In biology, a species (plural: species) is ane o the basic units o biological classification an a taxonomic rank. A species is eften defined as the lairgest group o organisms capable o interbreedin an producin fertile offspring. While in many cases this defineetion is adequate, the difficulty o definin species is kent as the species problem. Differin measures are eften uised, such as similarity o DNA, morphology, or ecological niche. Presence o specific locally adaptit traits mey further subdivide species intae "infraspecific taxa" such as subspecies (an in botany ither taxa are uised, such as varieties, subvarieties, an formae).