Mimicry

In evolutionary biology, mimicry is the evolved resemblance of an organism to something else, often another organism of a different species. Mimicry may evolve between different species, or between individuals of the same species. In the simplest case, as in Batesian mimicry, a mimic resembles a model, so as to deceive a dupe, all three being of different species. A Batesian mimic, such as a hoverfly, is harmless, while its model, such as a wasp, is harmful, and is avoided by the dupe, such as an insect-eating bird. Birds hunt by sight, so the mimicry in that case is visual, but in other cases mimicry may make use of any of the senses. Most types of mimicry, including Batesian, are deceptive, as the mimics are not harmful, but Müllerian mimicry, where different harmful species resemble each other, is honest, as when species of wasps and of bees all have genuinely aposematic warning coloration. More complex types may be bipolar, involving only two species, such as when the model and the dupe are the same; this occurs for example in aggressive mimicry, where a predator in wolf-in-sheep's-clothing style resembles its prey, allowing it to hunt undetected. Mimicry is not limited to animals; in Pouyannian mimicry, an orchid flower is the mimic, resembling a female bee, its model; the dupe is the male bee of the same species, which tries to copulate with the flower, enabling it to transfer pollen, so the mimicry is again bipolar. In automimicry, another bipolar system, model and mimic are the same, as when blue lycaenid butterflies have 'tails' or eyespots on their wings that mimic their own heads, misdirecting predator dupes to strike harmlessly. Many other types of mimicry exist.

Etymology

Use of the word mimicry dates to 1637. It derives from the Greek term mimetikos, "imitative", in turn from mimetos, the verbal adjective of mimeisthai, "to imitate". "Mimicry" was first used in zoology by the English entomologists William Kirby and William Spence in 1823. Originally used to describe people, "mimetic" was used in zoology from 1851.

History

Ancient

wrote in his History of Animals that partridges use a deceptive distraction display to lure predators away from their flightless young:
The behaviour is recognised as a form of mimicry by biologists.

19th century

In 1817, William Kirby and William Spence, in their book An Introduction to Entomology, used the term "mimicry" informally to depict the way that the structure and coloration of some insects resembled objects in their environments:
The English naturalist Henry Walter Bates worked for several years on butterflies in the Amazon rainforest. Returning home, he described multiple forms of mimicry in an 1862 paper at the Linnean Society in London, and then in his 1863 book The Naturalist on the River Amazons. The term "Batesian mimicry" has since been used in his honour, its usage becoming restricted to the situation in which a harmless mimic gains protection from its predators by resembling a distasteful model. Among the observations in Bates's 1862 paper is the statement:
The German naturalist Fritz Müller also spent many years studying butterflies in the Amazon rainforest. He first published a journal article on mimicry in German in 1878, followed in 1879 by a paper to the Entomological Society of London. He described a situation where different species were each unpalatable to predators, and shared similar, genuine, warning signals. Bates found it hard to explain why this should be so, asking why they should need to mimic each other if both were harmful and could warn off predators on their own. Müller put forward the first mathematical model of mimicry for this phenomenon: if a common predator confuses the two species, individuals in both those species are more likely to survive, as fewer individuals of either species are killed by the predator. The term Müllerian mimicry, named in his honour, has since been used for this mutualistic form of mimicry.
Müller wrote that

Overview

Evolved resemblance

Mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry functions to protect from predators. Mimicry systems have three basic roles: a mimic, a model, and a dupe. When these correspond to three separate species, the system is called disjunct; when the roles are taken by just two species, the system is called bipolar. Mimicry evolves if a dupe perceives a mimic as a model, and is deceived to change its behaviour to the mimic's selective advantage. The resemblances can be via any sensory modality, including any combination of visual, acoustic, chemical, tactile, or electric. Mimicry may be to the advantage of both organisms that share a resemblance, in which case it is mutualistic; or it can be to the detriment of one, making it parasitic or competitive. The evolutionary convergence between groups is driven by the selective action of a dupe. Birds, for example, use sight to identify palatable insects, whilst avoiding noxious ones. Over time, palatable insects may evolve to resemble noxious ones, making them mimics and the noxious ones models. Models do not have to be more abundant than mimics. In the case of mutualism, each model is also a mimic; all such species can be called "co-mimics". Many harmless species such as hoverflies are Batesian mimics of strongly defended species such as wasps, while many such well-defended species form Müllerian mimicry rings of co-mimics. In the evolution of wasp-like appearance, it has been argued that insects evolve to masquerade wasps since predatory wasps do not attack each other, and that this mimetic resemblance has had the useful side-effect of deterring vertebrate predators.
Mimicry can result in an evolutionary arms race if mimicry negatively affects the model, in which case the model can evolve a different appearance from the mimic.^p161 Mimics may have different models for different life cycle stages, or they may be polymorphic, with different individuals imitating different models, as occurs in Heliconius butterflies. Models tend to be relatively closely related to their mimics, but mimicry can be of vastly different species, for example when spiders mimic ants. Most known mimics are insects, though many other examples including vertebrates, plants, and fungi exist.

