View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Lake Forest College Publications Eukaryon Volume 4 Article 24 3-16-2008 The volutE ion of Mimicry Ethan Helm Lake Forest College Follow this and additional works at: http://publications.lakeforest.edu/eukaryon Part of the Animal Sciences Commons, Biodiversity Commons, and the Evolution Commons Disclaimer: Eukaryon is published by students at Lake Forest College, who are solely responsible for its content. The views expressed in Eukaryon do not necessarily reflect those of the College. Articles published within Eukaryon should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author. This Review Article is brought to you for free and open access by the Student Publications at Lake Forest College Publications. It has been accepted for inclusion in Eukaryon by an authorized editor of Lake Forest College Publications. For more information, please contact [email protected]. Eukaryon, Vol. 4, March 2008, Lake Forest College ____________________________Review Article The Evolution of Mimicry Ethan Helm* deduced. This review is a comprehensive analysis of Department of Biology primary literature focusing on addressing these Lake Forest College evolutionary issues, while also evaluating the two-step Lake Forest, Illinois 60045 hypothesis and the gradual hypothesis of Müllerian mimicry. Introduction Mathematical Modeling of Mimicry Cryptic species have evolved camouflage, which enhances survival by decreasing their visibility and thus Mathematical and computerized modeling systems are protecting them from would-be predators. Conversely, commonly used to mimic the evolution of mimicry. This aposematic species have evolved vibrant colors which methodology is advantageous because it enables enhance visibility. These warning signals work by scientists to study avoidance learning. Modeling helping unpalatable, toxic, evasive, or stinging prey studies require many assumptions, because it is hard to stand out from more favorable prey. Thus, predators create a system exactly like the wild. For mimicry, learn to generalize the appearance of prey which taste studying the interactions and development of avoidance bad or can inflict pain (Balogh 2005). learning is extremely difficult in the wild. This is the Yet, all species with aposematic coloration do biggest single advantage for mathematical models. not have other unfavorable features. These species, known as Batesian mimics, have evolved to look like Gradual Evolution or the Two Step Hypothesis of model species which are unprofitable to predators Evolution of Mimicry (Mappes 1997). Thus, Batesian mimicry enhances Balogh and Leimar (2005) created a computerized survival of otherwise unprotected species, while mathematical model to investigate gradual evolution as increasing the attack rate of the model species and defined by Fisher (1927) through utilizing a Müllerian deceiving the predator (Ritland 1991). As such, mimicry predator spectrum. In the model, predators Batesian mimicry is part of an evolutionary exploitative generalize their experience with unpalatable prey relationship, in which the mimic derives benefit at the causing them to avoid prey similar in appearance to the prey and predators expense. unpalatable prey (Balogh 2005). The model predicted Conversely, Müllerian mimicry, in which two that gradual evolution, as defined by Fisher (1927) (or more) unfavorable prey species share similar occurs in Müllerian mimicry systems by showing that physical characteristics, actually benefits all of the evolution toward mimicry occurred through directly involved species. Since the co-mimics are predominantly small peak shift mutations. Having a unfavorable and similar in appearance, Müllerian variety of predators with different generalizing mimics are less likely to be consumed by predators. specificities increased the gradual shift (Balogh 2005). The predators benefit because they do not have to Thus, ecosystems with multiple predators are more suffer the consequences of consuming the unfavorable likely to demonstrate gradual evolution of Müllerian prey. Thus, Müllerian mimicry is a complex mimicry, and coevolutionary changes frequently play a evolutionary mutualism which enhances survival of the role in Müllerian mimicry evolution. involved prey and predators alike (Gavrilets 1997). Through replicating Balogh and Leimar’s There are many questions regarding the (2005) model and modifying it slightly, Franks and evolutionary pathway of mimicry which have not yet Sherratt (2007) studied gradual evolution of multiple been elucidated. Two theories have been proposed to components. Through this, their model suggested that explain the pathway of Müllerian mimicry. The first gradual evolution is only possible when only a single theory, developed by Nicholson in 1927, is known as component or characteristic is being mimicked or the the two-step hypothesis. In this theory, evolution predators generalize widely over all components. In occurs first due to a large mutational change which other words, multicomponent Müllerian mimicry tends to causes a mimic to appear more similar to a model. evolve via the two step hypothesis of evolution, which After this large mutational change, the second step is a states that mimetic evolution occurs initially as a result gradual change (Balogh 2005). Conversely, Fisher of a large mutational change in prey which closely (1927) proposed the gradual hypothesis which is driven resembles a model species, and over time, smaller by predator generalization. This theory states that mutations refine the phenotypic similarity to the model mimicry occurs from many small mutations each of species (Turner 2000). Still, under certain ecological which slightly increase the similarity between the mimic circumstances, gradual evolution is the rule, and in fact, and the model. While eighty years have passed since the two theories are not necessarily mutually exclusive. these two theories were developed, the evolutionary path for Müllerian mimicry has still not been elucidated. Ecosystem Dynamics as a Selective Force for the Many other questions regarding the evolution Evolution of Mimicry of mimicry also still exist. For instance, the role of To evaluate the selective forces involved in the selective forces such as varying mortality rates among selection of Müllerian mimicry, Beatty (2004) simulated mimics and models in Batesian mimicry, population predator/pray relationships via a computer program dynamics and associated ecological interactions, and which examined avoidance learning in human predators the driving force for aposematic coloration have still not by allowing them to search a virtual environment for been elucidated. Furthermore, the role of imperfect prey. The study consisted of five separate mimicry, if any, in the evolution of mimicry has not been experiments. The first experiment utilized a simple ____________________________________________ system in which predators were exposed to an equal amount of profitable and unprofitable prey. The *This author wrote the paper for Biology 483: Plant and Animal profitable prey was all green, while the unprofitable prey Interactions, taught by Dr. Lynn Westley. 56 was split into nine different frequencies of green and investigate whether Müllerian and Batesian mimicry blue prey. In all, the experiment demonstrated that rare based on difficulty to capture is theoretically possible; forms of unprofitable prey are eaten more frequently Ruxton and associates (2004) created two homologous than common forms of unprofitable prey (Beatty 2004). mathematical models. The model demonstrated that Because of this, there is a selective force for Batesian mimicry is theoretically possible when uncommon unprofitable prey to mimic common predators have another food source, and pursuing unprofitable prey. The second experiment also utilized evasive prey is energetically unfavorable. Conversely, a simple system in which profitable prey was yellow, Müllerian mimicry is most likely to occur in situations in non-focal unprofitable prey was blue with a stripe, and which evasion is costly to the prey, predators learn the focal prey varied per trial (solid pink, black dot with avoidance slowly, and the abundance of evasive prey pink background, or pink with black stripe). Among the species is not the same. Thus, based on these three focal prey, each was selected equally mathematical models, the evolution of evasive mimicry demonstrating that Müllerian mimicry is not favored in should be common under the right conditions. simple communities (Beatty 2004). Thus, in communities in which predators have limited prey Signal Accuracy and Initiation of Mimic Evolution options, there is little to no driving force toward The widespread occurrence of aposematic coloration Müllerian mimicry. has been linked to predator learning (Lindström 2001). Beatty (2004) also analyzed Müllerian Sherratt and Beatty (2001) investigated this claim by mimicry within communities including predators with using a computerized model utilizing human predators multiple prey options. In experiment three, six different and studying the responses of human predators to profitable prey shared a common characteristic in which defended and undefended computer-generated prey they were all solid colored. Likewise, the six non-focal species. The findings suggested that the evolution of