Expected relationships between developmental divergence and the strength of selection. (a) Two potential patterns of conservation at the level of embryonic development. The grey line represents the classical pattern of early developmental conservation described by von Baer with the earliest stages of development being more constrained than later stages. The black line shows the 'hour-glass' pattern of conservation described by Raff . (b-d) Three predictions about the relationship between these patterns of divergence and natural selection (see text). Grey and black lines show, respectively, the action of natural selection acting on the genome at different times during development under von Baer's and Raff's models of embryological divergence and constraint. (b) Constraint results from negative selection on developmental variation. (c) Developmental buffering results in relaxed constraint on highly buffered developmental stages. (d) Divergence in development is the result of adaptation.