Given the great diversity of living beings that surround us, human beings have tried to classify them into discrete categories, according to a series of criteria that have changed between utilitarianism and kinship relations, throughout history.
The first classifications divided living beings according to very coarse traits: if they moved, they were animals, if not, plants; and within them, many groups were defined according to an evident behavior, a series of visible traits or according to their usefulness for the human being.
Brief history of the classification of living beings
But this very basic approach was revolutionized when Carl N. Linnæus developed his binomial nomenclature in 1753, which designated species according to two words: a generic name, capitalized, and a specific epithet, always lowercase. Thus, in the species Homo sapiens , Homo is the genus, and sapiens the specific epithet. In addition, he created higher categories —families, orders, classes…— that grouped living beings according to their similarities.
Today this rigid scheme of Linnæus is crumbling with the entrance of genetics. It was Willi Hennig , well into the 20th century, who introduced the concept of cladistics : the study and classification of living beings based on their phylogenetic relationships, that is, on the branches of the evolutionary tree to which they belong.
This is currently the new way of classifying living beings; for this reason we say that birds are dinosaurs —because they belong to the same branch—, with the same sense that we affirm that humans are primates . But it is still preserved, inheritance of Linnæus, that binomial name, that way of describing the species. The species is, in the end, the basic unit of classification of living beings, and every living being belongs to one, and only one.
The definition of ‘species’
Darwin said, in The Origin of Species , that «no definition [of ‘species’] has yet satisfied all naturalists; yet every naturalist vaguely knows what he means when he speaks of a species.’
Since he wrote those lines more than a century and a half ago, we have come a long way in understanding living beings. But when it comes to the definition of ‘species’, and contrary to what might be expected, everything has become much more complicated.
Currently, there are multiple definitions of ‘species’ , but none are satisfactory enough. Some are so restrictive that they do not include all entities that biologists consider to be species. Others are too broad, including entities that biologists attribute to categories higher than species. In addition, there are some examples of populations that could hardly fit the definition of species, as it is understood, or that could be considered two species at the same time. Something that would go against that basic principle: that every living being belongs to one, and only one species .
One of the clearest examples is that of the so-called ‘ ring species ‘, groups of populations of organisms that follow each other, like a chain around a biogeographical barrier, in which a genetic flow is maintained throughout the entire the chain, but whose populations at the ends have changed so much with respect to each other that they cannot hybridize and produce fertile offspring.
If we were to assume reproductive capacity as a defining feature of the species —which is usually done—, all the populations in the chain would be of the same species. However, if the intermediate populations were to become extinct and only those at the extremes remained, it would necessarily be necessary to speak of different species. Ring species are, therefore, formed by populations that may belong to a single species or to two different species, depending on the criteria used .
What if the species did not exist?
The problem with the term ‘species’ is that it is still a way of classifying into discrete categories, something that has a continuous character, such as groups of populations of living beings. The case of the ring species is an obvious case, which allows us to observe this continuity in space : how one ‘species’ can end up being another, without producing a fracture point in its area of extension. A kind of gray scale that does not allow you to indicate exactly where white ends and black begins.
This character of continuity is also observed in time. Populations change over time, in a process called biological evolution , and these changes are gradual. Except for very particular exceptions —such as polyploidy in plants—, there is no specific generation in which a female of one species has an offspring belonging to a different species. The change process happens over multiple generations and there is no real barrier.
Real populations and lineages in nature are not limited by those barriers that we conceptually construct to make our classifications more comfortable. Species, understood as discrete entities, are effectively not real. They are human constructs that facilitate the work of classifying, studying and understanding living beings.
In fact, philosophers of science, such as Mark Ereshefsky , advocate a pluralistic approach to ‘the species’: assuming that there are different types of species, corresponding to different concepts of ‘species’, depending on how they are defined. Always bearing in mind that, like Linnæus’ classes and families, they are still artificial and utilitarian categories, reflecting part but not all of the complexity of the natural world.
Coyne, JA et al. 2004. Speciation . Sinauer Associates.
Darwin, C. 1859. On the origin of species by means of natural selection, or, Thepreservation of favoured races in the struggle for life /. John Murray, Albemarle Street,. DOI: 10.5962/bhl.title.82303
Ereshefsky, M. 1992. Eliminative Pluralism. Philosophy of Science, 59(4), 671-690. Hull, D. L. 1976. Are Species Really Individuals? Systematic Zoology, 25(2), 174. DOI: 10.2307/2412744
Patten, M. A. et al. 2009. The Song Sparrow, Melospiza melodia, as a ring species: Patterns of geographic variation, a revision of subspecies, and implications for speciation. Systematics and Biodiversity, 7(1), 33-62. DOI: 10.1017/S1477200008002867