In biology, as in other sciences, interdisciplinarity has been steadily increasing in the last decades, but with a diversity of levels, intensity and outcome. The transfer of concepts, problems, and tools between disciplines is often strongly polarized, with a receiving discipline adopting them from a donating discipline [11], but sometimes this is a two-way affair. In this case we can characterize the process as one of integration, either epistemic or organizational, or both [11]. Moreover, it is an accepted notion that in modern science many key concepts are shared by traditionally separate disciplines, and these are often concepts that do not lend themselves to precise definitions [12].

The biological concepts whose definition has proved more problematic and is still controversial are probably those of species, homology, gene, and individual. In the first case the controversy is particularly strong within the single biological discipline of systematics [13] while affecting also evolutionary biology ^[1]; in the second case it involves different disciplines (morphology, phylogenetics etc.), mostly insofar as that these are united by the adoption of the comparative method [14]; see [15] for a broader perspective on these cases. The definition of gene has an overt transdisciplinary value, involving genetics in its various declinations, evolutionary biology, developmental biology, and the philosophy of biology [16,17,18,19]; the same applies for the definition of individual [20,21,22,23,24,25].

The need to address seriously, in a flexible and pluralistic way, the problem of a re-determination of the boundaries between biological disciplines is demonstrated by the number of concepts that in recent decades have assumed the value of nomadic concepts [26,27]. This term was proposed to describe concepts for which meaning and domain of application change with the new contexts into which they migrate. This has soon proved true also of the very notion of the nomadic concept [28,29,30]. I argue that the current relationships between anchor disciplines (e.g., developmental biology, evolutionary biology, biology of reproduction) and nomadic concepts wandering between them is worth being explored from a reciprocal perspective, by selecting suitable anchor concepts around which disciplinary fields can flexibly move. Examples could be a generalized anchor concept of generation that may suggest new perspectives on development and reproduction and a species concept as unit of representation of biological diversity that would suggest a taxonomic pluralism requiring suitable adjustments of current nomenclature rules.

It is legitimate to think that the lack of shareable definitions for the terms listed in the penultimate paragraph is not only a consequence of progress in the disciplines in which each of them originated, but also evidence of the disputable delimitation of biological disciplines. Eventually, we must acknowledge the historical specificity of individual disciplines, and possibly also the historical specificity of our own concepts of discipline [31] (p. 51).

The emergence of a new hybrid discipline may require gross conceptual rearrangements. This happened, in the last two decades of the 20th century, at the interface between evolutionary biology and developmental biology.

Interestingly, these two disciplines had been diverging more and more during most of the century. In the mind of authoritative evolutionary biologists, development was a black box between genotype and phenotype, whose content could be ignored [32]. On the other hand, the transition from the descriptive embryology of the 19th century (which had provided valuable contributions to the understanding of phylogenetic relationships) to the experimental embryology of the following century had seen a progressive loss of interest in the comparative aspects of developmental processes and a growing focus on experimental work restricted to a very small number of model species [33,34]. Eventually, however, formidable technical advances in the second half of the 20th century made it possible to implement a developmental genetics program. One of the most sensational results was the discovery of the involvement of homologous genes in the development of such different organisms as mouse and fruit fly. The increasingly accessible contents of the black box between genotype and phenotype proved to be of utmost interest not only for developmental biology, but also for evolutionary biology. The emergence of a new research field in this interface area is conventionally fixed by two books whose publication dates and titles respectively mark the completion of the maturation phase and the first full expression of the new discipline. In 1983, Rudy Raff and Thomas Kaufman published a book [35], the title of which (Embryos, Genes, and Evolution) clearly identified the subject, approach, and problems of this discipline, while Evolutionary Developmental Biology, the title of the book published nine years later by Brian K. Hall [36], provided the name (often abbreviated as evo-devo) by which the latter was definitively identified [37,38].

