Embryos in Deep Time: The Rock Record of Biological Development

2. Evo-Devo, Plasticity, and Modules

34 The discipline that brings together the fields of developmental genetics and evolution has been baptized “evo-devo.” Few if any new aspects of evolutionary biology have received as much attention from practitioners and philosophers of biology. Evodevo is purported to provide a new kind of synthesis of knowledge to understand the origin of biodiversity. For this reason, it is important to ponder how paleontology contributes to this area. The central focus of evo-devo is contested. Some see it as explaining the evolution of the capability to evolve, or evolvability . Many see it as explaining evolutionary novelties or innovations in the sense of truly new, large steps of morphological change—for example, the evolution of eyes, teeth, or limbs, or the turtle shell.1 Others see evo-devo as a “passing comet,” a discipline that will be superseded by a newer merging of fields or by a new ordering of groups of researchers and topics, perhaps dictated by the rise of genomics.2 Whatever the case, there is no denying that these discussions have brought much reflection and impulse to evolutionary studies of development. two Evo-Devo, Plasticity, and Modules Evo-Devo, Plasticity, and Modules / 35 The beginnings of evo-devo were very much about what many distantly related groups, such as flies and humans, or disparate features, such as teeth and limbs, have in common. That the eye of a fly and the eye of a vertebrate are formed via similar molecular mechanisms is a major discovery. That several molecules are shared in the development of teeth and limbs is also a significant achievement. But these structures, once they appear, have different shapes and colors, and that is what makes them fascinating. Evo-devo has changed the emphasis from trying to understand why organisms are so different to addressing how this disparity and diversity could have arisen given the widespread genetic conservatism characterizing these organisms. Understanding the relation between the genotype and phenotype is central in this endeavor. Many biologists in the past have assumed that there is a oneto -one relationship between genotype and phenotype, whereby a viable mutation leads directly to a new phenotype. At all levels of organization, even the molecular one, this has proven to be incorrect . Already one hundred years ago some biologists wrote about a reaction norm, the phenomenon that one and the same genotype is able to produce different phenotypes, depending on external environmental inputs.3 This idea was largely forgotten by the mainstream of evolutionary biologists but is now at the core of developmental studies of evolution. The newly baptized field of ecological developmental biology, as masterfully presented in a textbook by Scott Gilbert and David Epel, summarizes much about the interaction between the environment and individual development. Two aspects of diversity which have become central topics of evo-devo investigations are phenotypic plasticity and modularity . What these important concepts mean and how palaeontology can address them is discussed in what follows. 36 / Evo-Devo, Plasticity, and Modules PhenotyPiC PlastiCity and variation Development is not a river of genetic information flowing inexorably downstream toward the creation of biological form, but rather many rivers, tributaries, and eddies—a turbulent, cyclical process involving gene regulation and protein synthesis. Mark S. Blumberg, Freaks of Nature Nothing in variation makes sense except in the light of development. Jukka Jernvall Variation has a nongenetic component, and phenotypic plasticity refers to that variation induced by environmental effects. Such effects can be physical factors such as temperature, or biotic factors such as interactions with other species, or stimuli such as diet. This is of course very important, as variation is what natural selection acts upon. More variation can mean more evolution and more diversification.4 Phenotypic plasticity in growth patterns coupled with environmental factors has been reported for several dinosaur species, including the prosauropod Plateosaurus. In periods of abundance or favorable climate, animals grew faster. Such kind of plasticity is not characteristic of all prosauropods; it is absent, for example, in the closely related Massospondylus. This kind of investigation can be undertaken only for those species for which large quantities of fossils are known, enough to produce growth curves using paleohistology to estimate the age of individual specimens. Phenotypic plasticity has been hypothesized for several groups of Paleozoic early land vertebrates based on careful anatomical and paleoecological studies of hundreds of specimens. Rainer Schoch in Stuttgart demonstrated with exceptional fossils of a temnospondyl that the...