When comparing the characteristics of the two creatures for evolve, you always compare power to power and toughness to toughness.

Whenever a creature enters the battlefield under your control, check its power and toughness against the power and toughness of the creature with evolve. If neither characteristic of the new creature is greater, evolve won't trigger at all.

If evolve triggers, the comparison will happen again when the ability tries to resolve. If neither characteristic of the new creature is greater, the ability will do nothing. If the creature that entered the battlefield leaves the battlefield before evolve resolves, use its last known power and toughness to determine whether the creature with evolve gets a +1/+1 counter.

If multiple creatures enter the battlefield at the same time, evolve may trigger multiple times, although the comparison will take place each time one of those abilities tries to resolve. For example, if you control a 1/2 creature with evolve and two 2/2 creatures enter the battlefield, evolve will trigger twice. The first ability will resolve and put a +1/+1 counter on the creature with evolve. When the second ability tries to resolve, neither the power nor the toughness of the new creature is greater than that of the creature with evolve, so that ability does nothing.