Naming life (I): what taxonomy is and why it matters

01What is taxonomy?

The terms "taxonomy" and "systematics" are often used interchangeably, although they do not mean exactly the same thing, and how to define them has long been debated (Simpson 1961; Mayr & Ashlock 1991; Winston 1999). In a now classic definition, Simpson (1961) described systematics as the scientific study of the kinds and diversity of organisms and of any and all relationships among them, and taxonomy as the theoretical study of classification, including its bases, principles, procedures and rules. Mayr & Ashlock (1991) adopted the same definition of systematics, but defined taxonomy somewhat more broadly, as the theory and practice of classifying organisms. In practice, taxonomic work rests on three closely linked activities (Winston 1999):

• Identification: assigning a specimen to a group that has already been classified and named.
• Classification: arranging organisms into groups according to their similarities and differences.
• Nomenclature: naming those groups according to rules established for the purpose.

These groups, or taxa, are organised in a hierarchy of ranks (species, genus, family, order, class, phylum, kingdom), with the species as the essential unit (Fig. 1). Today most biologists aim for this hierarchy to reflect the evolutionary history of organisms, and to do so they rely on homologous characters, whether morphological, chemical, physiological or genetic (Winston 1999).

Defining exactly what a species is, however, is more complicated than it seems, and it is one of the longest-running debates in biology. Some 30 different species concepts have been proposed in the literature (Zachos 2016), each emphasising different criteria, such as whether populations can interbreed, whether they can be told apart by their characters, or whether they form a lineage with its own common ancestor. One very influential proposal holds that all of them agree on the essential point, that a species is a separately evolving lineage of populations, and that these criteria are not rival definitions but different lines of evidence for recognising one (de Queiroz 2007). Where the boundary between one species and another is drawn has very practical consequences, as several of the cases in chapter 4 will show.

A scientific name is only useful if it is unique and unambiguous, that is, if it refers to a single kind of organism and is understood in the same way anywhere in the world. A species exists for science, in a formal sense, when its binomial name (genus and specific epithet) is published together with a description of the characters that distinguish it, following the rules of the relevant code of nomenclature (ICZN 1999, art. 13). Curiously, nobody is obliged to follow these codes. Their authority rests on the voluntary cooperation of the scientific community, and a work that ignores them is unlikely to be published in a reputable journal (Winston 1999).

Describing a species, therefore, is not simply a matter of "giving it a name". It requires reviewing the previous literature, sometimes more than two centuries old, examining museum specimens (especially the type specimens of species already described), choosing and studying diagnostic characters, designating new types and depositing them in an accessible collection (Winston 1999). It is slow, painstaking work, and its quality determines whether any other researcher will be able to tell, decades or even centuries later, which organism was meant.

02A little history: from folk taxonomies to international codes

We are all taxonomists

Classifying our surroundings is a deeply human behaviour, much older than science. Ethnobiology has shown that traditional societies all over the world organise plants and animals into hierarchical classifications with shared features, in which the central level is that of "folk genera", which for the higher vascular plants and larger vertebrates often match genera or species of scientific classification (Berlin 1992). But the level of detail is not uniform; it depends on the cultural importance of each group. Artisanal fishers in Paraíba (Brazil) use 308 local names corresponding to 127 scientific taxa of fish, organised into four hierarchical levels (Medeiros et al. 2022). In Zululand (South Africa), by contrast, six folk names are enough for the 58 amphibian species in the area, and these categories are closer to genera or families than to species (Phaka et al. 2019). We classify in most detail what matters most to us. And in some cases, telling similar species apart has very serious consequences. The Florentine Codex (1577), the encyclopaedia of the peoples and culture of central Mexico compiled by Bernardino de Sahagún, drawing among others on the knowledge of indigenous physicians, separates the mushrooms of the hills, which "are good to eat", from some "small mushrooms" called teonanácatl, which "harm the throat and intoxicate" and cause visions "at times frightening" (my translations from the Spanish text). The former appear in the section devoted to mushrooms, and the latter among the herbs that intoxicate (Sahagún 1577; Fig. 2).

Linnaeus and binomial nomenclature

Before Linnaeus, European naturalists often designated each species with a Latin name that was a descriptive phrase of several words, because the name was expected to serve at the same time to name, describe and identify the species (Winston 1999).

