Viceroy (butterfly) - Evolution of Admiral Butterflies (Nymphalidae: Limenitis)

Evolution of Admiral Butterflies (Nymphalidae: Limenitis)

The world is divided into eight biogeographic areas called ecozones: Palearctic, Nearctic, Afrotropic, Neotropic, Australasia, Indo-malaya, Oceania, and Antarctica. Palearctic includes most of Eurasia and North Africa while Nearctic includes most of North America. Limenitis butterfly wing patterns are much more diverse in the Nearctic than the Palearctic. Three lineages of mimetic butterflies occur in North America and the evolution of mimicry may have played a large role in the diversification of this group. In order for butterflies to travel from the Palearctic region to the Nearctic region of the world, the migration must have occurred during a time period when Beringia, the land bridge between Eurasia and North America, was still above water. Based off crude divergence rate calculations, the colonization of the Nearctic Leminitis dates back approximately four million years. Whether the migration event was a single or multiple occurrence event has a significant effect on how we look at the evolution of mimicry. A history of multiple migrations would suggest that speciation occurred before the evolution of mimicry, meaning mimicry was the result of speciation instead of the driver of speciation.

However, much evidence supports that a single event colonization is the best explanation. One theory of Nearctic colonization states that the reason for the colonization was a larvae host plant shift. The position of the Poplar admiral (L. populi), a Palearctic species, in a phylogenetic tree confirms that the Poplar is the closest existing relative of the Nearctic taxa and is consistent with the theory that the host plant had a large effect on the evolution of North American admirals. Just like the wing-pattern of the Palearctic butterflies has little evidence of divergence, the host plant use of these species also shows no sign of divergence. These species only feed on different species of honeysuckle (Lonicera ssp.) The exception is the Poplar that feeds exclusively on aspen (Salicaceae: Populus tremulus). All North American Limenitis feed on Salicaceae as well, suggesting that an (ancestral host plant shift) expansion of a novel host plant across the Bering land bridge could have driven the colonization of the Nearctic. Species level phylogenies based on the mitochondrial gene COI and the gene EFI-α of Nearctic and Palearctic species also indicate a single colonization of the Nearctic species. The phylogenies produced indicate that a white-banded ancestor similar to the species L. arthemis. established itself in North America and resulted in several major lineages, three of which involved mimicry independently of each other. Given the present monophyly of the Nearctic species, it is likely that a single migration and subsequent expansion of the population was the foundation of the Nearctic butterflies.