With the Extended Evolutionary Synthesis (EES, here the Theory of Niche Construction and Evolutionary Developmental Biology), theoretical biology undeniably enters into a post-Synthesis period. We will show that causal asymmetry is inherent to the very notion of evolution by natural selection (in its Darwinian Moderns Synthesis version), according to which environmental factors differentiate fitness and selectively act on variation, the latter being thus selectively causally passive. We will argue that EES’s understanding of natural selection makes a step toward a symmetric causality by showing that variation can determine its own fitness and thus its evolutionary fate. We will propose a model for symmetry in evolutionary causation (Relativity in Evolutionary Causation, REC); we use as an illustration the concept of causality underlying the General Theory of Relativity. We also will propose an individual-based simulation evaluating REC’s predictions about the role of individual variation (measured by species abundance distribution) and especially of ontogenetic variation (e.g. stemming from learning) in the origin and spread of evolutionary novelty and speciation (measured by diversification rate). We will discuss REC’s possible consequences for sympatric speciation as well as for the Baldwin effect and studies devoted to the evolution of language and social capacities.