Linnaeus (Fig. 3) is often credited with inventing binomial nomenclature, that is, the system that names each species with two words, such as Homo sapiens. But two-word names were already in use before him, especially in genera with few species (Winston 1999). An example helps to understand his contribution. In Species Plantarum (1753), catnip appears with the phrase Nepeta floribus spicatis, verticillis subpedicellatis, foliis petiolatis cordatis dentato-serratis, that is, "Nepeta with flowers in spikes, whorls on very short stalks, and leaves stalked, heart-shaped and with toothed-serrate margins". The phrase distinguished it from the other species in the genus, but it was long and awkward to use. Linnaeus kept it as a description, but added a single word in the margin, cataria, so that the plant could be called simply Nepeta cataria (Linnaeus 1753). In this way, each species came to have a short name to name it and a description to recognise it. The key point is that he did this not occasionally but systematically, for plants in Species Plantarum (1753) and for animals in the tenth edition of Systema Naturae (1758) (Winston 1999). That is why these two works are today the official starting point of scientific names. In zoology, for example, no name published before 1 January 1758 is taken into account (ICZN 1999, art. 3).

From chaos to rules

Linnaeus's system was quickly adopted, but over the following century names proliferated out of control. On top of the flood of new species arriving from the great expeditions, authors renamed species that had already been described or altered existing names according to their own criteria. The need for common rules became ever more obvious (Winston 1999).

The first code intended for general use was the one Hugh Strickland presented in 1842 to the British Association for the Advancement of Science, drawn up by a committee whose members included Charles Darwin and Richard Owen. The Association adopted it in 1846 (Winston 1999; ICZN 1999). From the first International Congress of Zoology (Paris, 1889) onwards, an international code was promoted. In 1895 the International Commission on Zoological Nomenclature was created, in 1905 the Règles internationales were published, and in 1961 these were replaced by the first edition of the International Code of Zoological Nomenclature. The current edition, the fourth, was published in 1999 and came into force in 2000 (ICZN 1999). In botany, the process began with Alphonse de Candolle's "laws" (1867), and the first international botanical code was approved in Vienna in 1905 (Winston 1999). Its successor, now called the International Code of Nomenclature for algae, fungi, and plants, is revised at each International Botanical Congress; the most recent version is the Madrid Code (Turland et al. 2025; Fig. 4). Besides these two, there are other independent international codes: one for bacteria and archaea (Parker et al. 2019), complemented since 2022 by the SeqCode for those known only from their DNA (Hedlund et al. 2022), one for viruses (ICTV 2025) and one for cultivated plants, which governs the names of garden and crop varieties (Brickell et al. 2016).

Both codes rest on two key principles. The first is priority: the first validly published name for a taxon is the correct one. The second is typification: each name is tied to a particular specimen (the type; Fig. 5), which makes it possible to know objectively which organism a name refers to, even when ideas about its classification change (Winston 1999). This second principle, which seems obvious today, took a long time to become compulsory. In botany, citing a type has been a requirement for publishing a new name since 1958, and in zoology only since 2000. In zoology it was until then just a recommendation, although many authors had been designating type specimens long before and journals usually required it (Winston 1999). Since then, every new animal species must include the explicit designation of a holotype or syntypes (ICZN 1999, art. 16.4).

The codes keep adapting to the times. Since 2012, for example, the zoological code has accepted works published only in electronic form, provided that the work is registered in ZooBank, the official register of zoological nomenclature, and that the register names an archive, other than the publisher, responsible for preserving it permanently (ICZN 2012). In practice, however, these requirements are not always met, and Bocak et al. (2025) identified 43 works published in electronic-only journals whose new names are technically unavailable under the Code. For all their imperfections, the rules try to reconcile two opposing needs: having stable names and allowing classification to change as more is learnt about organisms (Winston 1999).

03How much do we really know?

When Winston (1999) published her manual of taxonomic procedure, some 1.4 million species of eukaryotes were known, and estimates of the species still to be described ranged from 5 to 50 million. The most striking figure came from Terry Erwin, who, on the basis of the beetles collected by fogging the canopy of Luehea seemannii trees in Panama, projected that there might be some 30 million species of tropical arthropods (Erwin 1982).

