Functional thresholds in evolutionary progress

Analysis of some phylogenetically important stages in the evolution of graptoloids, articulate brachiopods, and the earliest mammals suggests that large scale evolutionary progress did not proceed linearly but that biological improvement was stepwise, achieved by rapidly crossing successive functional thresholds which were interpolated between gradual morphological changes (here termed proximations) towards the condition at which the structures became predisposed to the next threshold crossing. Crossing of many functional thresholds involved morphological discontinuities or functional instabilities that had to be bridged abruptly, over dithyrial populations. The principle of scalariform evolutionary progress is believed to be of fairly general application, but our current ability to analyse many critical evolutionary events in functional terms is still at a primitive stage because the functional significance of many important morphological changes are unclear. It is stressed that the evolutionary success of any ‘hopeful monster’ is determined at the population level, and that therefore, if extreme cases are discussed, the emphasis must be on ‘monstrous families’.