Darwin proposed the biological concept of species.
According to him, species is a group of individuals which resemble each other in morphological, biological, physiological and behavioral characters. These individuals are capable of breeding with each other under natural conditions, but are unable to breed successfully with members of other species.
Ernst Meyr gave the modern concept of biological species.
According to him, species are the groups of actually interbreeding natural population of closely resembling individuals that are reproductively isolated from other such groups.
This concept is biological because interbreeding is purely a biological phenomenon.