Later estimates have not reached agreement (Fig. 6). Mora et al. (2011) calculated some 8.7 million species of eukaryotes, 86% of which would still be undescribed. Costello et al. (2013) proposed a lower figure of 5 ± 3 million. For insects, Stork (2018) estimated around 5.5 million species, of which only one million have been described, and considered figures of 30 million or more implausible. However, a recent study based on more than 1.6 million insects from Costa Rica identified by DNA barcoding suggests that there could be between 14 and 20 million insect species, and presents this range as a lower-bound estimate (Colwell et al. 2026). Whatever the real figure, even the lowest estimates imply that a large part of the planet's biodiversity still has no name. And some of it may disappear before it gets one. According to one study, between 15% and 59% of recent extinctions, depending on the group and region, would involve species that were never described, so that actual extinctions could be up to twice the number recorded (Tedesco et al. 2014). In the coastal forests of Brazil, for example, some birds may have become extinct before their existence was known (Lees & Pimm 2015). The scale of the problem is still debated (Costello et al. 2013; Mora et al. 2013).

Nor is it true that the pace of discovery is running out. The highest description rates in history, above 16,000 new species a year, have all been recorded since 2015, with a peak of 17,044 in 2020 (Li et al. 2025). And discoveries are not limited to megadiverse regions. Even in Europe, more than 770 new species of terrestrial and freshwater animals have been described on average every year since the 1950s (Fontaine et al. 2012b). An example close to home is Pijnackeria recondita Valero & Ortiz, 2015 (Fig. 7), a stick insect from the Sierra Nevada (Granada, Spain) that I found by chance during some Lepidoptera surveys and that we described using a combination of morphology and DNA barcoding (Valero & Ortiz 2015).

Another revealing figure is the time a species spends "on the shelf". Fontaine, Perrard & Bouchet (2012a) took a random sample of the species described in 2007, obtained the date on which the first specimen was collected for 570 of them, and found that on average 20.7 years passed before their formal description, with cases ranging from zero to 206 years. For flowering plants, Bebber et al. (2014) estimated a lag of about 35 years. In other words, many "new" species are already sitting in museum drawers, waiting for someone with the necessary training to study them.

Stick insects illustrate this situation well. With just over 3,600 valid species (Brock et al. 2026), they are a modest group compared with beetles or wasps, and yet many remain to be described, even in territories with a long history of entomological exploration. In French Guiana, for example, the stick insect fauna had hardly been revised since Lucien Chopard studied it in 1911, despite the abundant material held in European museums. By studying that material together with material from recent surveys, we described a new genus and 16 new species. Half of them had specimens in their type series that had been waiting in collections for decades. These included a paratype collected in Bolivia in 1949, the holotype of one species collected in 1976, and several specimens from the collection that M. E. Le Moult gathered in 1908, such as the holotype of Cladomorphus guianensis, described more than a century later (Conle et al. 2020; Fig. 8).

04What is taxonomy for?

"And what is that for?" is the question any taxonomist hears most often. The answer is not always obvious, because taxonomy almost always works in the background, as a preliminary step on which other decisions depend. It lies behind agriculture, when a pest has to be identified and its natural enemies found; behind environmental management, when an invasive species threatens an ecosystem; behind what we eat and drink, when we need to know which species is really being sold or how to protect a crop; behind public health, when we need to tell which mosquito transmits a disease; behind medicine, when we need to know which snake has bitten a patient; and behind conservation, because red lists, protection laws and the management of natural areas all work with species, and changing the way species are delimited can alter conservation priorities. Between 1996 and 2000, for example, 27% of the changes to the global list of threatened birds were due to taxonomic revisions rather than to real changes in their conservation status (Mace 2004). In all these fields, a correct name gives access to everything that is known about an organism; a wrong name, or no name at all, can lead to wrong decisions. The cases that follow, one per field and all well documented, show with figures what is at stake, both when taxonomy gets it right and when it arrives too late.

When taxonomy gets it right

Agricultural pestsThe papaya mealybug

The papaya mealybug, Paracoccus marginatus Williams & Granara de Willink, 1992, was described in 1992 from material from Belize, Costa Rica, Guatemala and Mexico. It is considered native to Mexico and Central America, where it has never become a serious pest, probably thanks to its local natural enemies. When it reached the Caribbean, by contrast, it became a pest. Since 1994 it has been recorded in 14 Caribbean countries, and in 1998 it reached Florida. With the species described and its area of origin established, from 1999 the United States Department of Agriculture collected several species of parasitoid wasps in Mexico which, once released, reduced mealybug populations by 99.7% at the study sites in the Dominican Republic and by 97% at those in Puerto Rico (Walker et al. 2003). Years later the pest reached India. In the state of Tamil Nadu, farmers had been suffering its damage since 2006, but the culprit was not identified as P. marginatus until 2008. In 2010 three of those parasitoids were imported from Puerto Rico, and one of them, Acerophagus papayae, was mass-reared and released, achieving excellent control of the pest within five months (Myrick et al. 2014). According to Myrick et al. (2014), the economic benefits were between 121 and 309 million US dollars a year, and between 524 and 1,340 million over five years. The project cost about 200,000 dollars in 2010 and 100,000 dollars a year for the following three years (Sharma & Muniappan 2022; Fig. 9).

