This paper deals with an evolutionary design method of structure using a genetic algorithm. In order to establish this method, virtual trees, which are able to adapt to various design requirements, are created in simulated environments. Assuming that the growth of the trees is a repetition of the self-similar shape of the structure (fractal), the tree growth is computer simulated in a virtual environment taking into account the effects of the sun, wind, gravity force and so on, where we introduce the concept of a "seed" representing the tree design method. Using a fractal-like calculation to translate the genotype of the seed into the phenotype of a virtual tree, one seed corresponds to the creation of a biotypical structure adapting to the environment. Using this idea, we evolve seeds representing the design methods by studying two cases: (A) stationary virtual environment and (B) nonstationary virtual environment. We examine the adaptability of seeds A and B in different environments, and demonstrate that seed B is more adaptive to changes in the virtual environment. We discuss the applicability of the evolutionary design method to the engineering design of structures.