We have reported for the first time that by using a generic magnesium-free saline buffer, simply composed of tris-acetate buffer supplemented by a suitable concentration of NaCl, highly multicomponent mixtures of DNA strands spontaneously self-assembled at room temperature and isothermally evolved toward their equilibrium state consisting in properly shaped objects. We demonstrated that this isothermal self-assembly could be operated in a broad range of temperatures (15 -60 °C) and salt concentrations (50 -250 mM), and was compatible with various designs that could be either scaffolded (origamis of different shapes) or scaffold-free (SST assemblies, DNA nanogrids), producing highly multicomponent motifs of well-defined size and shapes (origamis, SST) or self-repeating units extending in space (DNA nanogrids), while allowing in situ site-specific functionalization by proteins. With such a versatile and robust character, this isothermal self-assembly method is not only a novel and user-friendly way to produce functional nanostructures but also holds great perspectives to be combined with more sophisticated designs to generate tridimensional structures and/or larger-scale assemblies. Contrarily to conventional thermal annealing-based procedures, this isothermal self-assembly process was shown to be thermodynamically driven. With a buffer allowing reconfigurability and folding/assembly pathway exploration, kinetic traps were avoided and isothermal conditions let the DNA mixture progressively evolve until reaching the most stable state defined by the properly assembled nanostructure. This feature opens up new horizons for autonomous structure selection and evolution in competitive systems. For instance, we demonstrated here that a complex mixture of competitive DNA strands coding simultaneously for different shapes led to the successful and selective formation of the most stable shape only, thermodynamically unfavored competitive shapes, chimera or mis-folded structures being avoided. Demonstrated here with only two different competing shapes, it would be interesting to explore more complex mixtures, either to test models with designed strand mixes or to discover new set of DNA strands producing structures with better shape or functionality, for example to recognize and/or capture a target protein added to the self-assembling strands. Finally, we showed that this competitive selection capability was not restricted to a mixture of initially unbound DNA strands but was also effective starting from preformed assemblies to produce new dynamic responses. We demonstrated in particular that preformed DNA origamis exposed to a full set of competitive staples could spontaneously evolve and transform into a radically different shape by the exchange of all its constitutive DNA staples through a massive strand-displacement reaction. Thermodynamically driven and adjustable through staple composition and temperature, this process allows for complex DNA assemblies to morphologically evolve each time a new free energy minimum appears, for instance in response to a change in their composition or environment. This constitutes a novel and valuable route to build smart nanomachines capable to morphologically evolve with an encoded predefined dynamics or self-adapting to a changing environment. This opens particularly interesting perspectives for dynamic operations in living systems where the temperature is fixed, as well as for nanostructure discovery by directed evolution-inspired protocols where improved structures of complex shapes (e.g., protein binder) could emerge and be selected from large libraries of DNA components.