The action of stabilizing selection on the perfection of new characters in evolution

In birds, fusion of the carpal elements to a carpometacarpus during morphogenesis takes place during the evolutionary process at different rates, i.e. heterochronically, in different groups (developmental radiations), like the elements of the nasal apparatus in Sauropsida. This is because new characters spread and are pushed back at different rates from the terminal to the initial levels of morphogenesis. The shift is most likely automatic, in that it does not require further mutation, but it is set in motion by selection. Selection does not take effect only when an adequate character (which at first is very imperfect) is accepted; it also causes the character to grow, pushes it back towards the threshold of morphogenesis and thereby increases its perfection and its fitness. The path along which the shift takes place is determined by the morphogenetic route of recapitulation of the ancestral structure and since this movement is caused by protracted stabilizing selection, we can describe it as orthoselective movement. This means that recapitulation is flexible. At the same time, cellular and epigenetic interactions with surrounding structures are reciprocally influenced. The shift continues until adequate adaptation and perfection have been achieved. Stabilizing selection then ceases to act. The shift shows the evolutionary trend or further developmental possibilities. Its chief role is probably in the development of specializations.