Müllerian mimicry is a form of biologicalresemblance in which two or more dangerous or toxic organisms exhibit similarwarning systems. These organisms, that may or may not be closely related, facilitatepredator learning by mimicking each other’s warning signals, such as the same brightly colored wingpattern, to their mutual benefit.
Because a predator that haslearned to avoid an organism with a given warning system will avoidall similar organisms, the resemblance between Müllerian mimics acts as aprotective mechanism for participating organisms. Through a variety oftechnical approaches in developmental genetics and the construction of geneticlinkage maps, researchers determine gene expression and selective agents as sitesthat controls phenotypic traits related to mimicry. Müllerianmimicry relies on aposematism, or warning signals that make harmfulorganisms more identifiable by predators. Organisms with these signals areavoided by predators, which learn through experience not to hunt the same unpalatableprey again. Researchersspeculated that noxious organisms that developed a resemblance to each other mayhave had a selective advantage over organisms with unique warning signals. If different prey species with a mutual predatoremployed their own distinct warning signal predators would have to learn toidentify several different unpalatable prey groups through experience. Unalikeprey groups would suffer a greater loss of individuals due to the increasednumber of learning experiences required for predators to learn each warningsignal. Prey that evolve to share a similar appearance or mannerism share thecosts of predator education and reduce the overall number of experiencesrequired for predators to learn a common warning signal.
Because prey speciesthat appear similar to unpalatable species often escape predation incomparison to their conspecifics, natural selection drives preyspecies toward a single warning language. This mutualistic arrangement subsequentlyleads to multiple species joining the protective cooperative and the evolutionof Müllerian mimicry. Müllerian mimicry was firstidentified in tropical butterflies that shared colorful wing patterns.Some insight into the evolution of mimetic color mimicry in longwing (Heliconius) butterfliesin particular can be seen through the study of the genes associated with colorpatterns in wings.
The butterflies’ signature red wing patterns, which signalto predators of its toxicity, is largely controlled by the Optix gene. Bysharing this distinct coloration pattern Heliconius butterfly increases itschance of survival through association with other poisonous red wingedbutterflies the predator may have previously established as unpalatable. Thegenetic linkage maps of many related species of Heliconius butterfliesshow that the evolution of a single transcription factor (Optix gene) plays acrucial role in driving the convergent evolution of composite color patterns indistantly related species. Because the hypothesized reason behind phenotypic mimictraits is the evolution of a non-coding piece of DNA that regulatesthe transcription of nearby genes it is difficult to determine if the traitis homologous or simply the result of convergent evolution.However, the results of previously complied studies revealed that an allelic substitutionat the Optix locus in the Numata longwing butterfly (H. numata) and in the postman butterfly (H. melpomene) showed no obvious phenotypic homology.
One proposed mechanism for Müllerianmimicry is the “two step hypothesis”. The hypothesis consists of an initialmutational leap that establishes an imprecise resemblance of the mimic to themodel, followed by smaller changes establish a more accurate semblance. A simulationof a model population was compared to a mimic population on a feature traitscale and determined that without an initial mutational leap toward likeness inphenotype, a similar simulation did not result in gradual mimicry evolution inthe mimic population. These results led to the conclusion that drastic changesin qualities used by predators to determine palatability of prey is likely beessential in the initiation of Mu?llerian mimicry evolution.
The implicationsof this conclusion suggest that the evolutionary path to mimicry becomes morelikely when the mimic population has a predisposition to be similar to themodel population. A shortcoming in the two-step process is that the process relieson the assumption that a mimic trait is governed by a single gene, which is unlikelygiven the complexity of the coloration patterns observed in Müllerian mimics. The two-step hypothesis on Müllerianmimicry laid the foundation for the hypothesis of advergent evolution in mimicpopulations. In advergent evolution, the selection pressure of predation influencesthe mimicking species to emulate the model more and more precisely. There is noimplicit mutualism, as previous studies originally theorized for convergentevolution, and any initial advantage is thus to the mimic. Once high similarityin a particular trait has been established in the model and mimic, some degreeof mutual protection becomes likely.
The original convergence theory adopted byearlier studies hypothesized that all mimicking species in an area shouldconverge on a single pattern of coloration. This scenario does not appear tohappen in nature as more recent research reveals that Heliconius butterfliesform several clusters of mimicry species known as rings in a singlegeographical region. The finding implies that mimicry rings initially formthrough a process of advergence of one species of mimics to another beforeconvergence of larger rings of Müllerian mimics. Thetheory of evolution of Müllerian mimicry finds basis in both genetics andevolutionary biology. Studies suggest that the predominant driving force of superficialresemblance in two or more noxious species is likely the predation of organismswith uncommon warning signals. Because phenotypic mutations towards mimicry areonly beneficial once resemblance is established it is hypothesized that theevolution of mimicry occurred in a two-step process with advergence first occurringfollowed by later convergence of mimic rings.