What+special+code+evolves?

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 * REF!: This is explored here in a model study on tRNA hairpin evolution (REF). Hogeweg??

=What special code evolves?=

Besides evolving to parts of the landscape that allow for greater mutational [|robustness], evolution can also select for more specific types of coding. This is explored here in a model study on tRNA hairpin evolution (REF). In the model, fitness is defined as the length of a hairpin.

In the case of selection for one hairpin, results show evolution towards greater robustness and this is achieved by long strechtes of [|pyrimidines] and [|purines] which bind together. This is a robust result and occurs for all mutation rates. In other words, evolution choose for the coding structure of pyrimidines and purines which is a fail safe way of forcing RNA to fold neatly into a single long hairpin. Pretty smart.

Now consider the case of two hairpins where the fitness criterion is the multiplication of hairpin lengths. In this case there is a different solution to the problem. For one of the hairpins the solution is as above, but on the other the system evolves to [|oligo] stretches of alternating pyrimidine and purine stretches. These hairpins do not fold that perfectly, but they prevent the two hairpins from interfering with each other, and therewith maintain the two independent hairpins. Also clever.

With this study we can therefore go beyond the more vague concept of a population evolving to a flatter part of the landscape. Here we can understand what that means in terms of coding, i.e. pyrimidine and purine strechtes are selected because they fold best into long hairpins, and are robust to mutations. Moreover, given a alternative fitness criterion, the system finds an alternative coding structure to accomodate that. Therefore, this study with a real coding structure makses the issues more grabable. It also illustrates that what a sequence looks like depends on drift to more robust structures. Therefore, sequences one might find may be the result of drift to more robust structures, i.e. an **evolutionary signature**.

Next: Constructed landscapes and landscapes as metaphors


 * References**