Part 6: The binomial system
Figure 1: Carl Linnaeus, by Alexander Roslin, 1775 (oil on canvas, Gripsholm Castle).
Figure 2: Malus sieversii, a wild relative of the domestic apple, in Zhongar-Alatau National Park, Kazakhstan. (Fedorov, 2017; Creative Commons Attribution-Share Alike 4.0 International)
Sharing thoughts in writing is an act of hope. It involves opening up and being exposed to the audience’s reception of the narrative that is being offered to them as a commentary on, or summary of, a broader reading of information that the author hopes others can engage with.
I write this article less burdened with self-doubt than I might have otherwise been, having just been informed that my short series of articles has not only been well received but their content has been recognised by the Scottish Branch committee with this year’s Tom Hall Literary Award. This is an honour for me personally, as Dr Hall was widely regarded within the industry when I first began my career in this sector, his own writings being offerings of a pioneer who helped to progress arboricultural knowledge, something that I personally benefitted from in my formative years. His influence as an indirect mentor was to share a constructive lead, benefitting many people in this field before formal institutions of higher education had embraced the subject at degree level. I recall reading many of the papers in the issues of the Arboricultural Journal that he edited and was guided to read many books through the reviews he contributed to those same journals over many years. His articles were a most useful resource during my plant science degree and then while I was studying my RFS Cert Arb, AA Tech Cert and later preparing for the challenge of the old PD Arb (RFS).
In the spirit of the Tom Hall Literary Award, and for those who voted for me at the Scottish Branch, I will endeavour to honour his name and continue to share my insights through this thread I have offered.
This exploration of how plants have been classified has so far taken us on a journey through time and across quite some word count, as we have explored how different cultures have perceived and made sense of the world around them. The narrative has attempted to unpick some of the documented evidence, and where no direct evidence can be found, to search out other ways to make sense of the world by interpreting some of the material that others have left through their embodied experience, as anthropological or linguistic research and writings.
An inductive reasoning process that seeks to inform not only how plant names have come about but why is not always self-evident from a cursory reading. Examples of how the subtle transition in the evolution of the words we use to name plants, as understood by linguists, unveil a deeper understanding of the development of plant classification by different cultures long before the origin of the scientific binomial system of classification consolidated by Carl Linnaeus in 1753 (Fig. 1). More specifically, we have learned how original names have changed in meaning and been purposefully changed throughout their long cultural histories, sometimes leaving a track we can follow in the written or oral historical records that we can use to help build an understanding of how our landscapes once were. This includes gleaning what remains from the underlying ecology of plants and the communities they are part of – such as the shade intolerance of the oaks, which are found naturally regenerating in open-grown meadows but not under the cover of wall-to-wall woodlands as we were once taught as part of an early 20th-century misinterpretation of the pollen records, as you will recall from the first article in this series (winter 2021, issue 195).
We now find ourselves at a point in the story where we can dig deeper into the modern scientific tradition of plant naming while focussing on the practical application of its underlying process: how it can be utilised but also how it may be limiting.
The international code for the naming of names
I don’t know if any readers are aware of or have even seen the 203-page International Code of Nomenclature for Algae, Fungi and Plants — known as ‘The Shenzhen Code’ for the city where this latest version of the guidelines was adopted in 2018. Its principles try to administer a universal system that can be independent of a native language or culture but will also result in names that are more than mere monikers. They provide information such as other plant relatives of a species and, in some instances, can note where it was found, the name of the discoverer (the authority), or a striking characteristic. The code also imposes order on the technical task of converting a person’s name into a Latin or Greek scientific one: following convention, discoverers often name new species after people they know or admire.
Today, nomenclature is regulated by the International Association for Plant Taxonomy, based in Bratislava, Slovakia, whose Committee for Vascular Plants works to ensure scientists everywhere use the same standards. It also wrestles with knotty issues such as renaming plants, which can be a challenge given that scientists currently recognise almost 400,000 different plant species and attribute a name using the binomial scientific system of plant classification to each one, sometimes without universal agreement.
