Evolution
Though the common layman may call the change from Zombie to Drowned, or Skeleton to Husk, or some other variant, evolution, this would in fact be a misnomer. These changes are in fact caused due to environmental stimulus, though the exact mechanics of the change are unknown. Evolution would instead be better observed in the minute changes domesticated mobs have from generation to generation. Measuring the gradually increasing top speed of a horse or the shortening time a sheep requires to regrow a wool coat after being sheared would be far more indicative of evolution than simple monster adaptation.
Throughout history new mobs and materials would periodically be discovered or encountered. Entire biomes could be discovered in some far-off land laden with strange materials and mobs. Sometimes this would be cause for celebration, such as when alpacas and llamas were tamed and domesticated. Other encounters, however, were far more tragic. Thousands of lives were lost when Phantoms first flew across the skies, and it is unknown how many miners perished due to Silverfish. Despite all efforts, scientists remain unable to prove any of these mobs and materials existed before their discovery. They named these puzzling changes 'Updates', for each discovery changed the world.