Angiosperm Phylogeny Group

The Angiosperm Phylogeny Group is an international group of systematic botanists who have come together to try to establish a consensus view of the taxonomy of flowering plants in the light of the rapid rise of molecular systematics.

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; a revised version was published in 2003 (APG, 2003), and is 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.

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.

References

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)).

Botany | Plant taxonomy

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