یکشنبه 28 اسفند 1390 06:39 ب.ظ
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), theTakhtajan 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). As of 2010, 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, 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 Stace) is based on the APG III system. The previous edition was based on the Cronquist 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.
[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 orders and families 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 clades 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 classification.
Other features of the proposed classification include:
- 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 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 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 (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.
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.
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