Gilbert and Burian’s early summary [39] that evo-devo “is both a synthesis between evolutionary biology and developmental biology and an ongoing negotiation between these two disciplines” (p. 61) is still up-to-date [40]. Winther [41] proposed to characterize evolutionary developmental biology as a trading zone, a catching term introduced by Galison [42] to indicate those interdisciplinary areas in which specific adoption and redefinition of both the concepts and the environment where they are implemented allow successful conceptual transfers [12,30].

For sure, conflicts cannot be avoided even in the best trading zones. In the case of evolutionary developmental biology, Love [43] has pointed to a hardly erasable tension between the two ‘souls’. Repeatability of experimental results requires the highest possible uniformity in the strains (often long inbred laboratory lines) used in the tests. But this choice potentially deletes all the intraspecific variation on which evolution is deemed to happen. In this tug of war between developmental biology and evolutionary biology, it is not surprising that studies effectively coupling developmental genetics and population genetics (devgen-popgen [44]) are still few, despite a few excellent examples such as the already classic studies on the beaks of Darwin’s finches revisited from an evo-devo perspective [45,46]).

In these years that witness so much talk about the need to broaden the traditional neo-Darwinian vision of evolution to move towards an extended synthesis [47,48,49] it is necessary, in my opinion, to make an even more generous and adventurous effort and to seek, in an ever wider trading zone, to refresh the relationships between biological disciplines.

An overarching question is the relationship between the disciplinary articulation of the natural sciences and a hierarchical vision of nature structured into levels of organization.

The starting point of a debate that continues to this day [50,51], with an unceasing renewal of points of view and arguments, is a 1958 article by Oppenheim and Putnam [52] postulating both an articulation of nature in terms of levels of organization, and a close reciprocal correspondence between levels of organization in nature and the sciences that describe their components and develop theories on the relationships between them [53]. From these premises, Oppenheim and Putnam derived an entire reductionist program.

A description of reality in terms of levels of organization seemed useful even to those who were ready to recognize that theories are not always “limited to single levels, that levels are always well defined, or that two or more entities can always be unambiguously ordered with respect to level” [54] (p. 215). But in more recent times, the progressive move away from the reductionist program of Oppenheim and Putnam was one of the reasons for the decreasing favor of the very notion of levels of organization [55,56,57], until its total rejection by some authors [58]. Others have brought their arguments against these dismissionary positions, while nevertheless proposing new interpretations (and new epistemic roles) for the notion of organizational levels. While rejecting an ontological interpretation according to which the world would be structured by levels of organization, Brooks and Eronen [59]; see also [60,61] nevertheless save this notion as useful in the abstract description of systems and as a guide in the search for new areas of investigation to be explored. On the other hand, DiFrisco [62,63] rejects the criteria thus far used in identifying organization levels, in terms of compositional relationships or spatial scale, and suggests a dynamic approach that recognizes levels defined on the basis of rates or time scales of processes. Baedke [64] challenges the general acceptance of a never changing existence of levels of organization such as cells, tissues, organs, and individual organisms and points to the necessity of addressing their dynamical nature over developmental time and in evolution.

An overlooked consequence of the generalized acceptance of a vision of the living world in terms of compositional levels organized in part-whole relations [65] is the creation of disciplines through a copy-and-paste process. If in the study of humans and, more generally, of animals, it has proved useful to recognize a science of cells (cytology), a science of tissues (histology), a science of embryonic development (embryology) etc., this disciplinary articulation was accepted as sensible for all animals and even for multicellular organisms at large and corresponding disciplines were created for plants. Many biologists may take for granted, for example, the legitimacy of a plant embryology, but this should be resisted. In plant science, a technical use of the term embryo for the future seedling still enclosed within the seed casings was virtually unknown until 1788, when Gaertner [66] successfully introduced it in his treatise De fructibus et seminibus plantarum (On Plant Fruits and Seeds). Gaertner borrowed a number of terms and concepts from animal embryology. Some of these terms have remained in use for both plants and animals, but nobody would venture today to say, for example, that the placenta of plants is homologous to the placenta of mammals. Unfortunately, instead, the idea of an equivalence between what is called an embryo in either kingdom is still widespread, even among professionals [67].