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Variations upon variations

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Let's step back a bit to understand how the variation of already occurring themes is so important. Most thinking about molecular development begins with a paradox. How could the same materials that created a specific stage of an organism’s life create other stages as well? Insects, such as fruit flies, not only grow  larger, they also dramatically change their forms as they grow. The untrained eye can find little similarity between the soft grub of a juvenile fly and the well-articulated, chitin armored appearance of the adult. Even humans go through subtle chemical metamorphoses, as some lactose intolerant adults lose the ability to metabolize the sugar lactose, a metabolic skill they relied upon as an infant. Why did they lose this ability (or better yet, what happened to allow others to keep this juvenile state?) Or, in more biologically precise terms, how could a limited nucleic acid code account for all the changes organisms exhibit during their lives? Clearly, something was happening to allow some parts of the code to be expressed at one time and not at another. This implied that genes not only needed to be turned on, they had to be turned on at the right time and in the right place.

In his 2005 book, Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom, Sean Carroll summarizes the conceptual innovations that needed to occur to explain developmental changes. Key for Carroll was the recognition that development occurred as a “logic of making a series of initially similar modules and then making them different from one another.” (34). Key to this realization is that bodies varied twice in the evolutionary and developmental logic. The first variation occurs at the level of segmentation of each animal, as animal bodies occur as a series of varied body segments. Thus each segment  is similar to but not identical to the next segment. In fruit fly bithorax complex, for instance, one segment makes wings and the next makes halteres. The second crucial variation, was that each of these segments could vary as well. This would explain the development of homeo-mutants, such as the four winged fly. Depending on some form of mutation the segment that was supposed to be an abdominal segment could be transformed into a thoracic segment. 

This remarkable view of animal development also explains how differences and similarities occur between species. Imagine now that this basic module was not just for a specific species of animal, but for all types of animals. This would suggest an even third level of variation, where segments varied from animal to animal. Now all creatures could be explained through the variation of segmental construction. Morphological changes between species, such as the changes between the human forelimbs of arms and a birds forelimbs of wings are variations that occur in how the same materials and processes are organized. This is the importance of gothic moments, these are the moments during development when significant and irreversible changes occur.

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  1. From Sensory Bristles to the Spots on a Butterfly's Wing Phillip Thurtle

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