This special issue of Biological Theory is focused on development; it raises the problem of the temporal and spatial boundaries of development. From a temporal point of view, when does development start and stop? From a spatial point of view, what is it exactly that "develops", and is it possible to delineate clearly the developing entity? This issue explores the possible answers to these questions, and thus sheds light on the definition of development itself.

Regeneration capabilities are found in most or all animals. Whether regeneration is part of the development of an animal or a distinct phenomenon independent of development is a debatable question. If we consider regeneration as a process belonging to development, similarly to embryogenesis or metamorphosis, the existence of regenerative capabilities in adults can be seen as an argument in favor of the theory that development continues throughout the life of animals. Here I perform a comparative analysis of regeneration versus “classical” (...) developmental processes in animals in order to determine to what extent these processes are inclusive or distinct. I identify the existence of regeneration-specific processes, i.e., processes that occur during the regeneration, but not the initial development, of a given structure. In addition, I find that seemingly similar processes acting during development and regeneration may have differential molecular and cellular bases. I thus conclude that there are significant differences between regeneration processes in adult animals and developmental processes occurring during earlier phases of the life cycle. The existence of regenerative capabilities in adult animals can therefore not be used as an argument in favor of the idea that development spans the whole life. (shrink)

Is it possible, and in the first place is it even desirable, to define what "development" means and to determine the scope of the field called "developmental biology"? Though these questions appeared crucial for the founders of "developmental biology" in the 1950s, there seems to be no consensus today about the need to address them. Here, in a combined biological, philosophical, and historical approach, we ask whether it is possible and useful to define biological development, and, if such a definition (...) is indeed possible and useful, which definition can be considered as the most satisfactory. (shrink)

It is presumed that the evolution of morphological diversity in animals and plants is driven by changes in the developmental processes that govern morphology, hence basically by changes in the function and/or expression of a defined set of genes that control these processes. A large body of evidence has suggested that changes in developmental gene regulation are the predominant mechanisms that sustain morphological evolution, being much more important than the evolution of the primary sequences and functions of proteins. Recent reports1, (...) 2 challenge this idea by highlighting functional evolution of Hox proteins during the evolutionary history of arthropods. BioEssays 24:775–779, 2002. © 2002 Wiley Periodicals, Inc. (shrink)