The molecular mechanisms that bring about biological form in modern-day embryos … should not be confused with the causes that led to the appearance of these forms in the first place … selection can only work on what already exists. (G. B. Muller and S. A. Newman 2003: 3, Origination of Organismal Form: Beyond the Gene in Developmental and Evolutionary Biology)— Cited in Minelli and Fusco 2008: xv. Evolving Pathways: Key Themes in Evolutionary Developmental Biology.
The evolution of organismal form consists of a continuing production and ordering of anatomical parts: the resulting arrangement of parts is nonrandom and lineage specific. The organization of morphological order is thus a central feature of organismal evolution, whose explanation requires a theory of morphological organization. Such a theory will have to account for (1) the generation of initial parts; (2) the fixation of such parts in lineage-specific combinations; (3) the modification of parts; (4) the loss of parts; (5) the reappearance of lost parts [atavism]; and (6) the addition of new parts. Eventually, it will have to specify proximate and ultimate causes for each of these events as well.— Muller, Gerd B. (2003) Homology: The Evolution of Morphological Organization. In Origination of Organismal Form: Beyond the Gene in Development and Evolutionary Biology. (eds., Gerd B. Muller and Stuart A. Newman). The Vienna Series in Theoretical Biology. MIT Press. p. 51.
Only a few of the processes listed above are addressed by the canonical neo-Darwinian theory, which is chiefly concerned with gene frequencies in populations and with the factors responsible for their variation and fixation. Although, at the phenotypic level, it deals with the modification of existing parts, the theory is intended to explain neither the origin of parts, nor morphological organization, nor innovation. In the neo-Darwinian world the motive factor for morphological change is natural selection, which can account for the modification and loss of parts. But selection has no innovative capacity; it eliminates or maintains what exists. The generative and the ordering aspects of morphological evolution are thus absent from evolutionary theory.
What is evo-devo? Undoubtedly this is a shorthand for evolutionary developmental biology. There, however, agreement stops. Evo-devo has been regarded as either a new discipline within evolutionary biology or simply a new perspective upon it, a lively interdisciplinary field of studies, or even necessary complement to the standard (neo-Darwinian) theory of evolution, which is an obligate step towards an expanded New Synthesis. Whatever the exact nature of evo-devo, its core is a view of the process of evolution in which evolutionary change is the transformation of (developmental) processes rather than (genetic or phenotypic) patterns. Thus our original question could be more profitably rephrased as: What is evo-devo for? (Minelli and Fusco 2008: 1)
(….) Evo-devo aims to provide a mechanistic explanation of how developmental mechanisms have changed during evolution, and how these modifications are reflected in changes in organismal form. Thus, in contrast with studies on natural selection, which aim to explain the ‘survival of the fittest’, the main target of evo-devo is to determine the mechanisms behind the ‘arrival of the fittest’. At the most basic level, the mechanistic question about the arrival of the fittest involves changes in the function of genes controlling developmental programs. Thus it is important to reflect on the nature of the elements and systems underlying inheritable developmental modification using an updated molecular background. (Minelli and Fusco 2008: 2)