A population of morphologically and genetically similar organisms that can reproduce sexually among themselves but that cannot produce fertile offspring when mated with organisms belonging to another species.

Adapted from http://highered.mcgraw-hill.com/sites/0070294267/student_view0/glossary_s-z.html

species on planed [cite]

Mora, C; Tittensor, DP ; Adl, S; Simpson, AGB; Worm. AUG 2011. How Many Species Are There on Earth and in the Ocean? PLOS BIOLOGY,  Volume: 9 Issue: 8. DOI: 10.1371/journal.pbio.1001127