The increase in the number of protein functions per gene and the growing complexity of interactions between regulatory gene networks, probably encouraged by genomic duplications, undoubtedly enabled the emergence of vertebrate animals. But this general complexification of processes also led to these systems becoming less flexible and increasingly complicated to modify. From a developmental point of view, this translates into the reinforcement of the presence of internal constraints that prevent certain possibilities from being realized for lack of internal mechanistic coherence. The effects of this "internal selection", as Lancelot Whyte called it, were already documented at the turn of the 19th century, notably by William Bateson, but the impact of this vision in evolutionary theory has remained relatively moderate.
In this fifth lecture, some illustrations of this notion of internal constraints are discussed in the light of current knowledge in developmental biology and molecular genetics. These constraints may affect particular structures or systems, but also vertebrate embryos as a whole, which seem to have to pass through a bottleneck during which the fundamental characteristics of vertebrates are put in place. The nature of the mechanisms potentially responsible for this "philotypic hourglass" is investigated, as is the notion of homology, which will be reassessed in the light of the existence of internal constraints.
