Modeling Evo-Devo: Broken Hierarchies and Multiple Scales of Organization and Complexity

Abstract

Evo-Devo, the science that puts together in a common framework the dynamics of evolution with the processes of embryonic development is inherently multiscale. The hierarchical organization of life phenomena also contributes to the possibility of reducing its complexity to workable modules. However, the emphasis on a compositional, or blocks-within-blocks kind of hierarchy, implies a reductionistic perspective on multiscaling that ignores the irreducibility of some levels. The notion of generative hierarchies tackles this problem, introducing an organicist perspective that, while keeping levels of organization, acknowledges the existence of breaks in the hierarchy at the genomic, cellular, individual, and species levels. Whereas independent modeling in development or evolution has been done at each scale of organization, no multiscale approaches have so far been worked out that can account for the relationship between these two fundamental mechanisms that have shaped biodiversity throughout the history of life on Earth. Organization and Complexity in Evo-devoDeparting from evolutionary theory and developmental theory, the field of evolutionary developmental biology (evo-devo) has flourished in recent years, fuelled by the discovery of the so called "developmental genetic toolkit," a suit of genes used during development shared by most animals [1]. Before this renaissance, evo-devo had also a rich research tradition starting back in the 19th century with the French anatomist Etienne Geoffroy St Hilaire, and the German embryologists Karl von Baer and Ernst Haeckel (both famous for their law-like semiempirical developmental observations that ended up in Haeckel's linkage of ontogeny with phytogeny), and later in the 20th century, outstanding figures such as Garstang, De Beer, Waddington, Gould, and Alberch [2,3 and references therein]. In essence, this "old" evo-devo, was focused on morphological issues in a comparative framework. The emphasis of this morphological evodevo was on how anatomical parts differed in related species as a result of specific growth rates. This kind of research was subsumed under the all encompassing theme of heterochrony, which has still a very active role in the field. However, the morphological evo-devo tradition has reinvented itself, going beyond heterochrony as a main focus, to embrace other issues such as modularity, innovation and emergence of morphological traits, and phenotypic plasticity [4]. In the background of this tradition is the question of biological organization and biological complexity. Whereas organization has been "solved" by resourcing to hierarchy and modularity, biological complexity is one of those concepts for which there are no universal metrics; hence, it has rarely been used as a proxy for evolution and never to infer systematic relationships [5,6,7].