A recent example of a challenge involved three Chinese botanists who proposed that the scientific name of the apple tree should be changed from Malus domestica to Malus pumila. Malus domestica is the older name and otherwise met naming guidelines, so the new name was rejected by the committee and the older one retained as Malus domestica (Suckow) Borkh – a name first published in Theoretisches-praktisches Handbuch der Forstbotanik und Forsttechnologie (2:1272) in 1803. The Shenzhen Code’s ‘rule of priority’ was upheld; the first officially published scientific name is the one that remains ‘accepted’.
An arborist or horticulturist looking to find the correct name to use for a plant can be confronted with a minefield to step through. In an attempt to provide a way to negotiate this challenge, it is interesting to explore the story of the cultivated apple. Currently, as we have seen, the scientific name for the cultivated apple is a ‘contested’ name. What this means in practice is that other scientific names have been proposed over time based upon new scientific evidence and committed to print and also used online. Former scientific names, as proposed or published in the past, even if this was only once, may still be in the public domain and incorrectly used as synonyms. Some of these anomalies come about through the recognition that any domesticated plant is derived from a cultivated form, and therefore is the product of years of selection from something that would otherwise be found in the wild. In the case of the cultivated apple, we would call these original wild plant populations the ‘wild progenitors’.
For apple, in addition to the accepted name Malus domestica, we also find a proposed name in circulation – Malus sieversii L. –and this name is accepted by some learned scholars whom we call ‘authorities’. As you’ll recall, the L. following an italicised name reminds us it was first recognised and described using the binomial system by Linnaeus himself. As diligent arborists, we look to explore this wild species further, to ensure we include the correct name in a report, and so we delve a little deeper into the detail online or in book references, and we find that not only is Malus sieversii L. believed to be the wild relative of Malus × domestica Borkh., but it has been described by ecological scholars as being native to central Asia where it is still found today. Further, we find that it breeds true to its type and can reproduce by cross-pollination, in line with the concept of what defines a species (i.e. it freely interbreeds and results in fertile offspring), but also it is geographically isolated from other Malus and forms a dominant tree mosaic in the mountains of southern Kazakhstan (Morgan and Richards, 1993). From a conservation perspective, the wild population of Malus sieversii was last ecologically surveyed in 2007 and was then classified by the International Union for the Conservation of Nature as part of its Red List of globally endangered species. It is close to extinction in its former range but is described as being widely grown in its domesticated and interbred form (IUCN, 2007). The IUCN recommended in 2022 that the wild population needs to be revisited as it remains at risk of declining further.
Unfortunately, our story of Malus sieversii L. as the accepted name for our apple is not yet over. While it is a name accepted by some authorities, it is not the only name that you will uncover if you go back through the literature. Indeed Kew currently recognises 305 synonyms (alternative names), mostly other Malus species names, but those who read deep enough will find that it has also previously been described as a pear and given the name Pyrus sieversii Ledebour. This name was proposed by the German naturalist Carl Friedrich von Ledebour, a man who saw the tree growing in the Altai Mountains in 1833 and was either unaware of the previous attempts at its classification or not in agreement with the previous author’s treatment of the species, so he decided to classify it differently. He made what has been subsequently determined by many other taxonomists to be an unconvincing alignment with pears. In his defence, pears are actually very similar to apples, and also to Sorbus and quinces and a whole host of small trees that are all members of the subfamily of Maleae (incorrectly Pyreae). This is a tribe of plants that you may come across as members of the rose family (Rosaceace) which includes herbs, shrubs and trees amongst its number. Most species in this family are deciduous, but some are evergreen. They have a worldwide distribution but are most diverse in the northern hemisphere.
Flowers of plants in the rose family are generally described as ‘showy’. They are radially symmetrical (which means body parts are arranged around a central axis) and almost always hermaphroditic – with male and female parts found in the same flower. Rosaceous plants generally have five sepals, five petals and many spirally arranged stamens. These later traits were used by early botanists to classify these plants into similar-looking groups based on their floral make-up (Fig. 3). Within this rose family are found a group that have similar fruit formation, for which they became known as ‘pome fruit’, a group of economically important plants that are valuable for providing fruit crops for human nutrition, vitamin supply and health. Where these species of pome-fruit trees fail the ‘species test’, i.e. do not freely interbreed and produce fertile offspring, they are usually given separate species names (e.g. Malus domestica, Malus sylvestris etc.).
