Evolution and Monarchs
Posted on Sunday, January 1, 2006 at 11:40 am in Monarch Biology
So, what secrets does the story of monarch evolution hold? How has this tenacious insect developed over time to become the butterfly we know today? What are its closest ancestors and how did it split from them along the evolutionary tree? How has it developed the characteristics which make it so unique, like it’s migratory life cycle and mating strategies? And how can these concepts be integrated into the classroom?
The Class Insecta, to which monarchs belong, appeared in the fossil record during the Devonian period, around 400 million years ago. The earliest forms were small and flightless, but because insects can rapidly adapt to selective pressures and produce many offspring to pass on these adaptations successfully, they quickyl radiated (evolved) into different forms. for instance, the first winged insects (the pterygotes appear quite explosively in the fossil record-- evidently because the ability to fly gave them such an enduring survival advantage.
The order Lepidoptera, which includes today's butterflies and moths, emerged in the fossil record during the cretaceous Period (146-65 million years ago). it was a member of the Endopterygotaclade, which was successful through the adaptation of complete metamorphosis. In these "holo-metabolous" insects a larval stage is specialized for eating and growing, and an adult stage specialized for mating and reproducing. Usually this adult form is winged, whereas the larval form is not. Instead the wings develop internally and mature during a resting or pupal stage (as in the familiar monarch life cycle).
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The closest relatives to the Lepidoptera are in the order Tricoptera (caddisflies). Caddisflies also undergo complete metamorphosis, but their arvae are aquatic and spend this stage of their life in fast-moving streams. They also pupate under water. Only when they emerge as adults do their similiarities to moths and butterflies become obvious. Adult caddisflies look like small, simple moths, and their wings are covered in hair-like scales like Lepidoptera. However, the caddisfly adult only lives for a few days. Often its mouthparts are reduced or absent and they cannot eat.
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The Lepidoptera have the distinct advantage of being able to feed as adults (with a few exceptions). Their characteristic proboscis allows them to suck nectar from flowers, thus obtaining the nergy to live longer and reproduce more effectively. This trait led to one of the most interesting evolution stories of all. The process of co-evolutino between the higher insects and flowering plants is a striking phenomenon, with 20% of all insects feeding on flowers, pollen, or nectar and 2/3rds of flowering plants relying on insects for pollination.
The monarch belongs to the family Nymphalidae, a group of butterflies characterized by a reduced first pair of legs. Other members of this family include the beautiful blue morpho butterfly and the painted lady. Monarchs are in the sub-family Danainae, which contains butterflies that have evolved to feed on the poisonous milkweed plant, actually assimilating the toxins from the milkweed into their bodies. This unique development has given the Danainae a distinct advantage; they are distasteful to many predators. Some species of the Danainae clade closely resemble the monarch, but are brilliant blue instead of orange, like the glassy blue tiger to the left. Many of these blue species are endemic to Africa and Asia.
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