On novelty, heterochrony and developmental constraints in a complex morphological theory of recapitulation

Darwin proposed natural selection as the main evolutionary mechanism in 1859. However, he did not think that this was the only process by which new species were generated. It was the so-called Modern Synthesis who established natural selection as the only mechanism responsible for evolution. Since then, the evolutionary process is explained by the pair mutation-adaptation: new species are generated by the appearance of new mutations, which in case of allowing new adaptations to the environment, they will be fixed and organisms will survive, therefore resulting in new species. An alternative view to the plasticity promoted by the adaptationist program is to think organisms as truly organized structures, having different levels of structural organization, which would mean that not every form is possible, but only those that correspond to a certain building plan. This would be reflected in the appearance of structural constraints, showing the limits imposed to the organism during its evolutionary development. In this work, I studied the ontogeny and development of three species of the genus Trophon by geometric morphometrics, in order to clarify important concepts in evolutionary developmental biology (Evo-Devo). Integrating theoretical and empirical investigations, I could propose a new conceptual framework for heterochrony in a context of a complex theory of recapitulation. Furthermore, I could detect a developmental constraint in Trophon, which provided an opportunity to reconstruct the concept of constraint and propose a synthesis between heterochrony and constraint that explained evolution as a process fueled by them, that is, as directive and driving force.

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