Concept of Species

The idea of species as biology's fundamental unit of classification has evolved considerably across the history of the discipline. The term "species" was coined by the English naturalist John Ray, who in his three-volume 1693 work Historia Generalis Plantarum described species as a group of morphologically similar organisms arising from a common ancestor — an early definition grounded purely in shared appearance and descent.

Carolus Linnaeus, in his book Systema Naturae, treated species as the basic unit of classification within his newly developed hierarchical system, cementing its place as biology's foundational taxonomic rank. Building further on this, a species is generally defined as a group of organisms that share similar morphology and physiology and that can interbreed to produce fertile offspring — combining Ray's morphological criterion with a functional, reproductive test. …