Vertebrate evolution: the developmental origins of adult variation

Many biologists assume, as Darwin did, that natural selection acts mainly o n late embryonic or postnatal development. This view is consistent with von Baer's observations of morphological divergence at late stages. It is also suggested by the conserved morphology and common molecular genetic mechani sms of pattern formation seen in embryos. I argue here, however, that diffe rences in adult morphology may be generated at a variety of stages. Natural selection may have a major action on developmental mechanisms during the o rganogenetic period, because this is when many adult traits are specified. Evolutionary changes in these early developmental mechanisms probably inclu de subtle shifts in the timing of gene expression. Changes of this kind hav e little or no gross effect on the anatomy of the embryo; they are only phe notypically expressed, or readily detected, when amplified at later stages. The phylotypic stage, the developmental hourglass, modularity, and von Bae rian divergence are reassessed in terms of these arguments. (C) 1999 John W iley & Sons, Inc.