We've heard about harmless animals "camouflaging" themselves by imitating deadly animals. Müllerian mimicry doesn't work that way. Instead, deadly animals form an alliance and train their predators. Maybe.
On the Origin of Species was published in 1859, but its ideas were available to a certain subsection of the public well before that. Charles Darwin discussed his theories with a good number of scientists across the globe, including one German expatriate living in Brazil. Johannes Friedrich Müller was struck, both in Germany and in Brazil, by the number of species that had come to resemble each other.
Fritz Müller versus Henry Bates
When we learn about mimicry, we generally hear about harmless animals imitating their deadly counterparts. A harmless king snake will develop markings like a venomous coral snake, and predators will avoid them both. This is called Batesian mimicry, after Henry Walter Bates. It's easy to explain the engine behind this form of evolution. The harmless species gains a clear advantage as it comes to resemble its venomous betters.
Müller observed an altogether more difficult form of mimicry. Two species, both "unprofitable" to their predators, often came to resemble each other. Müller turned his focus on insects so, to support his claims, he found butterflies that looked alike, moths that resembled wasps, and flies that looked like bees. Occasionally whole groups of unprofitable insects came to resemble each other, in extended rings of self-defense. Sometimes they tasted bad, sometimes they had spines or venom, but each of the animals had something to deter predators, all on their own. Why would these species evolve to resemble each other if they could flaunt their inedibility alone?
The Enduring Question
This is a question people are still asking, and it's the reason why Müller never got the same credit that Bates did. At first Müller believed that sexual selection might drive two species together. Humans admire beauty in animals other than ourselves, and seek to imitate them. (As far as I can tell, this is the only explanation for the proliferation of zebra-print leggings.) Why wouldn't a butterfly find the patterns on a moth's wings beautiful and be drawn to mates with similar markings in its own species?
Soon Müller set the idea of sexual selection aside and proposed that the two animals derived extra protection from predators by coming to resemble each other. An animal eats, and is poisoned by, a butterfly. It learns to avoid similar butterflies. If a poisonous wasp doesn't resemble the butterfly, the animal will then have to eat the wasp before it avoids them as well. A resemblance between the two species takes away the need for a sampling of each.
There is another explanation. A recent study involving computer-generated hunting scenarios shows that this kind of mimicry isn't just about a predator not being able to tell two species apart. In the study, people hunted computer-generated prey. The researchers found that confusing predators might not always be helpful. In some cases it actually puts more pressure on the similar-looking prey. Instead attempting to confuse predators, the species might be training predators in what to expect. A group of butterflies and moths don't use bright red, orange, and yellow interspersed with black to attempt to blend together, but to show every animal in range that certain colors in a certain order represent a universal danger signal. Everything that's orange and black is bitter. Everything that's bright red and green makes you nauseous. Müllerian mimicry, perhaps, doesn't confuse. Instead it allows prey animals to use basic colors to train their own predators.
Top Image: PLOS