Incipience is a classic problem in evolutionary theory. How can a new structure arise through Darwinian evolution, when its incipient stages could not be useful? Preadaptation is the classic solution; an existing structure gradually acquires a new function, and consequently, a new size and shape. This is fine, but it sheds no light on the origin of structural elements in the first place.
If we subscribe to a theory of vertebrate evolution which admits the gradual elaboration of articulate skeletons, as in the traditional idea of tetrabod bones being elaborated from a simpler crossopterygian limb, preadaptation is not the answer. Such a theory must find a response to the criticism that random variations, even though sorted out by natural selection, cannot alone account for the emergence of new structural elements in useful positions, complete with interrelated parts of various cell types.
Some observable mutations are elaborative, such as polydactyly. Under the proposed model, these mutations are not evolutionary novelties but are rare varieties, representing the extreme of morphological variation in the species. All such supernumerary structures are always there, in a sense, in the embryo; they are just rarely expressed or realized. If they are evolutionary novelties of the kind which are supposed to be springing up randomly, why are they so nicely structured and positioned? It is easier to believe that random variation does not create such new structures, and that in the long term skeletons only diminish in number of parts, as the fossil evidence implies.
This requires the postulation of a beginning, when many-segmented organisms were rapidly formed, even though this may coincidentally and unfortunately appeal to creationists. And this requires giving up the idea that evolution is infinitely capable in any context; we must accept that within a particular time frame, however long, and within a particular biological system, however predominant, it may be that evolutionary change is limited to certain kinds of transformation, and that even these limited kinds of change are bought at a price--the loss of parts which cannot be replaced. The alternative to postulating such a beginning for segmented organisms is to believe that complex new structures emerge in fortuitous places though random variation.
forward to Specialization