Before revision:  An important biological example is the absolute discrimination between different ligand "qualities", such as immune decisions based on binding times to T cell receptors (TCRs) or Fc . In evolutionary simulations, the phenomenon of absolute discrimination is not achieved without detrimental ligand antagonism : a "dog in the manger" effect in which ligands unable to trigger response prevent agonists to do so. A priori it seems paradoxical to improve ligand discrimination in a context of increased ligand antagonism, and how such contradictory phenotypes can be disentangled is unclear. Here we establish for the first time a direct mathematical causal link between absolute discriminationand ligand antagonism.Inspired by the famous discussion by Gould and Lewontin, we thus qualify antagonism as a  "phenotypic spandrel": a phenotype existing as a necessary by-product of another phenotype.
After revision:  An important biological example is the absolute discrimination between different ligand "qualities", such as immune decisions based on binding times to T cell receptors (TCRs) or Fc We consider the general problem of absolute discrimination between categories of ligands irrespective of their concentration. An instance of this problem is immune discrimination between self and not-self. We connect this problem to biochemical adaptation, and establish that ligand antagonism - the ability of sub threshold ligands to negatively impact response - is a "phenotypic spandrel": a phenotype existing as a necessary by-product of another phenotype.