Biased predation could promote convergence yet maintain diversity within Müllerian mimicry rings of Oreina leaf beetles

Müllerian mimicry is a classic example of adaptation, yet Müller’s
original theory does not account for the persistence of diversity in
mimicry rings, which has been a thorn in the side of one of the oldest
models in evolutionary biology. Here, we aimed to assess how well
classical Müllerian mimicry can account for the color polymorphism found
in chemically defended Oreina leaf beetles, specifically testing its
predictions for predator behavior. We also evaluated whether
thermoregulation can explain diversity between Oreina mimicry rings. We
found that frequencies of each color morph were positively correlated
among species, a critical prediction of Müllerian mimicry. Predators
learned to associate color with chemical defenses. Learned avoidance of
the green morph of one species protected green morphs of another species.
Avoidance of blue morphs was completely generalized to green morphs, but
surprisingly, avoidance of green morphs was less generalized to blue
morphs. This asymmetrical generalization should favor green morphs:
indeed, green morphs persist in blue communities, whereas blue morphs are
entirely excluded from green communities. We did not find a correlation
between elevation and coloration, rejecting thermoregulation as an
explanation for diversity between mimicry rings. Biased predation could
explain within-community diversity in warning coloration, providing a
solution to a longstanding puzzle. We propose testable hypotheses for why
asymmetric generalization occurs, and how predators maintain the
predominance of blue morphs in a community despite asymmetric
generalization.