The study of the relationship between development and evolution has deep historical roots but has only recently emerging as a distinct academic discipline.  Evolutionary developmental biology is a trans-discipline in the sense that it fuses elements of evolutionary and developmental biology as well as genetics around a set of questions, perspectives and concepts that are distinct from these three fields. Along with Hendrikse and Parsons, I have previously argued that the central question of evolutionary developmental biology is the developmental basis for evolvability [1]. A key element of this argument is that fields orient themselves around overarching questions and that much of the work that falls under the banner of this new field of evolutionary developmental biology centers on this key question.  We argue that evolutionary developmental biology seeks to generate explanations of evolution and biological diversity that draw upon the generation of variation by developmental processes or to modify such explanations in light of how variation is generated by development. To accomplish this, evolutionary developmental biology draws upon a set of concepts that, together, form the scaffold on which these explanations are built. 

What are these core concepts that form the scaffold for explanations about evolution and development?  Do these concepts relate to each other in a coherent way? Is that even a question to which there is an objective answer? This is the question that I plan to tackle in a monograph on the conceptual foundations of evolution and development. This would address an important gap in knowledge because the epistemology of this new field that lies at the intersection of evolution and development is still very much unresolved. The core concepts abut incongruously in the current literature.  Their use is rife with conflation of pattern and process as well as definitional drift that diminishes the utility of these concepts for meaningful explanations.  Examples of conflation due to such drift include the delineation of canalization versus phenotypic plasticity, modularity versus integration, modules versus characters, and integration or allometry versus heterochrony. It would be naïve to think that there is one objective way to impose coherence on the relations among these concepts. However, this does not mean that attempting to create a coherent account of the conceptual foundations of evolution and development is without value. Doing so will help resolve areas of confusion that impede the use of these concepts. It may also reveal concepts that may have outlived their utility or are in need of significant revision as well as areas of uncertainty that would benefit from further debate. Finally, this effort is likely to stimulate further discussion related to defining the conceptual foundations of evolution and development, which will bring a much-needed focus on the emerging epistemology of this important field.

This project is timely because evolutionary developmental biology is new and its conceptual foundations are fluid. There is also a broader trend towards the blurring of disciplinary lines in both biological and social sciences research. When this happens, conceptual frameworks can collide or misalign, particularly when key differences in concepts across fields are not recognized. This happens frequently in evolution and development because the conceptual frameworks of evolutionary biology, genetics and developmental biology are often incongruent. Variation is similarly positioned in evolutionary biology and genetics, but not in developmental biology. Epistasis means something quite different to a developmental biologist compared to a population geneticist. Most importantly, though, central motivating questions can be orthogonal. Defining and then working through these issues of incongruence for evolutionary developmental biology can serve as a relatively simple case study for transdisciplinarity that is simpler than other such projects that are currently attempting to blend social science and biomedical frameworks, for example.

An account of the conceptual foundations of evolution and development cannot be divorced from the history the component concepts. Concepts are continually reinvented and redefined in light of current knowledge as well as shifting research agendas and priorities. Morphological integration may draw upon Darwin’s laws of correlations of growth [2, p. 80], but this is not really the same idea articulated by Olson and Miller [3]. Integration is different again in the hands of Wagner [4] and yet again when viewed through my palimpsest model [5, 6] or by Mitteroecker [7]. As they are reinvented and as research priorities shift, relations among concepts also change. Allometry and integration have distinct origins, for example, but within the palimpsest model, allometry is simply a special case of integration where the covariance patterns result from processes that influence overall growth [8]. Canalization and developmental stability are difficult to disentangle in Waddington’s [9] or Schmalhausen’s work [10] and yet these concepts have emerged as foci of quite distinct research agendas with different practitioners. Epigenetics originally referred to the ‘cybernetics” or system-level properties of development [9] and yet now means something completely different [11]. Many of the individual concepts tackled in this project have been the subject of historical analysis. However, this work will add to this existing literature by emphasizing the history not just of these concepts but the evolving connections among them that forms the conceptual framework for evolution and development.

References

1.         Hendrikse, J.L., T.E. Parsons, and B. Hallgrimsson, Evolvability as the proper focus of evolutionary developmental biology. Evolution & Development, 2007. 9(4): p. 393-401.

2.         Darwin, C., The Origin of Species (reprinted 1975). 1859, New York: Avenel.

3.         Olson, E.C. and R.A. Miller, Morphological Integration. 1958, Chicago: University of Chicago Press.

4.         Wagner, G.P., A comparative study of morphological integration in Apis mellifera (Insecta, Hymenoptera). Z. zool. Syst. Evolut. - forsch., 1989. 28: p. 48-61.

5.         Hallgrimsson, B., et al., Evolution of covariance in the mammalian skull. Novartis Found Symp, 2007. 284: p. 164-85; discussion 185-90.

6.         Hallgrimsson, B., et al., Deciphering the Palimpsest: Studying the Relationship Between Morphological Integration and Phenotypic Covariation. Evolutionary Biology, 2009. 36(4): p. 355-376.

7.         Mitteroecker, P. and F. Bookstein, The conceptual and statistical relationship between modularity and morphological integration. Syst Biol, 2007. 56(5): p. 818-36.

8.         Hallgrímsson, B., et al., Integration and the Developmental Genetics of Allometry. Integrative and Comparative Biology, 2019. 59(5): p. 1369-1381.

9.         Waddington, C.H., The Strategy of the Genes. 1957, New York: MacMillan Company.

10.       Schmalhausen, I.I., Factors of Evolution. 1949, Chicago: University of Chicago Press.

11.       Hallgrimsson, B. and B. Hall, Epigenetics: Linking Genotype and Phenotype in Development and Evolution. Epigenetics: Linking Genotype and Phenotype in Development and Evolution. 2011.