Evolve is a triggered ability that makes a creature grow when bigger allies arrive. Whenever a creature enters the battlefield under your control, compare its power and toughness to the evolve creature's. If the new creature has greater power or greater toughness, the evolve creature gets a +1/+1 counter. Only one counter is added per entering creature, even if both stats are higher. The ability cares about either stat being larger, not the total.
In play, evolve rewards mixing small evolve creatures with larger threats. A humble one-power creature can quickly become a threat as you cast bigger creatures later. It shines in blue-green decks that already like +1/+1 counters, and it can trigger multiple times across a turn. Some cards even grant evolve to other creatures, letting a whole team grow together.
