"Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species."
– Charles Darwin, On the Origin of Species, Chapter 2.
"I have just been comparing the definitions of species […] It is really laughable to see what different ideas are prominent in various naturalists' minds, when they speak of 'species'; in some, resemblance is everything and descent of little weight – in some, resemblance seems to go for nothing, and Creation the reigning idea – in some, descent is the key — in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable."
– Charles Darwin, in a letter to Joseph Hooker, 1856.
Faced with the rich diversity of living beings around us, humans have proven unable to resist the temptation to try to organize and categorize them. We have a natural tendency to classify things, a habit that's deeply rooted in our cognition and use of language. Our brain excels at recognizing patterns (and thus finding meaning where it doesn't exist), an ability that allows us to interact with the world using names — like "chair" — that we might be hard-pressed to properly explain. In fact, it's surprisingly difficult to define even a seemingly straightforward word like "chair" in a way that would let us recognize everything that should be included (from office chairs and recliners to stools and wheelchairs) but nothing that shouldn't (like tables, tree stumps, or other things we might decide to sit on).
Despite these difficulties, we've been classifying organisms throughout the history of human thought, from Aristotle's division between plants and animals to modern scientific nomenclature. The modern classification system is based on grouping organisms into units called 'species'; species, in turn, group together into a larger units called genus, family, order, and so on through the nested hierarchy of life. What make a species, though? Why should a particular group of organisms be thought of as a unit and given a distinct name? How do we decide which organisms make up a species?
Biologists have struggled with these questions (the "species problem") since before Darwin's time. Over the years, they've come up with a cornucopia of different answers, or species concepts. It's important for students of the natural world to appreciate and consider the different ways that "species" is used, since its meaning can change in different contexts. To me, it often seems that no single definition will ever suffice to capture the variety of ways that wildly different sorts of creatures (like plants and animals) manage to maintain their identity as a species or to form a new one.
Written By: Sedeer el-Showkcontinue to source article at nature.com