APG - classification by consensus

Classifications of plants were historically compiled by one or a few people, who often disagreed with each other on which characters should be emphasised. Technological improvements in DNA sequencing in the 1990s allowed botanists to break away from this tradition and led to the APG system, the fourth iteration of which has just been published. 

Linnaeus’ classification system 

When Linnaeus classified the flowering plants (angiosperms) in his book Species Plantarum (1753), he used the numbers of male and female parts in flowers as the main characters. He realised that this inevitably led to an artificial system, with unrelated plants being put together in many cases, and many of his contemporaries were shocked by the focus on sexual organs. 

Over the next two and a half centuries, many botanists endeavoured to come up with more natural systems, using more characters, with the intention of recognising groups that more closely reflected relationships. However, even readily recognisable groups such as orchids or legumes were treated as one family in some classifications and as several families in others. 

Following important works on plant classification by members of the de Candolle family, de Jussieu and others, Kew botanists, George Bentham and Joseph Hooker, developed a system of classification in the 19th century that was in use in Kew’s Herbarium and elsewhere until a few years ago. In the 20th century, Cronquist (in the USA) and Takhtajan (in Russia) developed widely used systems. 

Development of the APG system 

Three further versions of the APG classification were published in 2003, 2009 and 2016, each with multiple compilers and contributors. The most recent version, APG IV published this month, has ten compilers and 15 contributors from six countries and is the result of a workshop hosted by Kew in September 2015. An online survey also provided useful background about the views of botanists and other users of the classification around the world (Christenhusz et al., 2015). 

Each version of APG has included a list of families and/or genera of uncertain position – in 1998, 25 families fell in this category, but by 2016 the list had shrunk to seven genera, demonstrating the increase in knowledge as more plants have been sampled and more DNA regions have been investigated. The number of higher-level groups, formal and informal, has also increased, as a reflection of increasing confidence in the relationships of families to each other. Changes in APG IV on the basis of published studies include placement of Petenaea in its own family (Petenaeaceae), recognition of Kewaceae for the genus Kewa (previously included in Hypertelis in Molluginaceae) and reorganisation of some families that were known to be problematic. For example, several genera had been moved from Icacinaceae into the previously monogeneric Metteniusaceae. 

Many botanic gardens and herbaria (including Kew) have adopted the APG system (e.g. Wearn et al., 2013) - APG IV will not be the final version, but the decreasing number of changes means that reorganising collections to reflect changes is becoming less arduous. The resulting system allows greater predictability than previous classifications because the groups that are recognised reflect evolutionary relationships of the flowering plants.