Species

• 1. Introduction

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  A species, in biology, is the basic unit of classification, a rank that sits just below the genus. It sounds like the most settled idea in all of life science. Yet when the biologist R. L. Mayden sat down to count the working definitions of what a species actually is, he reached about 24. The philosopher of science John Wilkins, doing the same exercise, counted 26. How can the foundation stone of an entire discipline come in more than two dozen flavours? The classic textbook answer says a species is the largest group of organisms in which two individuals of the right sexes can produce fertile offspring. Listen closely and that definition starts to crack. What happens to it among bacteria, which trade genes freely with strangers? What happens among fossils, which can never be bred? What happens to the carrion crow and the hooded crow, classed apart yet interbreeding where they meet? And how can scientists claim there are between 8 and 8.7 million kinds of eukaryote on Earth when they cannot agree what one kind even is?

• 2. The Binomial Name

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  Boa constrictor tells you two things at once. The first word, Boa, names the genus the snake belongs to. The second, constrictor, is the specific name, sometimes called the specific epithet. Every species except viruses carries this two-part label, a binomen, and the system is built to be unique and universal across the whole planet. The reason for that universality is the chaos it replaces. The single creature Puma concolor answers to puma, cougar, catamount, panther, painter and mountain lion in different corners of America. Worse, the word panther also gets attached to the jaguar, Panthera onca, of Latin America, and to the leopard, Panthera pardus, of Africa and Asia. A scientific name cuts through that fog. The word cat can mean the domestic animal, Felis catus, or the entire family Felidae, but Felis catus can mean only one thing. A name becomes official when a type specimen is formally described in an accepted publication, and that physical specimen is lodged in a permanent repository, often a major museum or university collection, so that any later scientist can check it. The International Code of Zoological Nomenclature even asks describers to pick names that are appropriate, compact, euphonious, memorable, and do not cause offence.

• 3. Two Dozen Concepts

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  Wilkins did more than count the definitions; he sorted them into seven basic kinds. They run from agamospecies for asexual organisms, through biospecies for reproductively isolated sexual ones, to ecospecies built on ecological niches, evolutionary species built on lineage, genetic species built on gene pools, morphospecies built on form, and finally taxonomic species, meaning whatever a taxonomist decides. The oldest of these is the typological or morphological idea, the one Linnaeus had to use because he could only describe what he saw. It works on fixed properties, a type, so reliably that pre-literate people often recognise the same groups modern taxonomists do, and field guides and identification websites such as iNaturalist still lean on it. Its weakness is plain. A four-winged Drosophila born to a two-winged mother is not a new species, even though it looks like one. In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath sharpened the approach into the phenetic species, clustering organisms by a numerical measure of similarity across many phenotypic traits at once. The most cited definition of all came from Ernst Mayr in 1942: groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. His little word potentially has been argued over ever since, with some readers excluding matings that only happen in captivity. Other thinkers pushed past form and reproduction toward lineage. George Gaylord Simpson proposed the evolutionary species in 1951, an entity that keeps its identity through time and space and has its own independent evolutionary fate.

• 4. Reading The Genome

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  A figure of 97 percent once drew the line. Microbiologists treated members of Bacteria or Archaea whose 16S ribosomal RNA gene sequences matched more closely than that as suspect, needing a DNA-DNA hybridisation check to confirm they were one species. In 2006 the threshold tightened to 98.7 percent. The 16S sequence is a single locus simple enough for non-specialists, and using one easy locus this way is called DNA barcoding. For eukaryotes, one barcode lives in a region of mitochondrial DNA inside the gene for cytochrome c oxidase, and the Barcode of Life Data System now holds barcode sequences from over 190,000 species. The single gene can mislead, though. Among bacteria, some very different genomes share 99.9 percent 16S identity, and scientists such as Rob DeSalle warn that barcoding and classical taxonomy draw species lines differently. Comparing multiple loci, usually fewer than 10, captures more mutations and sharpens the picture, approaching the gold standard of whole-genome comparison at modest extra cost. The surest method compares entire genomes outright. The average nucleotide identity method measures genetic distance across regions of about 10,000 base pairs, and it sorted out Pseudomonas avellanae in 2013 before being applied to all sequenced bacteria and archaea from 2020 onward. Strikingly, those values cluster with an ANI gap between 85 and 95 percent, a quiet hint that a real genetic boundary for a species concept may exist after all.

