Can you choose not to put a +1/+1 counter on a creature with evolve even if the entering creature has greater power or toughness?

Evolve is a triggered ability that reads: "Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature." Because the ability uses the word 'put' (not 'you may put'), it is a mandatory effect with no optional component.

Under CR 603.3, a triggered ability triggers automatically when its trigger condition is met. Once the ability resolves, its effect is applied without player choice. CR 701.5 confirms that putting counters is a defined game action that happens as written — there is no built-in option to decline it.

The only exception noted in the Evolve reminder text (and Gatecrash FAQ) is the comparison check at resolution: if the would-be evolved creature is no longer on the battlefield, or the entering creature no longer has greater power or toughness at the time the ability resolves, the counter is simply not placed. But this is a conditional check, not player choice.

Example: You control a 2/2 Cloudfin Raptor (with evolve). A 3/3 creature enters under your control. Evolve triggers. When it resolves, you cannot decide to skip the counter — you must place a +1/+1 counter on the Raptor, making it a 3/3.

In short, if the trigger fires and the conditions still hold at resolution, the counter placement is completely mandatory per CR 603.3 and the plain text of the ability.