Invasive speciesSalvinia molesta and its weevil

Salvinia molesta D.S. Mitch. (Fig. 10) is a floating fern native to south-eastern Brazil (CSIROpedia n.d.) which, outside its native range, has become one of the world's worst aquatic weeds (Thomas & Room 1986). It was described in 1972, but because it was sterile it was regarded as a hybrid of species of the S. auriculata complex with no native range of its own, until its area of origin in south-eastern Brazil was located between 1978 and 1982 (CSIROpedia n.d.). The first attempts at biological control used a weevil, Cyrtobagous singularis, collected from plants of the S. auriculata complex. It was released in 1971 in Botswana and Zambia and in 1979 in Fiji, but it did not help to control the weed. In June 1980 a weevil collected in south-eastern Brazil, the native range of S. molesta, was released in Australia; it was at first taken for a form of C. singularis, and within about 15 months it destroyed the fern mat (CSIROpedia n.d.). It turned out to be a different species, and in 1985 it was described as Cyrtobagous salviniae Calder & Sands, 1985 (Calder & Sands 1985). On the Sepik River (Papua New Guinea), where the fern came to cover hundreds of square kilometres of lagoons and channels on which thousands of people depend for food and transport, the release of the weevil in 1982 made it possible to reopen the waterways and for village life to return to normal (CSIRO n.d.). The paper that reported the case in Nature was, in fact, entitled "Taxonomy and control of Salvinia molesta" (Thomas & Room 1986).

ViticultureGrape phylloxera

In the second half of the 19th century, a tiny insect, phylloxera (Fig. 11), reached Europe from North America and caused huge losses to viticulture (EPPO n.d.). In France alone it destroyed around 40% of the vineyards, and the economic blow was so severe that it even showed up in people's height. The children of wine-growing families born in the affected regions and years were between 0.6 and 0.9 cm shorter than others at the age of 20, an effect the authors attribute to the impoverishment of these families during their children's early years. That may seem small, but over the whole 19th century the average height of the French increased by only about 2 cm (Banerjee et al. 2010). In France the insect was described as a new species, Phylloxera vastatrix Planchon, but it turned out to be the same species described earlier in North America as Pemphigus vitifoliae Fitch, now Daktulosphaira vitifoliae (Fitch) (EPPO n.d.). Its North American origin helps to explain the solution. The grapevine species native to North America, which live alongside the insect, are resistant to it. What prevailed, and is still the main method of control today, was grafting European varieties of Vitis vinifera onto rootstocks derived from American grapevine species (Granett et al. 2001), a practice that is widespread in Europe and the Mediterranean basin (EPPO n.d.).

FoodFish labelling

Hebert et al. (2003) proposed identifying animal species from the sequence of a fragment of the mitochondrial COI gene, a technique known as DNA barcoding that is now also used to detect food fraud. A meta-analysis of 141 studies on the labelling of fish and shellfish, a field that has grown enormously thanks to forensic DNA identification, estimated the average mislabelling rate per product at around 8% (Luque & Donlan 2019). It is worth remembering, though, that DNA barcoding can only assign a sequence to a species if a reference library exists, and its power depends on the quantity and quality of the records it contains (Hebert et al. 2003; Geiger et al. 2016).

Public healthDisease-carrying mosquitoes

Malaria is transmitted through the bites of some species of mosquitoes of the genus Anopheles, and in 2024 it caused some 282 million cases and 610,000 deaths worldwide (WHO 2025a). Dengue, Zika, chikungunya and yellow fever, by contrast, are transmitted by mosquitoes of the genus Aedes (WHO 2024), mainly Ae. aegypti in the case of dengue. In 2024 alone, 14.6 million dengue cases and more than 12,000 deaths were reported (WHO 2025b). Telling the two genera apart has practical consequences. Insecticide-treated nets are one of the core tools against malaria (WHO 2025a), but they protect people while they sleep under them, and the Aedes mosquitoes that transmit dengue bite during the day (WHO 2025b). Identification to species level matters too. What was long considered a single African species, An. gambiae, is in fact a complex of several species, distinguished by molecular characters and by their biology (Coetzee et al. 2013; Fig. 12). And the spread of An. stephensi, an invasive Asian species already present in eight African countries, which thrives in urban settings and is resistant to many of the insecticides used in public health, poses an additional challenge for malaria control on the continent (WHO 2025a).

