Müllerian mimicry
Müllerian mimicry is a natural phenomenon in which two or more well-defended
Müllerian mimicry was first identified in tropical
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 for example in velvet ants. Since the frequency of mimics is positively correlated with survivability, rarer mimics are likely to adapt to resemble commoner models, favouring both advergence and larger Müllerian mimicry rings. Where mimics are not strongly protected by venom or other defences, honest Müllerian mimicry becomes, by degrees, the better-known bluffing of Batesian mimicry.
History
Origins
Müllerian mimicry was proposed by the
Müller's mathematical model
Müller's 1879 account was one of the earliest uses of a mathematical model in evolutionary ecology, and the first exact model of frequency-dependent selection.[8][9] Mallet calls Müller's mathematical assumption behind the model "beguilingly simple".[10] 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,[8]
then species 1 loses a1n/a1+a2 individuals, and species 2 loses a2n/a1+a2 individuals.
Species 1 therefore gains n-a1n/a1+a2 = a2n/a1+a2 and species 2 similarly gains a1n/a1+a2 in absolute numbers of individuals not killed.
The proportional gain compared to the total population of species 1 is g1 = a2n/a1(a1+a2) and similarly for species 2 g2 = a1n/a2(a1+a2), giving the per head fitness gain of the mimicry when the predators have been fully educated.
Hence, Müller concluded, the proportion g1:g2 was a2/a1 : a1/a2, which equals a22:a12, and the rarer species gains far more than the commoner one.[8]
The model is an approximation, and assumes the species are equally unprofitable. If one is more distasteful than the other, then the relative gains differ further, the less distasteful species benefiting more (as a square of the relative distastefulness) from the protection afforded by mimicry. This can be thought of as parasitic or quasi-Batesian, the mimic benefiting at the expense of the model. Later models are more complex and take factors such as rarity into account. The assumption of a fixed number n to be attacked is questionable.[5] Müller also effectively assumed a step function, when a gradual change (a functional response[11]) is more plausible.[10]
Non-deceitful mimicry
Biologists have not always viewed the Müllerian mechanism as mimicry, both because the term was strongly associated with Batesian mimicry, and because no deceit was involved—unlike the situation in Batesian mimicry, the aposematic signals given by Müllerian mimics are (unconsciously) honest. Earlier terms, no longer in use, for Müllerian mimicry included "homotypy", "nondeceitful homotypy" and "arithmetic homotypy".[12]
Evolution
Aposematism, camouflage, and mimicry
Müllerian mimicry relies on
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[b] species will be safer than its conspecifics, enabling natural selection to drive the prey species toward a single warning language. This can lead to the evolution of both Batesian and Müllerian mimicry, depending on whether the mimic is itself unprofitable to its predators, or just a free-rider. Multiple species can join the protective cooperative, expanding the mimicry ring. Müller thus provided an explanation for Bates' paradox; the mimicry was not, in his view, a case of exploitation by one species, but rather a mutualistic arrangement, though his mathematical model indicated a pronounced asymmetry.[7][16][9]
Relationship to Batesian mimicry
The Müllerian strategy is usually contrasted with
Viceroy butterflies and monarchs (types of admiral butterfly) are both poisonous Müllerian mimics, though they were long thought to be Batesian. Mitochondrial DNA analysis of admiral butterflies shows that the viceroy is the basal lineage of two western sister species in North America. The variation in wing patterns appears to have preceded the evolution of toxicity, while other species remain non-toxic, refuting the hypothesis that the toxicity of these butterflies is a conserved characteristic from a common ancestor.[18]
Non-visual mimicry
Müllerian mimicry need not involve
Negative frequency-dependent selection
There is a
Genetics
Some insight into the evolution of mimetic color mimicry in Lepidoptera in particular can be seen through the study of the Optix gene. The Optix gene is responsible for the Heliconius butterflies' signature red wing patterns that help it signal to predators that it is toxic. By sharing this coloration with other poisonous red winged butterflies the predator may have pursued previously, the Heliconius butterfly increases its chance of survival through association. By mapping the genome of many related species of Heliconius butterflies "show[s] that the cis-regulatory evolution of a single transcription factor can repeatedly drive the convergent evolution of complex color patterns in distantly related species…".[20] This suggests that the evolution of a non-coding piece of DNA that regulates the transcription of nearby genes can be the reason behind similar phenotypic coloration between distant species, making it hard to determine if the trait is homologous or simply the result of convergent evolution.
