biology the main category of taxonomic classification into which genera are subdivided, comprising a group of similar interbreeding individuals sharing a common morphology, physiology, and reproductive process —note 1. there is generally a sterility barrier between species, or at least reduced fertility in interspecific hybrids —note 2. the species is the basic unit of taxonomy on which the binomial system has been established; the lower taxonomic hierarchy is species, subspecies, variety, and forma This definition last updated 10/07/2008