Does defeating Alphas unlock evolutions?

Alphas do two separate jobs in the evolution system, and it helps to keep them apart. The first is supply: Alpha Aniimo drop evolution stones and the other resources evolutions consume, and every evolution costs specific resources. The second is gating. Defeating certain Alpha Aniimo is itself the condition that opens a path, so the kill is the requirement rather than the reward. Only specific Aniimo work this way, so clearing an Alpha will not always move an evolution forward.

That gating sits alongside other requirements rather than replacing them. Some evolutions need conditions or progression milestones beyond simply hitting a level, and an Alpha fight can be one of those milestones stacked on top of a level and a material cost.

There is a shortcut worth knowing before you commit to the fight. Catching an Alpha boss gives you an already-evolved Aniimo, so you skip the evolution entirely. Beating it feeds your own creature's evolution. Catching it hands you the end result.

Celestis evolution, as the worked example

Celestis into Stellarys is the clearest case of all three requirement types at once. The evolution happens at level 18, it consumes Night Stones, and it requires you to have defeated an Alpha Stellarys. Miss any one of those and the evolution will not fire, so getting Celestis to level 18 on its own gets you nowhere.

If you are short on a material and do not know where to look, open it in the menu. Checking an evolution material there tells you which Aniimo drop it and where to obtain it, which usually points you straight back at an Alpha.

Evolutions Alphas have nothing to do with

Plenty of paths never touch an Alpha. Some evolutions are unlocked through the Pathfinder's character talent tree, and certain side quests unlock alternate forms and secret evolutions. Others hinge on the creature itself: Skippy evolves into Glacie only if the Aniimo is female.

If you bought the Advanced pack, its companion handbook includes evolution materials for the Aniimo family it bundles, which covers part of the resource side for those creatures without farming for it.

Not settled: Which Aniimo have Alpha-gated evolutions is a short list rather than a rule, and the beta build does not lay that list out anywhere in the menus. You find out by checking an individual evolution's requirements.