus is Asphodelus.
In the APG IV system (2016) for the classification of flowering plants, the name asterids denotes a clade. Common examples include the forget-me-nots, nightshades, the common sunflower, petunias, morning glory and sweet potato, coffee, lavender, lilac, olive, jasmine, honeysuckle, ash tree, teak, snapdragon, sesame, psyllium, garden sage, table herbs such as mint, basil, and rosemary, and rainforest trees such as Brazil nut.
Asparagaceae is a family of flowering plants, placed in the order Asparagales of the monocots. Its best known member is Asparagus officinalis, garden asparagus.
Peridiscaceae is a family of flowering plants in the order Saxifragales. Four genera comprise this family: Medusandra, Soyauxia, Peridiscus, and Whittonia., with a total of 12 known species. It has a disjunct distribution, with Peridiscus occurring in Venezuela and northern Brazil, Whittonia in Guyana, Medusandra in Cameroon, and Soyauxia in tropical West Africa. Whittonia is possibly extinct, being known from only one specimen collected below Kaieteur Falls in Guyana. In 2006, archeologists attempted to rediscover it, however, it proved unsuccessful.
Octoknemaceae is a monotypic family of flowering plants endemic to continental Africa. The APG III system of 2009 and the APG II system of 2003, do not recognize this family. The family is recognized by the Angiosperm Phylogeny Website, based on work since the publication of the APG III system.
Oncotheca is a genus of tree endemic to New Caledonia. There are two species, Oncotheca balansae and Oncotheca humboldtiana.
Mesangiospermae is a group of flowering plants (angiosperms), informally called "mesangiosperms". They are one of two main clades of angiosperms. It is a name created under the rules of the PhyloCode system of phylogenetic nomenclature. There are about 350,000 species of mesangiosperms. The mesangiosperms contain about 99.95% of the flowering plants, assuming that there are about 175 species not in this group and about 350,000 that are. While such a clade with a similar circumscription exists in the APG III system, it was not given a name.
Hydrostachys is a genus of about 22 species of flowering plants native to Madagascar and southern and central Africa. It is the only genus in the family Hydrostachyaceae. All species of Hydrostachys are aquatic, growing on rocks in fast-moving water. They have tuberous roots, usually pinnately compound leaves, and highly reduced flowers on dense spikes.
Magnoliidae is a subclass of Equisetopsida in the sense used by Mark W. Chase and James L. Reveal in their 2009 article "A phylogenetic classification of the land plants to accompany APG III." This subclass comprises the angiosperms or flowering plants.
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Note: This is a selected list of the more influential systems. There are many other systems, for instance a review of earlier systems, published by Lindley in his 1853 edition, and Dahlgren (1982). Examples include the works of Scopoli, Batsch and Grisebach.