The species does not exist

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In this article, we discuss the question "What is a species?". It's a concept you've probably already heard of, and you'll probably have no trouble identifying a few species, even if their Latin names escape you. However, what seems simple at first glance is in fact not so simple at all, especially when you look at it from a paleoanthropological perspective. But remember one thing: species don't exist, but the notion of species does.

Where does the term species come from?

The term species comes from Latin and means “appearance, aspect, type”. So, at first glance, a species refers to a group of individuals or objects with physical similarities. As early as 1734, Réaumur, a French physicist and naturalist, defined species as “all living beings, bodies, substances, figures or geometric shapes with similar properties” [9].

In the 18th century, European naturalists embarked on voyages across all oceans and continents, discovering hitherto unknown animals and plants. It then becomes essential for them to classify and name them. At that time, species classification was based mainly on physical criteria. These observations are therefore subject to a certain degree of subjectivity, as the perception of what is similar or different sometimes varies considerably from one observer to another. As a result, some species are given not only several names, but also different denominations in different languages.

Carl von Linné established the species as the basic heading of his classification system. In 1758, in the 10th edition of his Systema Naturae, he created the species Homo sapiens (our species) and classified it in the primate order. Linnaeus’ classification system is still in use today, although many corrections have been made to his original classification.

For Linnaeus, classification was simply a means of making the divine plan of creation intelligible, without any idea of evolution. In his view, “there are as many species as the Infinite Being produced forms in the beginning” [6]. Linné had a static vision of species, based mainly on morphological criteria.

How do you name a species?

During the 20th century, international botanical and zoological congresses sought to eliminate the confusion caused by the multiple names given to species and thus establish a nomenclature. According to this code, the name of a species is composed of the name of the genus, beginning with a capital letter, followed by the qualifier of the species, which begins with a lower case letter, followed by the initials or abbreviation of the name of its discoverer. Genus and species names are always written in italics.

ex: Homo sapiens -> Homo = genus, sapiens = species.

In addition, the principle of seniority has been established, meaning that the first name given to the species is the one that will always be retained. A species can change genus, but its name must remain.

e.g. Pithecanthropus erectus, which became Homo erectus.

Species definitions

From a classification point of view, the species represents the smallest entity, just above the individual. Nevertheless, many definitions of “this smallest entity” are proposed.

Classification of the living world with the species at the end
Figure 1: Classification system for the living world.

With Darwin, the static conception of the species as conceived by Linnaeus is no longer adequate. Indeed, according to Darwin and his book The Origin of Species (1859), the classification of organisms should reflect their evolutionary history.

Thus, at the end of the 19th century, the species can no longer be considered as fixed, perfectly defined, with immutable borders, existing since ever and for ever since :

– Species evolve(click here to read our article on evolution)

– More and more fossil species are being discovered- Species contours are difficult to establish in both space and time

From the twentieth century onwards, numerous definitions of the term species have emerged.

For example, in 1937, Theodosius Dobzhansky, biologist, geneticist and evolutionary theorist, was the first to propose a biological definition of species. According to him, a species corresponds to “the stage in an evolutionary process where several groups that were previously in an interbreeding relationship […] separate into at least two distinct groups, between which there can no longer be any interbreeding” [3] . Here, the term interbreeding is to be understood as interfertility. However, this definition has been criticized because it describes the mechanisms of speciation rather than what a species is in itself.

This has given rise to many other concepts of the species, such as the ecological concept of the species. This defines a species in terms of its ecological niche, i.e. the set of environmental conditions in which it lives and perpetuates itself.

In 1942, Ernst Mayr, ornithologist, biologist and geneticist, proposed a definition of biological species directly related to that proposed by Dobzhansky, and which is still taught in schools today. To do this, he relies on the criterion of interfertility. The idea is simple: if living beings can reproduce and produce fertile offspring, then they belong to the same species.

However, the definition of a species is not quite so simple. The interfertility criterion poses several problems. For example, this is not a truly operational criterion, as we can never really verify this inter-fertility, notably because of the geographical and temporal dimensions that we cannot “control”.

For example, until recently we thought that polar bears and grizzly bears couldn’t hybridize and produce fertile offspring, but that’s not the case! Indeed, due to global warming, these two species are increasingly crossing paths in the environment, and the hybrids born of these encounters (called grolars or pizzlies) are indeed fertile! Does this mean that polar bears and grizzly bears are the same species? No, this would mean wiping out thousands of years of separate evolution resulting in genetic, morphological and other differences between the two species.

Since then, other definitions have been formulated, such as the ecological species (Andersson, 1990) or the genetic species (Mallet, 1995). In 1997, R.L. Mayden listed at least 22 different concepts of species.

Why several definitions for a single “reality”?

The reason we find it so hard to agree on a single definition of species is quite simply that species does not intrinsically exist in nature. There are only notions or concepts of what a species might be. Indeed, the desire to name and categorize the world is something unique to us humans. The notion of species was therefore invented to help categorize living things into different boxes such as kingdom, order, family, etc. But in nature, there are only individuals. Whether you call the flower in your garden a “dandelion” or a “daisy” makes no difference to this individual, who will continue to exist no matter what you call him. So the concept of species is a human invention. However, nature is far more complex than simply classifying living things into distinct categories.

Living things are constantly evolving, but they also have unique intra-specific characteristics. There may also be other types of major differences within a single species, such as sexual dimorphism. For example, in great apes such as gorillas, there are significant differences between males (presence of a sagittal crest) and females (absence of a sagittal crest).