When taxonomy arrives too late or is ignored

Forest healthAsh dieback in Europe

Around 1995 a serious disease of ash trees was detected in Europe, and the fungus responsible was described from material from Poland (Baral et al. 2014). For years, the sexual stage of the fungus was identified as Hymenoscyphus albidus, a harmless species known in Europe since at least 1850. In 2011 it was shown that this name concealed two species considered indistinguishable on morphology: the native one and another, new and pathogenic, now called H. fraxineus (Fig. 13), which turned out to have been introduced from Asia (Baral et al. 2014). By then the epidemic had been spreading for more than fifteen years. In Britain alone, its cost over a century is estimated at £14.8 billion, a third more than the 2001 foot-and-mouth disease outbreak (Hill et al. 2019).

PoisoningToxic mushrooms in China

Gathering wild mushrooms is a widespread custom in much of the world, and many poisonings happen when toxic species are mistaken for very similar edible ones. These poisonings are estimated to kill at least 100 people a year worldwide, a figure that is probably an underestimate (Li et al. 2020), and more than 90% of those deaths are caused by species containing amatoxins (Tuğcan & Akpınar 2025). China is among the regions of the world most affected by these fatal poisonings (Yaneva et al. 2026). There alone, between 2010 and 2020, 10,036 outbreaks of mushroom poisoning were recorded, with 38,676 people affected and 788 deaths (Li et al. 2021). In 2019, the deadliest species in the country was Amanita exitialis Zhu L. Yang & T.H. Li, responsible for 13 of the 22 recorded deaths, with a case fatality rate of 52% (Li et al. 2020). And yet such a dangerous species was not described until 2001 (Yang & Li 2001). Even today, the species responsible for a poisoning often goes unidentified. Of those 10,036 outbreaks, only 38.6% were reported with the name of the mushroom, almost always a common rather than a scientific name, and 65% of the deaths occurred in outbreaks in which the mushroom was not identified (Li et al. 2021).

SnakebiteAn unsuitable antivenom in West Africa

The composition of snake venom can vary greatly from one region to another, and the geographical origin of the venoms used to make an antivenom influences how effective it is in each area (Ainsworth et al. 2020). In West Africa, the introduction of an antivenom made with venom from Indian snakes to treat, among others, bites from the carpet viper Echis ocellatus Stemmler, 1970 led to an increase in mortality among treated patients from 1.8% to 12.1% (Visser et al. 2008; Ainsworth et al. 2020; Fig. 14).

Species conservationThe two African elephants

For decades, African elephants were managed as a single species. In 2001, a genetic study provided evidence that there are in fact two: the savanna elephant, Loxodonta africana (Blumenbach, 1797), and the forest elephant, L. cyclotis (Matschie, 1900) (Roca et al. 2001). However, the IUCN did not assess them separately until 2021. That first assessment showed that the forest elephant had declined by more than 86% over 31 years, making it Critically Endangered, while the savanna elephant, with a decline of at least 60% over 50 years, was listed as Endangered. Until then they had been assessed together, and according to the Natural History Museum in London, separating them made it clear that both were doing much worse than had been thought (Natural History Museum 2021; Fig. 15).

Wildlife diseaseChytridiomycosis in amphibians

From the 1980s, numerous populations of frogs and toads began to disappear, most severely in Australia and the Americas (Scheele et al. 2019). For years, many of these declines were described as "enigmatic", because their cause was unknown (Van Rooij et al. 2015). We now know that behind many of them lay chytridiomycosis, a disease caused by fungi that has contributed to the decline of at least 501 amphibian species over the past half-century, 90 of them presumed extinct (Scheele et al. 2019). Declines peaked in the 1980s (Scheele et al. 2019), more than a decade before the pathogen was identified. When it was finally described, in 1999, it turned out to be a genus and species new to science: Batrachochytrium dendrobatidis Longcore, Pessier & D.K. Nichols, 1999 (Longcore et al. 1999).

05What these cases teach us

The cases above have something in common. When a species is well described and identified, it becomes possible to look for its natural enemies in its area of origin, choose the right treatment or assess its conservation status. When it is not, mistakes are paid for in harvests, forests, human lives and species. Taxonomy rarely makes the news, but many of these decisions depend on it. It is a kind of infrastructure that hardly anyone notices while it works, and whose importance is only appreciated when it fails.

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