retrogression

1. The act or process of deteriorating or declining.
2. Biology. A return to a less complex or more primitive state or stage.

noun

A return to a former, usually worse condition: regression, retrogradation, reversion. See forward/backward, repetition.

1. The act or process of deteriorating or declining.
2. A return to a less complex biological state or stage.

Degeneration; deterioration; regression; return to an earlier, less complex condition.

The noun has 2 meanings:

Meaning #1: passing from a more complex to a simpler biological form
  Synonym: degeneration

Meaning #2: returning to a former state
  Synonyms: regression, regress, reversion, retroversion