Müllerian mimicry

right|thumb|Two examples of Müllerian mimicry in [[Heliconius butterflies: In this image the top four are forms of Heliconius numata, which mimic species from the genus Melinaea, while the bottom four are H. melpomene (left) and H. erato (right), which mimic each other.]]

Müllerian mimicry is a type of biological mimicry in which two or more well-defended species, often foul-tasting and sharing common predators, converge in appearance to mimic each other's honest warning signals. This convergence of appearance achieves the following benefit to species that undergo it: predators need only experience a single unpleasant encounter with any member of a set of Müllerian mimics in order to thereafter avoid all creatures of similar appearance, whether or not it is the same species as the initial encounter. A ring of distinct species is thereby protected from their mutual predators by attempted predation upon any one of its members. The phenomenon is named after the German-Brazilian naturalist Fritz Müller, who proposed the concept in 1878, supporting his theory with a mathematical model of frequency-dependent selection, one of the first such models to be deployed in biology.

Müllerian mimicry was first identified in tropical butterflies that shared colourful wing patterns, but it is found in many groups of insects such as bumblebees, as well as in other animals such as poison frogs and coral snakes. The mimicry need not be restricted to that detected by vision—many snakes share auditory warning signals. Similarly, the defences involved are not limited to toxicity—anything that tends to deter predators, such as foul taste, sharp spines, or defensive behaviour can make a species unprofitable enough to predators to allow Müllerian mimicry to develop.

Once a pair of Müllerian mimics has formed other mimics may join them by advergent evolution (one species changing to conform to the appearance of the pair, rather than mutual convergence, forming mimicry rings. Large rings are found among the several thousand species of velvet ants (which are all, in fact, types of wasp). Because the frequency of mimics in a given environment is positively correlated with an individual mimic's survival, the rarer mimics come to resemble commoner models, favouring both advergence and ever-larger rings of Müllerian mimicry. Where Müllerian mimics are not strongly protected by venom or other defences, honest Müllerian mimicry transforms incrementally into the better-known bluffing of Batesian mimicry.

History

Origins

thumb|The [[viceroy butterfly (top) appears very similar to the noxious-tasting monarch butterfly (bottom)<!-- but viceroy has an extra postmedian black line across the hindwing-->. Although it was for a long time purported to be an example of Batesian mimicry, the viceroy has recently been discovered to be just as unpalatable as the monarch, making this a case of Müllerian mimicry.]]

Müllerian mimicry was proposed by Fritz Müller (1821–1897). An early proponent of evolution, Müller offered the first explanation for resemblance between certain butterflies that had puzzled the English naturalist Henry Walter Bates in 1862. Bates, like Müller, spent a significant fraction of his life in Brazil, which Bates described in his book The Naturalist on the River Amazons. Bates conjectured that these abundant and distasteful butterflies might have been caused to resemble each other by their physical environment. Müller had also seen these butterflies first hand and, like Bates, had collected specimens. Müller proposed a variety of other explanations. One was sexual selection—that individuals would choose to mate with partners with frequently-seen coloration, such as those resembling other species. However, if as is usual, females are the choosers, then mimicry would be seen in males, but in sexually dimorphic species, females are more often mimetic. Another was, as Müller wrote in 1878, that "defended species may evolve a similar appearance so as to share the costs of predator education."

Müller's mathematical model

Müller's 1879 account was the earliest use of a mathematical model in evolutionary ecology, and the first exact model of frequency-dependent selection. Müller presumed that the predators had to attack n unprofitable prey in a summer to experience and learn their warning coloration. Calling a₁ and a₂ the total numbers of two unprofitable prey species, Müller then argued that, if the species are completely unalike they each lose n individuals. However, if they resemble each other,

then species 1 loses

individuals,

and species 2 loses

individuals.

Species 1 therefore gains

and species 2 similarly gains

in absolute numbers of individuals not killed.

The proportional gain compared to the total population of species 1 is

and similarly for species 2

, giving the per head fitness gain of the mimicry when the predators have been fully educated.

Hence, Müller concluded, the proportion g1:g2 was

, which equals , and the rarer species gains far more than the commoner one.) is more plausible.

thumb|center|upright=2|[[Batesian mimicry|Batesian vs Müllerian mimicry: the former is deceptive, the latter honest.]]

Evolution

<!--thumb|An aposematic [[Lycidae|lycid beetle, Calopteron terminale]]

An aposematic zygaenid moth, Neoalbertia constans-->

Aposematism, camouflage, and mimicry

Müllerian mimicry relies on aposematism, or warning signals. Dangerous organisms with these honest signals are avoided by predators, which quickly learn after a bad experience not to pursue the same unprofitable prey again. Learning is not actually necessary for animals which instinctively avoid certain prey; however, learning from experience is more common. The underlying concept with predators that learn is that the warning signal makes the harmful organism easier to remember than if it remained as well camouflaged as possible. Aposematism and camouflage are in this way opposing concepts, but this does not mean they are mutually exclusive. Many animals remain inconspicuous until threatened, then suddenly employ warning signals, such as startling eyespots, bright colours on their undersides or loud vocalizations. In this way, they enjoy the best of both strategies. These strategies may also be employed differentially throughout development. For instance, large white butterflies are aposematic as larvae, but are Müllerian mimics once they emerge from development as adult butterflies.

Selective advantage

Many different prey of the same predator could all employ their own warning signals, but this would make no sense for any party. If they could all agree on a common warning signal, the predator would have fewer detrimental experiences, and the prey would lose fewer individuals educating it. No such conference needs to take place, as a prey species that just so happens to look a little like an unprofitable