Abstract
The assumption that natural selection alone is sufficient to explain not only which traits get fixed in a population/species, but also how they develop, has been questioned since Darwin’s times, and increasingly in the last decades. Alternative theories, linked to genetic and phenotypic processes, or to the theory of complex systems, have been proposed to explain the rise of the phenotypic variety upon which natural selection acts. In this article, we illustrate the current state of the issue and we propose a logical space based on phenotypic robustness that allows a classification of evolutionary phenomena and can provide a framework for unifying all these accounts.