Butterfly Mimics Yield Secrets of Natural Selection
After more than a century of debate, an Earthwatch scientist has verified a central evolutionary theory using an elegant field experiment. In 1879, German naturalist Fritz Müller suggested that two unrelated species of butterfly that are both unpalatable will evolve to share the same bright color patterns so can help each other avoid predation. By sharing the burden of educating predators to avoid their warning coloration, unpalatable butterflies with identical patterns would be favored by natural selection, reasoned Müller. But while this theory has stimulated 130 years of field studies by biologists, the selective advantage of müllerian mimicry has never been clearly demonstrated, until now.
Dr. Durrell Kapan, a researcher at the Universidad de Puerto Rico, Rio Peidras, reported the müllerian mimicry of the butterfly Heliconius cydno in the latest issue of the journal Nature. Kapan's results are based on several years of research in the western rainforests of Ecuador supported by Earthwatch teams participating in Ecuador's Rainforest Butterflies. His findings represent the first field experiment clearly demonstrating natural selection for müllerian mimicry.
"Testing the benefit one target species gains from müllerian mimicry with another species is difficult because the researcher must minimize or eliminate the educational effect the first species has on its own predators," said Kapan. "Thus, to understand how müllerian mimicry works you need to find predators that are relatively unfamiliar with your target species but that will ignore it based on its shared resemblance with a 'co-model' species."
Kapan's work takes advantage of an unusual mimicry system found in H. cydno, a species with two divergent color patterns, or morphs, mimicking two other unpalatable Heliconius species, H. sapho and H. eleuchia, with yellow, white, and iridescent blue markings. The two color morphs of H. cydno tend to vary in abundance with presence and abundance of the two co-models, so that H. cydno mimicking H. sapho were more common in areas of the forest where H. sapho was abundant. By this finely tuned polymorphic mimicry, H. cydno apparently avoided predation by local jacamars, motmots, and flycatchers. Kapan's experiments sought to verify this assumption.
By transferring samples of the two color morphs of H. cydno to areas where their respective co-models were uncommon, Kapan was able to test the prediction that they would be subject to more attacks by predators there. Control groups of the alternate morph were also transferred to areas where their co-model was common, and all experimental subjects were individually marked for observation later. The introduced samples that did not match the locally dominant co-model suffered significantly more attacks by predators than the controls that matched the common co-model, clearly demonstrating natural selection for divergent color patterns.
Kapan's findings hold important implications for the origin and maintenance of diverse warning colors in Ecuadorian butterflies, and the source of new mimetic species. His study also sheds light on the origins of species diversity in general, a key problem at the heart of ecology and conservation.
"My research goes one step towards understanding why we get diversity in mimicry by showing that one species can be influenced to display more than one color-pattern through mimicry with two differently colored species" said Kapan. "Thus, variation across Ecuador in species abundance generates variation in which species it is better to look like. This is just one strikingly obvious example of how 'divergent selection' due to environmental variation across sites can generate diversity within a species."
H. cydno and it's two co-models live in a region of Ecuador where less than 8 percent of the primary rainforest they depend on remains. The butterflies rely on the rainforest's uncommon resources for survival. For instance, H. sapho larvae feed on a species of canopy passion vine (Passiflora tina) that has only recently been described, the flowers of which were first documented in the field by Kapan and his Earthwatch team in 1998. The implications of this intricate mimicry system for species conservation could be profound. For instance, the loss of one mimic could lead to the rapid loss of others in the system.
"If forest destruction eliminates the canopy vine that H. sapho depends upon we might see the rapid loss of the white morph of H. cydno," said Kapan. "Since we do not know if the long-term survival of the polymorphism in H. cydno depends on mimicry we cannot say for sure that mimetic diversity would be decreased, but it seems likely."
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