A Darwinian demon is a hypothetical organism that is able to maximize all aspects of its fitness simultaneously. It can reproduce immediately after birth, produces the maximum number of young, and lives indefinitely.

No such organism exists, but biologists use this concept in thought experiments concerning life history strategies (such as giving birth to many offspring and not caring for them as opposed to giving birth to few offspring and caring for them) and evolutionary tradeoffs (cost paid in the currency of fitness when a beneficial change in one trait is linked to a detrimental change in another).