Conformational selection between pre-existing structural states of an oligomeric protein was the conceptual step behind the formulation of Monod-Wyman-Changeux (MWC) allosteric theory. Variations on the basic theme of allosteric control are briefly illustrated in this paper by reference to some hemoglobins from different species whose functional properties were found to respond to specific physiological requirements. In my opinion the enormous success of the allosteric theory may be attributed not only to its efficiency in accounting for data and its formal mathematical elegance, but also because the selective mechanism conforms to the founding concept of Darwinian evolution.