Angiosperm Phylogeny Group
Encyclopedia
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 plant
s (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.
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 1980's, 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:
Peter F. Stevens
, one of the authors of all three of the APG papers, maintains a web site, 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 features of the proposed classification include:
A major outcome of the classification is the disappearance of the traditional division of the flowering plants into two groups, monocots and dicots. The monocots are recognized as a clade, but the dicots are 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 proposed classification continues 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 say that they have generally accepted the views of specialsts, although noting that specialists "nearly always favour splitting of groups" regarded as too varied in their morphology.
APG II continues 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 Asparagaceae
family 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 are:
In 2007, a paper was published giving a linear ordering of the families in APG II, suitable for ordering herbarium specimens, for example.
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.
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 rank
s; previously only informal clade names were used above the ordinal level.
a = listed as an author; c = listed as a contributor
c = listed as a contributor
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
who came together to try to establish a consensus on the taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
of flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s (angiosperms) that would reflect new knowledge about plant relationships discovered through phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
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 systemEngler system
One of the prime systems of plant taxonomy, the Engler system was devised by Adolf Engler.According to Engler, Syllabus der Pflanzenfamilien the main groups of plants are:* I. divisio Schizophyta* II. divisio Phytosarcodina...
in continental Europe, the Bentham & Hooker system
Bentham & Hooker system
An early taxonomic system, the Bentham & Hooker system for seed plants was published in :The system recognises the following main groups:*DICOTYLEDONUM POLYPETALARUM*: Series 1. THALAMIFLORÆ*: Series 2. DISCIFLORÆ*: Series 3. CALYCIFLORÆ...
in Britain (particularly influential because it was used by Kew), the Takhtajan system
Takhtajan system
A system of plant taxonomy, the Takhtajan system of plant classification was published by Armen Takhtajan, in several versions from the 1950s onwards. It is usually compared to the Cronquist system. Key publications:-External links:* Takhtajan system at...
in the former Soviet Union and countries within its sphere of influence and the Cronquist system
Cronquist system
The Cronquist system is a taxonomic classification system of flowering plants. It was developed by Arthur Cronquist in his texts An Integrated System of Classification of Flowering Plants and The Evolution and Classification of Flowering Plants .Cronquist's system places flowering plants into two...
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
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
(particularly of their flower) and biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
(the kinds of chemical compounds in the plant).
After the 1980's, 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 KewRoyal Botanic Gardens, KewThe Royal Botanic Gardens, Kew, usually referred to as Kew Gardens, is 121 hectares of gardens and botanical glasshouses between Richmond and Kew in southwest London, England. "The Royal Botanic Gardens, Kew" and the brand name "Kew" are also used as umbrella terms for the institution that runs...
, 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, a recent photographic survey of the plants of the USA and Canada is organized according to the APG II system.
- In the UK, the latest edition of the standard flora of the British Isles (by StaceClive A. StaceClive Anthony Stace is a British botanist and botanical author. His academic career was based at the University of Leicester, where he held the post of Professor of Plant taxonomy...
) is based on the APG III system. The previous edition was based on the Cronquist systemCronquist systemThe Cronquist system is a taxonomic classification system of flowering plants. It was developed by Arthur Cronquist in his texts An Integrated System of Classification of Flowering Plants and The Evolution and Classification of Flowering Plants .Cronquist's system places flowering plants into two...
.
Peter F. Stevens
Peter F. Stevens
Peter Francis Stevens is a British botanist born in 1944.He is a researcher at the Missouri Botanical Garden and professor of Biology of the University of Missouri–St. Louis...
, one of the authors of all three of the APG papers, maintains a web site, APweb
Angiosperm Phylogeny Website
Angiosperm Phylogeny Website is a well-known web site dedicated to research on Angiosperm phylogeny and taxonomy.The site is hosted by the Missouri Botanical Garden website and maintained by researcher Peter F. Stevens, who is a member of the Angiosperm Phylogeny Group...
, 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.
Principles 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:- The Linnean system of ordersOrder (biology)In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
and familiesFamily (biology)In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
should be retained. "The family is central in flowering plant systematics." An ordinal classification of families is proposed as a "reference tool of broad utility". Orders are considered to be of particular value in teaching and in studying family relationships. - Groups should be monophyletic (i.e. consist of all descendants of a common ancestor). The main reason why existing systems are rejected is because they do not have this property, they are not phylogenetic.
- A broad approach is taken to defining the limits of groups such as orders and families. Thus of orders, it is said that a limited number of larger orders will be more useful. Families containing only a single genus and orders containing only a single family are avoided where this is possible without violating the over-riding requirement for monophyly.
- Above or parallel to the level of orders and families, the term cladeCladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s is used more freely. (Some clades have later been given formal names in a paper associated with the 2009 revision of the APG system.) The authors say that it is "not possible, nor is it desirable" to name all clades in a phylogenetic tree; however, systematists need to agree on names for some clades, particularly orders and families, to facilitate communication and discussion.
APG I (1998)
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 explains 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 are "outdated". The main reason why existing systems are rejected is because they are not phylogenetic, i.e. are not based on strictly monophyletic groups (i.e. groups which consist of all descendants of a common ancestor). An ordinal classification of flowering plant families is 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 classificationTakhtajan system
A system of plant taxonomy, the Takhtajan system of plant classification was published by Armen Takhtajan, in several versions from the 1950s onwards. It is usually compared to the Cronquist system. Key publications:-External links:* Takhtajan system at...
