Markedness values are asymmetric. The value MARKED means relative narrowness of conceptual scope, whereas its counterpart UNMARKED means relative breadth of conceptual scope. These basic definitions are aligned respectively with greater and lesser complexity. When languages change, there is a general tendency to change from the more complex value (= marked) to the corresponding less complex value (= unmarked).
This teleology can explain the tendency in contemporary American English speech (and in American dialects) to collapse the three ablaut vowels of strong verb conjugation, as in sink/sank/sunk, drink/drank/drunk, etc. such that the simple past vowel is eliminated and replaced by the vowel of the past passive participle. Thus one constantly hears speakers saying things like “Honey, I shrunk [instead of the correct shrank] the dog,” etc.
The markedness values of the vowels involved underwrite this change in grammar. The [ae] of the simple past is marked, whereas the schwa [ə] of the past participle is unmarked, and thus the teleology is unmistakably from marked to unmarked, which corresponds to the empirical facts.