You can see from this melee of apple nomenclature that it can be relatively easy for different authors to give different names over the years to the same plant. While it does make for a confusing story to unpick in the modern age of the internet, you can at least begin to understand why it is important for our botanists who specialise in taxonomy to follow an agreed international nomenclature code.
So how does the lay reader, who has acquired various tree books over the years or is exploring an online search engine to find the name of the cultivated apple, avoid getting lost in the potted history that this article has sought to expose? My first port-of-call is Plants of the World Online (POWO: powo.science.kew.org/), where you will find the current accepted name for the domestic apple plus its 305 synonyms, non-accepted or analogous names that may also be in use elsewhere as a result of prior designations over the years. POWO is an international collaborative programme with the primary aim of making available digitised data of the world’s flora gathered from the past 250 years of botanical exploration and research. A quick search of the POWO home page results in the return of the cultivated apple’s accepted name as Malus domestica (Suckow) Borkh.
The adoption and use of the international binomial system is not always an easy trail to follow. However, it does provide a universal approach, giving those of us who do not classify plants but who want to reference the correct name with the means to share and describe the same plant with a high degree of orthodoxy, so we can access the same information from the same sources with confidence to allow others to compare the metrics we report or details we have described. The important point here is that the authority, the initials and names after the binomial are just as important to record as the binomial itself if we are to truly enable the comparison of apples with apples (excuse the pun).
Figure 3: Structure of a pome, showing the corresponding parts of an apple flower and an apple fruit: sepals, the ovary and surrounding accessory tissue, and the pedicel (stalk). (Credits: Apple blossom (from Yearbook of USDA 1898, via ClipArt ETC, license); red delicious half (J. Smith, via Wikimedia Commons, CC BY-SA 3.0). Images modified from originals.)
The application of scientific names
The scientific classification of plants, like the classification of anything else, is based on the idea of grouping together things which are similar. So, we recall that species are plants (in our context) that have a range of similar characteristics and are capable of freely interbreeding with each other (i.e. pollen from the flower of one plant can successfully fertilise the egg cells of another plant so that they will grow to produce seed which will germinate and produce a new plant). Species are grouped together into collections which have fairly similar characteristics. These groups are called genera (singular: genus). Genera are grouped into collections of genera which have a relatively great similarity to each other: these are called families. Scientists name all plants using the system that describes the genus and species of the organism. To recap, the first word in the name indicates the genus and the second is the species. The first word is capitalised and the second is not; these words are usually italicised or underlined, and the initial or after-name which follows is the authority (the discoverer). The accepted name Quercus robur L. (for Linnaeus) was first published alongside 7087 others in the new binomial scientific format by Carl Linnaeus in his Species Plantarum in 1753 (Kew, 2022a and see ‘European oak’ below).
Attempts to classify species are based on the longstanding view that species are real biological entities. Ideas about the best way to define species have changed over time, due in part to the increasing amounts and new types of data available. Aristotle’s classification scheme focused on morphological (form and structure) similarities and differences, with species – a term that translates from Latin as ‘kinds’ – assumed to be entities that are unchanging and static from their inception and distinct from one another. Carl Linnaeus was among the first to note hybridisation as a possible mechanism for the initiation of new species.