.
Other features of the proposed classification include:
- Formal, scientific names are not used above the level of order, named cladeCladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s being used instead. Thus eudicotsEudicotsEudicots and Eudicotyledons are botanical terms introduced by Doyle & Hotton to refer to a monophyletic group of flowering plants that had been called tricolpates or non-Magnoliid dicots by previous authors...
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 FumariaceaeFumariaceaeFumariaceae is a family of about 575 species of herbaceous plants in 20 genera, native to the Northern Hemisphere and South Africa.-Flower shape:Plants in the fumitory family are easily recognised by their peculiar flowers with two dissimilar pairs of...
can either be treated as a separate family or as part of PapaveraceaePapaveraceaePapaveraceae, informally known as the poppy family, are an economically important family of 44 genera and approximately 770 species of flowering plants in the order Ranunculales. The family is cosmopolitan, occurring in temperate and subtropical climates, but almost unknown in the tropics...
.
A major outcome of the classification is the disappearance of the traditional division of the flowering plants into two groups, monocots and dicots. The monocots are recognized as a clade, but the dicots are 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'.
APG II (2003)
The second paper published by the APG presents an update to the original classification of 1998. The authors say that changes have been proposed only when there is "substantial new evidence" which supports them.The proposed classification continues 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 say that they have generally accepted the views of specialsts, although noting that specialists "nearly always favour splitting of groups" regarded as too varied in their morphology.
APG II continues 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 Asparagaceae
Asparagaceae
Asparagaceae is the botanical name of a family of flowering plants, placed in the order Asparagales of the monocots.In earlier classification systems, the species involved were often treated as belonging to the family Liliaceae...
family 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 are:
- 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 (ApodanthaceaeApodanthaceae
The family Apodanthaceae comprises 22 to 30 species of endoparasitic herbs. They live in branches or roots of their host , emerging only to flower. The only leaves present are several bracts at the base of each flower. The plants do not carry out any photosynthesis...
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
Agavaceae
Agavoideae is a subfamily of monocot flowering plants in the family Asparagaceae, order Asparagales. It has previously been treated as a separate family, Agavaceae. The group includes many well-known desert and dry zone types such as the agave, yucca, and Joshua tree...
) and the hyacinth family (Hyacinthaceae
Hyacinthaceae
Scilloideae is a subfamily of the monocot family Asparagaceae in the order Asparagales. The Scilloideae are bulbous flowering plants, which have sometimes been placed in a separate family, the Hyacinthaceae. The subfamily name is derived from the generic name of the type genus, Scilla...
) are no longer regarded as distinct from the broader asparagus family (Asparagaceae
Asparagaceae
Asparagaceae is the botanical name of a family of flowering plants, placed in the order Asparagales of the monocots.In earlier classification systems, the species involved were often treated as belonging to the family Liliaceae...
). 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.
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 rank
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
s; previously only informal clade names were used above the ordinal level.
Listed as "author" of one or more of the papers
| Name | APG I | APG II | APG III | Institutional affiliation |
|---|---|---|---|---|
| Birgitta Bremer | c | a | a | Swedish Academy of Sciences |
| Kåre Bremer | a | a | a | Uppsala University; Stockholm University |
| Mark W. Chase | a | a | a | Royal Botanic Gardens, Kew |
| Michael F. Fay | c | c | a | Royal Botanic Gardens, Kew |
| James L. Reveal | a | a | University of Maryland; Cornell University | |
| Douglas E. Soltis | c | a | a | University of Florida |
| Pamela S. Soltis | c | a | a | Florida Museum of Natural History |
| Peter F. Stevens Peter F. Stevens Peter Francis Stevens is a British botanist born in 1944.He is a researcher at the Missouri Botanical Garden and professor of Biology of the University of Missouri–St. Louis... |
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
Listed as "contributor" to one or more of the papers
| Name | APG I | APG II | APG III |
|---|---|---|---|
| Arne A. Anderberg | c | c | c |
| Anders Backlund | c | ||
| Barbara G. Briggs | c | ||
| Peter K. Endress | c | ||
| Peter Goldblatt | c | c | |
| Mats H.G. Gustafson | c | ||
| Sara B. Hoot | c | ||
| Walter S. Judd | c | c | |
| Mari Källersjö | c | c | |
| Jesper Kårehed | c | ||
| Elizabeth A. Kellogg | c | ||
| Kathleen A. Kron | c | c | |
| Donald H. Les | c | ||
| Johannes Lundberg | c | ||
| Michael J. Moore | c | ||
| Cynthia M. Morton | c | ||
| Daniel L. Nickrent | c | c | |
| Richard G. Olmstead | c | c | c |
| Bengt Oxelman | c | ||
| J. Chris Pires | c | ||
| Robert A. Price | c | ||
| Christopher J. Quinn | c | ||
| James E. Rodman | c | c | |
| Paula J. Rudall | c | c | c |
| Vincent Savolainen | c | c | |
| Kenneth J. Sytsma | c | c | c |
| David C. Tank | c | ||
| Mats Thulin | c | ||
| Michelle van der Bank | c | ||
| Kenneth Wurdack | c | c | |
| Jenny Q.-Y. Xiang | c | c | |
| Sue Zmarzty | c | c |
c = listed as a contributor