Studies of students' thinking about natural selection have revealed that the scenarios in which students reason evoke different types, magnitudes, and arrangements of knowledge elements and misconceptions. Diagnostic tests are needed that probe students' thinking across a representative array of evolutionary contexts. The ACORNS is a diagnostic test that treats different evolutionary contexts as unique scenarios worthy of focused assessment and targeted instruction. Our investigations revealed that ACORNS scores produce valid and reliable inferences about students' thinking about natural selection. We urge biology teachers at all educational levels to begin assessing and attending to their students' reasoning across a broader array of evolutionary contexts, as competency in one context is often not indicative of competency in another.