Crypsis

Crypsis, adaptation of an organism to avoid detection, has terminology and definitions that overlap mimicry. Crypsis may involve blending into an organism's environmental background while mimicry involves resembling another organism. Complications arise in cases like an insect which resembles a leaf, which is both another organism and part of the insect's background. For example, a katydid acts as a cryptic at a distance but up close it is a leaf mimic. Leaf mimicry in butterflies such as Kallima is a form of masquerade, protective resemblance.

Evolutionary explanations

It is widely accepted that mimicry evolves as a positive adaptation. The lepidopterist and novelist Vladimir Nabokov however argued that although natural selection might stabilize a "mimic" form, it would not be necessary to create it. The most widely accepted model used to explain the evolution of mimicry in butterflies is the two-step hypothesis. The first step involves mutation in modifier genes that regulate a complex cluster of linked genes that cause large changes in morphology. The second step consists of selections on genes with smaller phenotypic effects, creating an increasingly close resemblance. This model is supported by empirical evidence that suggests that a few single point mutations cause large phenotypic effects, while numerous others produce smaller effects. Some regulatory elements collaborate to form a supergene for the development of butterfly color patterns. The model is supported by computational simulations of population genetics. The Batesian mimicry in Papilio polytes is controlled by the doublesex gene.
Some mimicry is imperfect. Natural selection drives mimicry only far enough to deceive predators. For example, when predators avoid a mimic that imperfectly resembles a coral snake, the mimic is sufficiently protected.
Convergent evolution is an alternative explanation for why coral reef fish have come to resemble each other; the same applies to benthic marine invertebrates such as sponges and nudibranchs.

Living and non-living models

In its broadest definition, mimicry can include non-living models. The specific terms masquerade and mimesis are sometimes used when the models are inanimate, and the mimicry's purpose is crypsis. For example, animals such as flower mantises, planthoppers, comma and geometer moth caterpillars resemble twigs, bark, leaves, bird droppings or flowers. In addition, predators may make use of resemblance to harmless objects in aggressive masquerade, to enable them to approach prey. This wolf in sheep's clothing strategy differs from the more specific resemblance to the prey in aggressive mimicry, where the prey is both model and dupe.
Many animals bear eyespots, which are hypothesized to resemble the eyes of larger animals. They may not resemble any specific organism's eyes, and whether or not animals respond to them as eyes is also unclear. The model is usually another species, except in automimicry, where members of the species mimic other members, or other parts of their own bodies, and in inter-sexual mimicry, where members of one sex mimic members of the other.

Types

Many types of mimicry have been described. An overview of each follows, highlighting the similarities and differences between the various forms. A typology of mimicry is often based on function with respect to the mimic. Some cases may belong to more than one class, e.g., automimicry and aggressive mimicry are not mutually exclusive, as one describes the species relationship between model and mimic, while the other describes the function for the mimic. The terminology used has been debated, as typologies have differed or overlapped; attempts to clarify definitions have led to the partial replacement of old terms with new ones.

Name	No. of spp.	Function	Dupe finds Model	Deception	Description
Aristotelian	2	Protective	Agreeable	Deceptive	Brooding bird mimics itself with broken wing, luring predator away from nest
Automimicry	1 or 2	Protective	Agreeable	Deceptive	Multiple forms, e.g. one sex mimics the other, tail mimics head, etc.
Bakerian	2	Reproductive	Forbidding	Deceptive	Female flower resembles male flower, cheating pollinator
Batesian	3	Protective	Forbidding	Deceptive	Palatable mimic resembles distasteful model, deceives dupe
Browerian	2	Protective	Forbidding	Deceptive	Palatable butterfly resembles toxic member of same species
Emsleyan	3	Protective	Forbidding	Deceptive	Deadly snake resembles less deadly species, predators get chance to learn to avoid them
Gilbertian	2	Protective	Forbidding	Deceptive	Host/prey mimics and so repels parasite/predator
Kirbyan	2	Aggressive	Agreeable	Deceptive	Brood parasite adult or egg mimics host which raises the young as its own
Müllerian	3 or more	Protective	Forbidding	Honest	Distasteful co-mimics resemble each other, aposematically warning off predators
Pouyannian	2	Reproductive	Agreeable	Deceptive	Plant mimic resembles female bee, deceives male, gets itself pollinated
Vavilovian	3	Reproductive	Agreeable	Deceptive	Mimic resembles crop, deceives farmer
Wasmannian	2	Commensalist	Agreeable	Deceptive	Mimic resembles and deceives ant, lives in ant nest
Wicklerian	2	Aggressive	Agreeable	Deceptive	Predator or parasite resembles and attacks prey or host; parasite may get itself swallowed
Camouflage	2	Protective	Uninteresting	Deceptive	Mimic resembles background