Hi! Thanks for your feedback!
They also has different dances depends on their speed!
There are many hidden mechanics not stated in info panel. Maybe it would be better if we make an whole fullscreen widget for game info :)
Every evolve do what it says - Speed/Health/Damage/Score for every unit approaching tower. Only population is not tied to any unit.
More over it's meant to be like real world survival of the fittest evolution where mutations are random across population. So every time you click on some "mutation" all survived units get random change mutated stat value, something like +-20%.