In the mid-1800s, Charles Darwin proposed that species arise through gradual change over time, embracing the ideas that traits can evolve and that new species can form through the divergence of existing ones. This led to the notion that the evolutionary relationships among organisms can be depicted using a branching structure that is today known as a phylogenetic tree. Darwin’s view of evolutionary relationships among organisms remains the guiding principle for modern taxonomy. Indeed, modern species concepts have universally moved away from the type-specimen approach of early taxonomists, which used an individual specimen to establish the defining characteristics of a taxon, to focus instead on population-level features, especially the range of individual phenotypes (sets of observable characteristics). Modern concepts pay particular attention to the delineation of independently evolving lineages. Ecological, morphological and cytological (cell-level) data can all be of use in identifying lineages, but challenges can arise when data of different types suggest different conclusions. Modern genetic data and analytical tools can help to address these challenges. DNA sequence data complement ecological, morphological and cytological data by providing information on the genetic makeup of individuals and populations and making it possible to draw inferences about their genetic relatedness and history.
The biological species concept
John Ray was the first person to produce a biological definition of species, in his 1686 History of Plants: ‘no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species ... Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa’ (cited in Mayr, 1904:256).
This concept was taken up and improved upon by Ernst Mayr (1942), on whom we confer the status of the originator of the modern accepted concept of biological species, which sees species as groups of potentially or actually interbreeding individuals that are reproductively isolated from other such groups. The vast majority of biologists now agree on the value of this basic approach, but they also appreciate the complexities involved in it in light of emerging evidence that hybridisation is a key part of the history of many taxonomically recognised species. Species are one of the fundamental units of comparison in virtually all subfields of biology, from anatomy to behaviour, development, ecology, evolution, genetics, molecular biology, palaeontology, physiology and systematics.
The phylogenetic species concept
The phylogenetic species concept is largely compatible with the biological species concept, but it has an added emphasis on shared ancestry – that is, individuals are inferred as belonging to the same species if they are descended from a common ancestor and share lineage-specific mutations. As a natural consequence of the relentless processes of mutation and genetic drift (i.e., the random changes that occur in the frequency of alternative forms of a gene that arise by mutation due to chance), reproductively isolated species diverge at the DNA level over time, with or without changes in their morphology, or genetically. The central idea here is that when populations are geographically separated, they will diverge from one another, both in the way they look and genetically. These changes might occur by natural selection or by random chance (i.e., genetic drift), and in both cases they result in reproductive isolation.
The subspecies designation
Some classification schemes also recognise taxonomic groups below the species level. The earliest definitions of subspecies distinguished sets of populations whose members share pattern, colour or morphological attributes not found in other, geographically separated populations of the same species. Most modern concepts, including one suggested by Darwin, rely on the notion of a partial restriction of gene flow. Virtually all modern definitions of subspecies follow the spirit of these original definitions, with the general view being that subspecies are groups of actually or potentially interbreeding populations that are phylogenetically (i.e., in a way that relates to evolutionary development and diversification) distinguishable from, but reproductively compatible with, other such groups (Hamilton & Reichard, 1992).
Figure 4: The influence that people had on the distribution of oaks in Europe may have been far greater than is currently accepted. One argument put forward in an interesting article suggests that the northward spreading of oaks from their glacial refugia after the last ice age was actually the result of the northward spreading of humans from the same areas. (Source and further information: © Old European Culture Blog, 13 November 2014)
For most European tree species, the differentiation patterns are closely associated with the history of the postglacial recolonisation. Three main refugia (refuges) of oaks are supposed to have existed during the last glacial maximum – the Würm/Weichsel – (20,000 years ago); these were located on the Iberian, Apennine and Balkan peninsulas. Currently there are five recognised subspecies of Quercus robur which represent geographically isolated races that have been accepted into the International Plant Names Index and World Checklist of Vascular Plants 2023 (available at www.ipni.org and powo.science.kew.org):
- Quercus robur subsp. broteroana O. Schwarz – native to Portugal and Spain
- Quercus robur subsp. brutia (Ten.) O. Schwarz – native to Albania, Bulgaria, Italy, Yugoslavia
- Quercus robur subsp. imeretina (Steven ex Woronow) Menitsky – native to North Caucasus, Transcaucasus
- Quercus robur subsp. pedunculiflora (K. Koch) Menitsky – native to Bulgaria, Greece, Iran, Kriti, Krym, North Caucasus, Transcaucasus, Turkey, Turkey-in-Europe, Yugoslavia
The divergence of species during the Würm geological glacial stage began about 70,000 years ago and can be geologically divided into early, middle and late phases. Weichsel is the name for the palaeontological (fossil) stage, and it can be correlated with the Würm glacial stage timelines. Both mark the major division of late Pleistocene deposits and time in western Europe (the Pleistocene Epoch began about 2.6 million years ago and ended about 11,700 years ago). Science comes to our aid here in helping us follow the trail through time on the migration paths and transition in the three temporarily isolated refugia where trees became the geographically distinct subspecies of oak we recognise today (Fig. 4). Radiometric dating techniques and pollen analyses have provided an excellent chronology of Weichselian events including of fossil pollen evidence in Europe for oak’s progress north. A complex picture emerges at the end of the Würm stage: the retreat of the final glaciers was in fact a series of minor retreats and advances. It is currently accepted that oak started to emerge from refugia in southern Europe as the ice caps began to retreat at the end of the Younger Dryas period around 9500 BCE. Oaks arrived in north central Europe by the 8500 BCE and reached eastern Britain by 7500 BCE.
