A species is a group of living organisms that are all broadly similar and can, at least in principle, breed with one another. This seems straightforward enough, but the word “species” is actually fiendishly difficult to define. Despite decades of research, biologists do not agree on what constitutes a species. Several dozen definitions have been proposed.
On a common-sense level, it is obvious what constitutes a species. Chimpanzees are a species: we know what a chimpanzee looks like and they are clearly distinct from, say, orang-utans. Furthermore, when two chimpanzees mate the result is a new chimpanzee, and not a chicken.
This idea was formalised by Carl Linnaeus, a Swedish biologist who in his 1735 book Systema Naturae popularised what is now known as Linnaean taxonomy. This was the idea that every organism could be classified into increasingly specific groups, culminating in its species.
For example, chimps are Pan troglodytes. Pan is the name of the genus and troglodytes is the species. A genus can include several species: for instance, Pan also includes bonobos (Pan paniscus).
All species also belong to larger groups. For example, chimps belong to a family called the hominids, which also includes humans. The hominid family in turn belongs to a larger class, called mammals.
However, when you start to dig into the notion of a species, you run into problems. Bonobos were long thought of as a kind of chimpanzee, because they look so similar. It is only in recent decades that scientists have treated them as separate species, because their DNA and behaviours are distinct. But despite these differences, they have interbred with chimpanzees.
The idea that animals belong to different species if they cannot breed is known as the “biological species concept” and was proposed by the biologist Ernst Mayr. Today it is how many people understand the word, but it does not correspond to reality. Chimps and bonobos have interbred, so under Mayr’s schema they are not separate species. Similarly, polar bears can interbreed with brown bears, and red wolves in the US seem to be hybrids between grey wolves and coyotes.
The “hybrid” organisms produced by these inter-species matings often have health problems and reduced fertility, but regularly pass on their genes nonetheless. When a horse mates with a donkey, the result is a mule and mules are almost always infertile – but this is not the general rule for hybrids. Humans are arguably hybrids as many of us carry genes from extinct hominins like the Neanderthals.
There are alternative definitions for a species, for instance looking at their appearance or their DNA. But again it is not obvious where to draw the line.
The problem only becomes more difficult when we consider single-celled organisms. Bacteria do not have sex, so we cannot pick out which ones do and don’t mate. Instead they reproduce by dividing into two. To make things more complicated, they sometimes swap bits of DNA with distantly-related organisms, a process called horizontal gene transfer. That means many bacteria carry DNA from what are, on the face of it, different species.
