Two unrelated poisonous species evolve to look identical, purely for mutual benefit

Unlike a harmless species copying a dangerous one, Müllerian mimicry is when two genuinely toxic species converge on the same warning pattern, so predators only need to learn one lesson to avoid both. Fritz Müller worked out the logic in 1878: splitting the cost of educating predators between species means fewer individuals of either one have to die teaching the lesson. It's why so many unrelated wasps, and unrelated poisonous butterflies sharing a region, end up wearing near-identical colours.

— Fritz Müller, Ueber die Vortheile der Mimicry bei Schmetterlingen — Zoologischer Anzeiger, 1878