• 5. Where Definitions Break

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  The dandelion is not one plant but a crowd. Taraxacum officinale and the blackberry Rubus fruticosus are aggregates of microspecies, perhaps over 200 dandelion forms and around 400 blackberry forms in Britain alone, tangled by hybridisation, apomixis and polyploidy until gene flow becomes impossible to trace. These microspecies, with very little genetic variability, quietly dissolve the tidy picture of discrete kinds. Ring species twist the knife further. A connected chain of populations each interbreeds with its neighbours, yet the two end populations are too distantly related to breed, and they may even meet and close the ring. Proposed cases include the herring gull and lesser black-backed gull complex around the North Pole, the 19 salamander populations of the Ensatina eschscholtzii group in America, and the greenish warbler in Asia. Many supposed ring species, on closer inspection, turned out to be misclassifications, leaving open whether any truly exist. Then there is horizontal gene transfer, which defies every definition that assumes an organism inherits its genes from one or two parents. Strong evidence shows it between dissimilar prokaryotes, and at least occasionally between dissimilar eukaryotes, including some crustaceans and echinoderms. Confronted with all of this, the evolutionary biologist James Mallet concluded that species gaps can be verified only locally and at a point of time, and that any local integrity of species is greatly reduced over large ranges and long periods.

• 6. Viruses And Quasispecies

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  Viruses are the one group that gets no binomen at all. They are doubtfully alive, little more than a string of DNA or RNA in a protein coat, they exist in enormous populations, and they mutate so fast that conventional species concepts simply fail. In their place stands the quasispecies: a group of genotypes linked by similar mutations, competing inside a highly mutagenic environment, governed by a mutation-selection balance. The mathematics produces a counterintuitive result called survival of the flattest. A quasispecies sitting at a low but broad and well-connected region of the fitness landscape will outcompete one perched on a higher but narrower peak, because the rivals surrounding that peak are unfit. No one suggests this looks anything like a traditional biological species. To impose order, the International Committee on Taxonomy of Viruses has built a universal taxonomic scheme for viruses since 1962, and that effort has stabilised viral taxonomy.

• 7. The Stakes Of A Name

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  Surveys using a phylogenetic species concept reported 48 percent more species than surveys using nonphylogenetic concepts, and with them came smaller populations and smaller ranges. Critics called this taxonomic inflation, warning it could make the count of endangered species swell and create political and practical trouble. Defenders fired back. They call inflation a pejorative, label the opposing camp taxonomic conservatism, and argue that recognising smaller populations as full species lets them enter the IUCN red list more easily and attract conservation legislation and funding. The fight is not theoretical. In North America, the protected northern spotted owl hybridises with the unprotected California spotted owl and the barred owl, dragging lawmakers, land owners and conservationists into legal argument. Difficulty assigning organisms to a species is more than an academic nuisance; it can make a measurement of how abundant a species is in an ecosystem meaningless. The disagreement reaches back to the start. The ichthyologist Charles Tate Regan once remarked that a species is whatever a suitably qualified biologist chooses to call a species. The philosopher Philip Kitcher named that the cynical species concept, though Wilkins argued it usefully leads to an empirical taxonomy built on real experience. Some biologists would go further still and abandon the word species entirely, speaking instead of Least Inclusive Taxonomic Units.

• 8. Darwin's Doubt

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  Aristotle drew his lines with two Greek words. He used genos for a kind, such as bird or fish, and eidos for a specific form within it, such as the crane, eagle, crow or sparrow, and he believed every kind and form was distinct and unchanging. Those words crossed into Latin as genus and species, though the mapping was loose; what Aristotle filed under birds we now call a class, his cranes a family, his crows a genus. His scheme held sway until the Renaissance. Early modern naturalists then built systems of their own, some whimsical, grouping plants by yellow flowers or lumping snakes, scorpions and biting ants by behaviour. The Englishman John Ray offered the first biological definition in 1686, arguing that organisms springing from the seed of one and the same plant could only be the same species. Carl Linnaeus classified by shared physical characteristics in the 18th century, yet still pictured species as created by God within a fixed hierarchy, the scala naturae or great chain of being. Even Linnaeus, watching hybrids form, conceded species were not wholly fixed, though he stopped short of new ones arising. Jean-Baptiste Lamarck broke harder from Aristotle in his 1809 Zoological Philosophy, describing the transmutation of species over time. Then, in 1858, Charles Darwin and Alfred Russel Wallace explained how new species form, and Darwin showed that populations evolve, not individuals. The man who wrote the most famous book on the subject trusted the category least. He confessed he was much struck how entirely vague and arbitrary is the distinction between species and varieties, and he looked at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other.