Living forms in becoming between old constraints and unexpected opportunities of change

Abstract

What does it mean to understand a biological form? Traditional approaches have tried to generate families of form through generative algorithms, often mathematically elegant (e.g., fractals) but very far from biological reality, or to explain it in terms of adaptation. In recent times, a different reading of living forms has been fueled by progress in developmental biology. The key point is that natural selection can only act on the products of the development mechanisms actually operating in nature. There are biological forms that, had they appeared, would have been successful, but simply never saw the light. There are also reciprocal examples of ‘monstrous’ individuals whose chances of survival are uncertain and are not able to reproduce, yet they often reach adulthood, demonstrating that existing developmental mechanisms are capable of constructing forms other than normal ones. Thus, to understand living forms as these exist in nature we cannot be satisfied either with the functionalist logic of evolutionary biology, or with the explanations provided by developmental biology in terms of ontogenetic processes: separately taken, neither is sufficient to explain the biological forms we find in nature. However, we can attempt to integrate between the two approaches, following the recent program of evolutionary developmental biology (evo-devo). Within this discipline, an original program has taken shape, focusing on evolvability, modularity and the origin of evolutionary novelties. Evolutionary and developmental changes of living forms can be modular or systemic. Modularity allows different kinds of development reprogramming: heterometry, heterotopy, heterochrony, which involve, in the order, changes in temporal, positional and quantitative aspects of the production of individual body parts during development. Despite the explosive development of studies on the genetic control of developmental processes, a thorough knowledge of the genotype of an animal or plant is not sufficient to predict its structure. The expression of one gene is generally involved in many traits (pleiotropy), while identical or almost identical traits can be achieved even in the presence of differences in the genes or in the gene networks involved in their control (convergence or redundancy). Moreover, the phenotype that actually shows up depends also on influences from the environment in which the development takes place (phenotypic plasticity), and on often standing and specific interactions with other organisms, as in the fungus-alga symbiosis of lichens and the interactions of multicellulars – humans included – with their microbiome.