If you complete all the tasks of a Doodle's evolution line, you unlock additional bonuses when encountering Doodles of that evolution line in the wild. When all tasks of a Doodle is complete, it will be marked with a check in the Doodlepedia.
Completing all of a Doodle's tasks will grant one of the following rewards which vary in amount:
Every Doodle in the game has Tasks which can be found in the Doodlepedia under Tasks. Doodlepedia Tasks are optional quests a player can complete in return for a reward.