"group of organisms evolved from a common ancestor," 1957, from Greek klados "young branch, offshoot of a plant, shoot broken off," from PIE *kele-, possibly from root *kel- "to strike, cut" (see holt).
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.
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Harper, D. (n.d.). Etymology of cladistic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/cladistic