Wallace: a species is "a group of actually or potentially interbreeding natural populations, each population reproductively isolated from the others."
Campbell: a species is "a population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot successfully interbreed with members of other species."
The species concept is at best a compromise between our need to categorize. The definition of species is a quantum concept while nature is a continuum, one species melding into another through time or geography.
Problems with the present biological species concept include: