Biologists cannot agree on what a species is, and count over two dozen definitions
Species is biology's most basic unit, yet nobody has pinned it down. One researcher listed about 24 competing definitions and a philosopher counted 26. Hybrids blur boundaries, bacteria swap genes freely, and asexual lineages break the rules entirely, yet the imperfect idea stays indispensable for studying and protecting life.
The textbook version says a species is the largest group whose members can interbreed and produce fertile offspring. Alternatives lean on chromosomes, DNA sequences, anatomy, behaviour or ecological role, and palaeontologists, unable to test whether fossils could breed, use chronospecies. Every species gets a two-part Latin name, genus first: Boa constrictor belongs to the genus Boa. The best recent estimate puts eukaryotic species at 8 to 8.7 million, of which only about 14 per cent had been described by 2011.
Ideas changed dramatically over time. From Aristotle to the 18th century, species were fixed rungs on a great chain of being. Linnaeus could only describe what he saw, an approach later formalised as the morphological concept that field guides and iNaturalist still use, although looks can mislead: a four-winged fruit fly born to a two-winged mother is not a new species. Darwin's On the Origin of Species, in 1859, showed how natural selection creates new ones, and 20th-century genetics deepened the picture.
Ernst Mayr made reproductive isolation central, but that is often impossible to test for populations separated by distance or time. Later biologists refined it: mate-recognition species are defined by whether individuals recognise each other as partners, and cohesion species stay distinct if limited hybridising never fully merges their gene pools. In the 1970s Robert Sokal, Theodore Crovello and Peter Sneath proposed phenetic species, clustered numerically across many measurable traits. John Wilkins sorted the whole zoo of definitions into seven basic kinds.
Evolution itself creates the headache. Because species keep changing, taxonomists must decide when a fossil lineage has shifted enough to split, or when diverging populations deserve separate names. Hybridisation and polyploidy can produce new species quickly, genes sometimes jump between species horizontally, and viruses are best treated as quasispecies shaped by mutation and selection.
Source: Species
