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