Two step evolution
One proposed mechanism for Müllerian mimicry is the "two step hypothesis". This states that a large mutational leap initially establishes an approximate resemblance of the mimic to the model, both species already being aposematic. In a second step, smaller changes establish a closer resemblance. This is only likely to work, however, when a trait is governed by a single gene, and many coloration patterns are certainly controlled by multiple genes.[21]
Advergence versus mutualism
The mimic poison frog
R. imitator has thus apparently evolved in separate populations to resemble different targets, i.e. it has changed to resemble (adverged on) those target species, rather than both R. imitator and the other species mutually converging in the way that Müller supposed for tropical butterflies.[24]
Such advergence may be common. The mechanism was proposed by the entomologist F. A. Dixey in 1909[25] and has remained controversial; the evolutionary biologist James Mallet, reviewing the situation in 2001, suggested that in Müllerian mimicry, advergence may be more common than convergence. In advergent evolution, the mimicking species responds to predation by coming to resemble the model more and more closely. Any initial benefit is thus to the mimic, and there is no implied mutualism, as there would be with Müller's original convergence theory. However, once model and mimic have become closely similar, some degree of mutual protection becomes likely.[9][24] This theory would predict that all mimicking species in an area should converge on a single pattern of coloration. This does not appear to happen in nature, however, as Heliconius butterflies form multiple Müllerian mimicry rings in a single geographical area. The finding implies that additional evolutionary forces are probably at work.[22]
Mimicry complexes
Müllerian mimicry often occurs in clusters of multiple species called rings. Müllerian mimicry is not limited to butterflies, where rings are common; mimicry rings occur among
The relationships among mimics can become complex. For example, the poison fangblenny
Sets of associated rings are called complexes. Large complexes are known among the North American velvet ants in the genus Dasymutilla. Out of 351 species examined in one study, 336 had morphological similarities, apparently forming 8 distinct mimetic rings; 65 species in another study appeared to form six rings separable by both morphology and geography.[27][28]
Taxonomic range
Müllerian mimicry was discovered and has mainly been researched in insects. However, there is no reason why the mechanism's evolutionary advantages should not be exploited in other groups. There is some evidence that birds in the New Guinea genus
Many species of
Aposematic
In marketing
The evolutionary zoologist
See also
Notes
- Thomas Malthus's use of tables of numbers illustrating the limits to human population growthis one of the few earlier uses of a mathematical argument that could be called a model.
- ^ Unprofitability may consist of anything which makes prey not worth a predator's while to eat. Unpalatability on grounds of toxicity or foul taste is a common mechanism, but defences may include sharp spines; an aggressive nature; agility or speed in escape rendering the prey costly to catch; foul smell, and so on.[9]
- ^ Drones have no sting, but similar patterns, and may (more or less accidentally) benefit from automimicry of females of their own species.[9]
- ^ Sherratt notes that this use of red is shared with Walker's crisps, whereas the uses of blue and green are interchanged with respect to Walker's.[5]
References
- PMID 17048984.
- ^ Müller, Fritz (1878). "Ueber die Vortheile der Mimicry bei Schmetterlingen". Zoologischer Anzeiger. 1: 54–55.
- ^ Müller, Fritz (1879). "Ituna and Thyridia; a remarkable case of mimicry in butterflies. (R. Meldola translation)". Proclamations of the Entomological Society of London. 1879: 20–29.
- S2CID 28667520.
Viceroys are as unpalatable as monarchs, and significantly more unpalatable than queens from representative Florida populations.
- ^ PMID 18542902.
- ^ Forbes 2009, pp. 40–42.
- ^ a b Ruxton, Speed & Sherratt 2004, pp. 116–118.
- ^ a b c "Fritz Müller in 1891". Retrieved 18 November 2017.
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- ^ Wickler, Wolfgang (1998). "Mimicry". Encyclopædia Britannica. Vol. 24 (15th ed.). pp. 144–151.
- ISBN 978-90-6193-128-7.
- ^ Ruxton, Speed & Sherratt 2004, p. 126.
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- ^ Schulte, Rainer (1989). "Dendrobates imitator. Eine Neue Dendrobates-Art aus Ostperu (Amphibia: Salentia: Dendrobatidae)". Sauria (in German). 8 (3): 11–20.
- ^ a b Ruxton, Speed & Sherratt 2004, pp. 126–127.
- ^ Dixey, F. A. (1909). "On Müllerian mimicry and diaposematism. A reply to Mr G. A. K. Marshall". Transactions of the Entomological Society of London. 23: 559–583.
- ^ Edmunds 1974, pp. 127–130.
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- ^ ISBN 978-0226094366.
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Sources
- Edmunds, M. (1974). Defence in Animals. Longmans. ISBN 978-0-582-44132-3.
- Forbes, Peter (2009). ISBN 978-0-300-17896-8.
- ISBN 978-0-19-852860-9. Chapters 9 and 11 provide an overview.
Further reading
- ISBN 978-0-07-070100-7. Especially chapters 7 and 8.