VERB (3)

1. become dissimilar by changing the sound qualities;
- Example: "These consonants dissimilate"

2. make dissimilar; cause to become less similar;

3. become dissimilar or less similar;
- Example: "These two related tribes of people gradually dissimilated over time"

The Collaborative International Dictionary of English v.0.48:

Dissimilate \Dis*sim"i*late\, v. t.
   To render dissimilar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dissimilate
    v 1: become dissimilar by changing the sound qualities; "These
         consonants dissimilate" [ant: assimilate]
    2: make dissimilar; cause to become less similar [ant:
       assimilate]
    3: become dissimilar or less similar; "These two related tribes
       of people gradually dissimilated over time" [ant:
       assimilate]