Evolution Rules

Everything has rules.  Evolution works, but not because of faith or miracle, but thanks to rules.

Rule one: Sex.  Without procreation there is simply no evolution.
Converse: Barriers.  Separating populations results in speciation.

Rule two: Mutability.  Without variation improvement is impossible.  Failure to adapt means other species will improve and out-compete, forcing the unchanged to extinction. 
Converse: Stability.  Change can be dangerous.  Too many mutations drown positive, useful adaptations in a flood of useless limbs, maladaptations, and tumors.  It's a balancing act with extinction on both sides.

Rule three: Death.  It's no good for favorable mutations to occur if previous generations keep passing along the old-fashioned, less favorable genes.  Even asexual microbes must self-destruct to divide into the next generation. 
Converse: Survival.  Adaptation to the environment isn't enough; nature shakes things up now and again with extinction events.  To stay in the evolution game, species must possess some strategy to pass both tests and survive.