degenerationist (n.)

c. 1600, "loss or impairment of the qualities proper to the race or kind," also figurative, "descent to an inferior state," from French dégéneration (15c.) or directly from Late Latin degenerationem (nominative degeneratio), noun of action from past-participle stem of Latin degenerare "to be inferior to one's ancestors, to become unlike one's race or kind, fall from ancestral quality," used of physical as well as moral qualities, from phrase de genere, from de "down from, away from" (see de-) + genus (genitive generis) "birth, descent" (from PIE root *gene- "give birth, beget").

[Degeneration] means literally an unkinding, the undoing of a kind, and in this sense was first used to express the change of kind without regard to whether the change was to perfect or to degrade; but it is now used exclusively to denote a change from a higher to a lower kind, that is to say, from a more complex to a less complex organisation; it is a process of dissolution, the opposite of that process of involution which is pre-essential to evolution. [Henry Maudsley, "Body and Will,"  1884]

word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.

Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.