Arguably one of the most fascinating questions is that of understanding the basic mechanisms by which (molecular) structures or organisms evolve over time. |
The concept of "self design" of organisms by way of inheritance dates back to Lamarck (1800). This type of "adaption" will lead to evolution.Lamarck's
theory stipulates that all individuals of a species are indistinguishable
and adapt as a whole to an environment induced goal.Lamarck's
conceptualization exhibits many features of modern theories on evolution. The milestone in evolutionary theories is Darwin's theory of biological evolution (1859) presented in the "The Origin of Species". Darwin's theory is based on two fundamental principles, genetic variability caused by mutation and natural selection. |
In contrast to Lamarck he observed that there is hereditary diversity among offspring i.e. offspring is individualized and that survival is not just a random sample but those who are better adapted to the environment. Accordingly distinct individuals possibly take different routes eventually leading to speciation. Without doubt, this diversity of adaptive responses can be viewed as "neutral evolution". 1883 Weismann introduced the dualism of a "legislative" and "executive" which he located in the cell nucleus. Obviously, his conceptualization is still part of contemporary molecular biology. The work of Morgan, Fisher and Haldane showed that Darwin's theory of evolution is compatible with a theory of inheritance based on genes organized via chromosomes (potentially
being present in multiple variants). However, the biochemical identity of a
gene was still unknown.It was 1953 when Watson and Crick discovered the structure of DNA as two hetero polymeric strands with monomer-specific
hydrogen bonding. |
It was this structure itself that suggested that the molecule could replicate by growing a respective complement of each strand. As a surprise
then came the discovery of the triplet (trinucleotide) coding for amino
acids, the building blocks of proteins. |
It all came together: the trinucleotide code proved the linkage between Weismann's legislative and executive. Then it was discovered that RNA mediated the gene expression as a complementary, ribonucleic acid copy of a single DNA strand. It was shown that this copy is translated into the corresponding protein chain via the translation process, which in itself contains RNA. The central dogma was coined: from DNA to RNA to Protein. In extension of Darwin's theory of evolution Wright identified the role of stochastic processes. Here the idea is that a population of molecular sequences undergoes mutational changes on the level of individual sequences. Wright saw the importance of the genetic drift in evolution in improving the evolutionary search capacity of the whole population. |
A fundamental shift in paradigms was obtained by Kimura who proposed that the majority of changes that are observed in evolution at the molecular level were the results of random drift of genotypes. He stipulated that most mutants are neutral in fitness and the fixation of new genotypes is the outcome of a stochastic process. During the 70s and 80s Eigen and Schuster developed a theory of information in polynucleotide sequences undergoing erroneous replication. They introduced the concepts of the molecular quasispecies and the hypercycle which, assuming fixed mutation rate and sequence length, can be analytically described. It is the molecular quasispecies that is acted upon by evolution,the Darwinian process itself assures the maintenance of
variability.
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In the 1990s Schuster started changing the paradigm and took into account that RNA can be both: legislative and executive in one molecule. Schuster used the concept of RNA secondary structures pioneered by Waterman, and considered the mapping from sequences to structures in the context of evolution.His "RNA toy world" allowed to put the concept of molecular quasispecies to the level of molecular structures and to study in particular
one central aspect of evolution: neutrality.
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Neutral networks were discovered, i.e. sets of sequences all being mapped into a given structure and equipped with an adjacency notion induced by the Hamming graph they are embedded in. Neutral networks convey the ideal
combination of search capacity and robustness to mutations. Random graph
theory showed that neutral networks undergoes a phase transition with respect
to their connectivity and in addition that any two of them come close in
sequence space. As a result neutral networks allow populations to diffuse by
single nucleotide mutations without loosing a fit structure while constantly maintaining a hull of mutant-sequences through which eventually a fitter structure is discovered.
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