Abstract
The concept of developmental constraints has been central to understand the role of development in morphological evolution. Developmental constraints are classically defined as biases imposed by development on the distribution of morphological variation.This opinion article argues that the concepts of developmental constraints and developmental biases do not accurately represent the role of development in evolution. The concept of developmental constraints was coined to oppose the view that natural selection is all-capable and to highlight the importance of development for understanding evolution. In the modern synthesis, natural selection was seen as the main factor determining the direction of morphological evolution. For that to be the case, morphological variation needs to be isotropic (i.e. equally possible in all directions). The proponents of the developmental constraint concept argued that development makes that some morphological variation is more likely than other (i.e. variation is not isotropic), and that, thus, development constraints evolution by precluding natural selection from being all-capable.This article adds to the idea that development is not compatible with the isotropic expectation by arguing that, in fact, it could not be otherwise: there is no actual reason to expect that development could lead to isotropic morphological variation. It is then argued that, since the isotropic expectation is untenable, the role of development in evolution should not be understood as a departure from such an expectation. The role of development in evolution should be described in an exclusively positive way, as the process determining which directions of morphological variation are possible, instead of negatively, as a process precluding the existence of morphological variation we have no actual reason to expect.This article discusses that this change of perspective is not a mere question of semantics: it leads to a different interpretation of the studies on developmental constraints and to a different research program in evolution and development. This program does not ask whether development constrains evolution. Instead it asks questions such as, for example, how different types of development lead to different types of morphological variation and, together with natural selection, determine the directions in which different lineages evolve.
Highlights
One central tenet of evolutionary developmental biology, or evo-devo, is that development is important for understanding morphological evolution [1,2,3,4,5,6,7,8,9,10,11]
For natural selection to be the only important factor determining the direction of evolution, it is required that morphological variation is possible and likely in all directions [3, 9, 14, 19], at least by the small gradual changes favored in the modern synthesis
As discussed in the previous sections, the developmental constraint and developmental bias concepts, either in its positive or negative meanings, imply there is an expectation on possible morphological variation and that development is the cause for a departure from such expectation rather than the source of such expectation
Summary
One central tenet of evolutionary developmental biology, or evo-devo, is that development is important for understanding morphological evolution [1,2,3,4,5,6,7,8,9,10,11]. For natural selection to be the only important factor determining the direction of evolution, it is required that morphological variation is possible and likely in all directions [3, 9, 14, 19], at least by the small gradual changes favored in the modern synthesis. Another possible reason to expect the existence of some morphologies would be, as explained above, the isotropic expectation required for natural selection to be the only important factor determining the direction of morphological evolution.
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