Beyond intra-species differences, it can sometimes be difficult to determine the degree of difference at which a species is considered to be different.

How can the notion of species be used in paleoanthropology?

How can we distinguish between different species on the basis of fossil remains that are often fragmentary and badly damaged? This question is still at the heart of debates within the paleoanthropology community.

When studying fossils, certain criteria cannot be verified, such as inter-fertility, which Ernst Mayr has been criticized for. As a result, morphological characteristics are most often used to define a species. This list of characters is called a “diagnose”. It is based on a fossil chosen as a reference, called the “holotype”. Nevertheless, it’s sometimes difficult to know where to draw the line when integrating or rejecting a trait for a species.

Revolutionary concepts of the species were born to alleviate this problem. This is the case, for example, of paleontologist and evolutionary systematist Georges Gaylord Simpson, who defines a species as a phyletic lineage evolving independently of others, with its own distinct and unitary evolutionary roles and tendencies.

However, evolution is a slow process, taking place over many millions of years, so the morphological traits studied don’t change all at once. It takes several thousand years for the characteristics of a species to appear and become permanently fixed in the population. A striking example isHomo neanderthalensis. In fact, the first Neanderthal-type morphological features appeared as early as around 300,000 years ago in certain populations known as pre-Neanderthals. Nevertheless, the full range of morphological features specific to Neanderthal were present around 140,000 years ago, by which time the species Homo neanderthalensis was considered to have existed. Nevertheless, should we consider pre-Neanderthal populations as already being in some way Neanderthals or as belonging to another species, Neanderthal’s ancestor?

Moreover, it’s sometimes difficult to differentiate between sexual dimorphism and morphological variation within what we consider a species, or a different species. We then subjectively choose to set limits to define species by including or excluding certain fossils, and these choices can also evolve over time.

At the end of the 20th century, Cracraft (1983) formulated the phylogenetic concept of the species in order to overcome the difficulties outlined above. According to him, the species is the smallest diagnosable group of individual organisms within which there is a parental pattern of ancestry and descent. In 1990, Nixon & Wheeler reformulated Cracraft’s definition: “the smallest aggregation of populations (sexual) or lineages (asexual) that can be diagnosed by a unique combination of character states in comparable individuals (semaphoronts = an organism as seen in a certain period of time, however brief, but not a snapshot). A character state is an attribute inherited from a common ancestor and present in all comparable individuals”.

How does paleoanthropology work in practice?

At present, the definition of the species remains as vague as ever. However, the only concept of species that can be tested and falsified is the phylogenetic concept, even though it has been much criticized for the taxonomic inflation it engenders, i.e. the creation of additional distinct species and genera.

Nevertheless, the phylogenetic concept of the species is the only one that can test and prove the existence of an evolutionary lineage.

In paleoanthropology, the specimens studied are fossils. Thus, the distinction between species is necessarily based on morphological and possibly genetic characteristics. This is part of the paleontological concept of the species.

Nevertheless, we must be careful not to mix up the different concepts, especially when trying to apply biological concepts to fossil species, such as the criterion of inter-fertility. For example, Homo sapiens and Homo neanderthalensis are two different “paleontological” species. Once again, this does not rule out the possibility of hybridization between the two species! However, to lump them together as a single species would be to overlook their separate evolutionary histories.

In conclusion, it’s important to remember that if everything has the same name, it becomes difficult, if not impossible, to study evolutionary histories. Taxonomy and classification in themselves are artificial constructs. Only phylogeny, the history of life, has a biological reality. However, to study phylogeny, it is necessary to assign names to the entities under study. At the end of the day, the name doesn’t really matter. The most important thing is to describe the people you’re talking about in such a way that everyone, whatever they call them, can understand what you’re talking about.

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Bibliography :

[1] Arnould P., ” Biodiversité : la confusion des chiffres et des territoires “, Annales de géographie, vol. 651, no. 5, 2006, pp. 528-549.

[2] Buffon G., Histoire naturelle, générale et particulière. Tome II, 1749.

[3] Darwin C., On the Origin of Species by Natural Selection or the Preservation of Favored Races in the Struggle for Life, ed. orginale 1859, 2022, éditions Flammarion.

[4] Dreuil D., “Theodosius Dobzhansky”, in P. Tort (ed.), Dictionnaire du darwinisme et de l’évolution, PUF, 1996, tome 1, p. 1239-1255.

[5] Gontier T., Animal et animalité dans la philosophie de la Renaissance et de l’Age Classique, Éditions de l’Institut supérieur de philosophie, 2005.

[6] Linné C., Genera plantarum eorumque characteres naturales secundum nuemrum, figuram, situm & proportionem omnium fructificationes partiums, 1737.

[7] Linné C. Systema Naturae, 1758.

[8] Mayden R.L., “A hierarchy of species concepts: the denouement in the saga of the species”, in M. F. Claridge, H. A. Dawah, M. R. Wilson, Species: The units of diversity, London, Chapman & Hall, 1997, pp. 381-423.

[9] Réaumur, Insectes, Premier discours, Second mémoire, 1734, page 52, in Gallica

[10] Reed, et al, “Hominin nomenclature and the importance of information systems for managing complexity in paleoanthropology”, Journal of Human Evolution, vol. 175, 2023.

[11] Simpson, Wiley, Systematic Biology, Volume 27, Issue 1, March 1978, Pages 17-26,

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