Қорғаныш стратегиясы: Өзара тиімді мимикрия және эволюциялық даму

Mutually beneficial mimicry of strongly defended species

Müllerian mimicry is a natural phenomenon in which two or more well defended species, often foul tasting and sharing common predators, have come to mimic each other's honest warning signals, to their mutual benefit. The benefit to Müllerian mimics is that predators only need one unpleasant encounter with one member of a set of Müllerian mimics, and thereafter avoid all similar coloration, whether or not it belongs to the same species as the initial encounter. It is named after the German naturalist Fritz Müller, who first proposed the concept in 1878, supporting his theory with the first mathematical model of frequency dependent selection, one of the first such models anywhere 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, and other animals such as poison frogs and coral snakes. The mimicry need not be visual; for example, 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 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.

Müllerian mimicry was proposed by the German zoologist and naturalist 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 part of his life in Brazil, as 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, and he proposed a variety of other explanations. One was sexual selection, namely 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."

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.

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 unprofitableefn|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. 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.

Müllerian mimicry need not involve visual mimicry; it may employ any of the senses. For example, many snakes share the same auditory warning signals, forming an auditory Müllerian mimicry ring. More than one signal may be shared: snakes can make use of both auditory signals and warning coloration.

There is a negative correlation between the frequency of mimics and the "survivability" of both species involved. This implies that it is reproductively beneficial for both species if the models outnumber the mimics; this increases the negative interactions between predator and prey. 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.

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.

[[File:Formation of Müllerian mimicry rings. svg|thumb|upright=1.5|Formation of Müllerian mimicry rings by a process of advergence of one species or pair of mimics to another, presumably larger or more abundant. Evolution is shown on two axes denoting phenotypes for convenience; in practice there would be any number of dimensions (e. g. coloration features). The model predicts a single mimicry ring in an area, but this is not the case in Heliconius butterflies. thumb|left|A common morph of Ranitomeya (Dendrobates) imitator is aposematically striped black and yellow, but in other areas, other morphs imitate differently coloured species. 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. Such advergence may be common. The mechanism was proposed by the entomologist F. A. Dixey in 1909 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. 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.

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 Hymenoptera, such as bumblebees, and other insects, and among vertebrates including fish and coral snakes. Bumblebees Bombus are all aposematically coloured in combinations, often stripes, of black, white, yellow, and red; and all their females have stings,efn|Drones have no sting, but similar patterns, and may (more or less accidentally) benefit from automimicry of females of their own species.

thumb|upright|left|Pitohui kirhocephalus, from Duperrey, 1825–1839, appears to be a Müllerian mimic of Pitohui dichrous in some of its plumage types. 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 Pitohui are Müllerian mimics. Pitohui dichrous and Pitohui kirhocephalus "share a nearly identical colour pattern" where their geographic ranges overlap, but differ elsewhere; they are conspicuous; and they are chemically defended by a powerful neurotoxic alkaloid, batrachotoxin, in their feathers and skin. This combination of facts implies that the populations in these zones of overlap have converged to share honest warning signals. [[File:Ictonyx libyca multivittata. jpg|thumb|Several mammals including the Saharan striped polecat, Ictonyx libycus, are aggressive, aposematically coloured predators. They share black and white patterns, suggesting Müllerian mimicry. It has been proposed that spiny plants such as Cactaceae and Agave in the Americas, Aloe, Euphorbia, white thorned Acacia in Africa and spiny Asteraceae of the Mediterranean may form Müllerian mimicry rings, as they are strongly defended, are generally agreed to be aposematic, have similar conspicuous patterns and coloration, and are found in overlapping territories. Aposematic mammals in the families Mustelidae, Viverridae, and Herpestidae have independently evolved conspicuous black and white coloration, suggesting that Müllerian mimicry may be involved.

The evolutionary zoologist Thomas N. Sherratt suggests that different types of mimicry occur in brand and product marketing. He notes that distinctive forms like the Coca Cola bottle's shape are defended by businesses, whereas rival companies have often imitated such famous motifs to benefit from the investment and reputation of their well known competitors, constituting Batesian mimicry. Sherratt observes that the packaging of British supermarket own brands of potato crisps are consistently colour coded red for the ready salted variety, green for salt and vinegar, and blue for cheese and onion, across the major chains Sainsbury's, Tesco, Asda, and Waitrose. He argues that this sharing of pattern is very unlikely to have arisen by chance, in which case the resemblance is intentionally to inform customers reliably (honest signalling) of what each package contains, to mutual benefit in the manner of Müllerian mimicry.