Angiosperm Phylogeny Group
From Wikipedia, the free encyclopedia
The Angiosperm Phylogeny Group, or APG, refers to two international groups of systematic botanists who came together to try to establish a consensus view of the taxonomy of flowering plants that would reflect new knowledge in angiosperm relationships molecular systematics. Two papers resulted from these collaborations, commonly referenced as 'APG I (1998)' and 'APG II (2003),' and were largely attempts to deal with the deficiencies in prior angiosperm classifications (Cronquist 1981, Thorne 1992 and 2001, Takhtajan 1997) as seen by phylogenetic theories based on analysis of DNA.
The first paper by the APG marked an important change in how angiosperm classifications were done, as prior classification systems had been done by one or two scientists, while this paper was the effort of 29 botanical systematists to make sense of the large number of molecular phylogenies being studied in the angiosperms. This made angiosperms the first large group of organisms to be systematically reclassified largely on the basis of molecular characteristics.[1]
The flowering plants (also known as angiosperms, Angiospermae, Anthophyta, Magnoliophyta), are one of the groups of organisms whose classification has been affected most radically as molecular data became available. The influential classification scheme published by American botanist Arthur Cronquist in 1981, the Cronquist system, had been increasingly challenged during the 1990s. The molecular data that have become available since around 1990, analysed by cladistic methods, have clarified our views of some relationships and radically changed others. This has made possible a much closer approach to the phylogenetic goal of making classification reflect descent.
The rapid increase in knowledge has led to many proposed changes in classifications, and these pose problems for all users of classifications (including encyclopaedists). By bringing together researchers from major institutions world-wide, and publishing jointly, the Angiosperm Phylogeny Group have sought to provide a stable point of reference, publishing the APG-system (1998). This system deals mostly with higher ranks and, as there are still severe limits to our knowledge, a firm classification is not possible in all cases.
The system is based on two chloroplast genes and one gene coding for ribosomes. This selection of genes from cell organelles is significant; zoological taxonomy similarly uses genes of mitochondria. The genome of cell organelles is separated somewhat from the nuclear genome, both chloroplasts and mitochondria having their own DNA, actually prokaryote DNA. The sequence of nucleotides is subject to a different rate of change compared to nuclear DNA.
The first APG classification was published in 1998 and was superseded in 2003 by a revised version (APG, 2003), known as APG II 2003 or just APG II. Its major innovations were:
- Not to use formal, scientific names above the level of order, but rather to have named clades, such as eudicots, monocots and rosids.
- To place a substantial number of taxa whose classification has traditionally been uncertain.
- To offer alternative classifications for some groups, in which for example a number of families can either be regarded as separate or can be merged into a single larger family. APG II refers to such groups as "bracketed" taxa.
Bracketed taxa are introduced to help cope with the transition from the older, morphologically based classifications to the newer, molecularly-based systems, since the process has tended to produce a number of rather small taxa, e.g. monogeneric families, which are inconvenient for users. As the APG authors note (p. 402), "We generally accept the opinion of specialists... but we also recognise that specialists nearly always favour splitting of groups...".
Independent researchers, including members of the APG, continue to publish their own views on areas of angiosperm taxonomy, and in any case no classification is ever final; it presents a view at a particular point in time, based on a particular state of research. New results are always appearing. Nonetheless the APG publications are increasingly regarded as an authoritative point of reference.
APG II 2003 was "compiled by Birgitta Bremer, Kåre Bremer, Mark W. Chase, James L. Reveal, Douglas E. Soltis, Pamela S. Soltis and Peter F. Stevens, who were equally responsible and listed here in alphabetical order only, with contributions from Arne A. Anderberg, Michael F. Fay, Peter Goldblatt, Walter S. Judd, Mari Källersjö, Jesper Kårehed, Kathleen A. Kron, Johannes Lundberg, Daniel L. Nickrent, Richard G. Olmstead, Bengt Oxelman, J. Chris Pires, James E. Rodman, Paula J. Rudall, Vincent Savolainen, Kenneth J. Sytsma, Michelle van der Bank, Kenneth Wurdack, Jenny Q.-Y. Xiang and Sue Zmarzty (in alphabetical order)."
Institutions represented among the principal authors of the APG II classification include:
- Royal Swedish Academy of Sciences
- Uppsala University, Sweden
- Royal Botanic Gardens, Kew, United Kingdom
- University of Maryland, College Park, USA
- University of Florida, Gainsville, USA
- Missouri Botanical Garden, USA
Contributions also came from many other institutions world-wide.
[edit] References
- ^ Soltis, DE; PS Soltis, PK Endress, MW Chase (2005). Phylogeny and Evolution of Angiosperms. Sunderland, Massachusetts: Sinauer, 237-248.
- Angiosperm Phylogeny Group (2003). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society 141: 399-436 (Available online: Abstract | Full text (HTML) | Full text (PDF)).
- The Angiosperm Phylogeny Group. (1998) An Ordinal Classification for the Families of Flowering Plants. Annals of the Missouri Botanical Garden. [1]
- Haston, E.; Richardson, J. E.; Stevens, P. F.; Chase, M. W.; Harris, D. J. (2007). A linear sequence of Angiosperm Phylogeny Group II families. Taxon 56(1):7-12.