Getting to grips with plant classification
I have personally found that the only way to gain a fair grasp of plant identification and culture is to learn plants by their family names as well as their genus and species names. As technical students at University of Cambridge Botanic Garden in the mid-1990s, my fellow students and I were familiarised with the characteristics of plant families, and this enabled us to place a particular plant in a family and make an educated guess as to its growing requirements, susceptibilities and other likely characteristics. Some plant families are of much greater significance and importance than others in amenity arboriculture, and taking the time to learn their characteristics is very worthwhile. It is to this group of plant names that we will turn next time before returning to the vagaries of the results of human intervention in manipulating the direction of plant development and selecting plants from the wild that were suitable for a specific purpose. Humans learned to modify plants that were not yet ideally suited to our purposes in diverse ways, leading to the origins of cultivated plants.
An, M., Deng, M., Zheng, S.-S., Jiang, X.-L., Song, Y.-G. (2017). Introgression threatens the genetic diversity of Quercus austrocochinchinensis (Fagaceae), an endangered oak: a case inferred by molecular markers. Frontiers in Plant Science, 21 February 2017: 8:229.
Cottrell, J., Munro, R.C., Tabbener, H.E, Gillies, A., Forrest, G.I., Deans, J.D. & Lowe, A. (2002). Distribution of chloroplast DNA variation in British oaks (Quercus robur and Q. petraea): the influence of postglacial colonisation and human management. Forest Ecology and Management 156 (1–3): 181–195.
Fischer, T.C., Malnoy, M., Hofmann, T., Schwab, W., Palmieri, L., Wehrens, R. & Martens, S. (2014). F1 hybrid of cultivated apple (Malus × domestica) and European pear (Pyrus communis) with fertile F2 offspring. Molecular Breeding 34(3), 817–828.
Hamilton, C.W. & Reichard, S.H. (1992). Current practice in the use of subspecies, variety, and forma in the classification of wild plants. Taxon 41(3): 485–498.
IUCN (2007). Malus sieversii – Red List (accessed: 18.01.2023) Available at: www.iucnredlist.org/species/32363/9693009
Kew (2022a). Plants of the world online (accessed: 20/12/2022). Available at: powo.science.kew.org
Kew (2022b). Plants of the world online: Malus domestica (accessed: 13.01.2023). Available at: powo.science.kew.org/taxon/urn:lsid:ipni.org:names:726282-1
Mayr, E. (1904). The Growth of Biological Thought. Cambridge, Massachusetts and London.
Mayr, E. (1942). Systematics and the Origin of Species. New York.
Morgan, J. & Richards, A. (1993). The Book of Apples. London.
Old European Culture (2014). How did oaks repopulate Europe? Online blog (accessed 13/01/2023) Available at: oldeuropeanculture.blogspot.com/2014/11/how-did-oaksrepopulate-europe.html
Kevin Frediani is the Curator of the Botanic Garden and Grounds, University of Dundee.
This article was taken from Issue 200 Spring 2023 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.