For the past five decades, evolutionary biology has been the stage for a persistent and ongoing scientific controversy. This controversy is about the question whether the dominant version of evolutionary theory – the modern synthesis – needs to be reformed and expanded. It has been characterized by the opposition between two broad camps: the reformists and the conventionalists. The reformists have argued that the modern synthesis has outlived its usefulness, in some important sense, and needs major reform. The conventionalists have opposed these claims, generally arguing that all empirically adequate reformist proposals can be incorporated into the modern synthesis without fundamentally changing that synthesis or our view of evolutionary theory. The result has been a continuous and complex scientific controversy about the structure and content of evolutionary theory – the ‘extension controversy’.

So far, the dynamics and longevity of the extension controversy have not yet been properly explained. Instead, historians and philosophers of science have tended to neglect this general controversy in favour of a narrower, more piecemeal focus. Accounts of the reformist-conventionalist controversy have been focused on specific sub-controversies during particular time periods, such as the role of developmental biology, or relationship between micro- and macroevolution; or they have been focused on only the most recent debate; or they have recast the wider controversy as a series of controversies over the relative significance of different evolutionary theories or causal factors; or they have explained the wider controversy as a consequence of the pluralistic nature of the biological sciences, the great variety in its methods, and its use of idealized models; or they have argued that the wider debate is largely naïve and unnecessary.

These types of accounts have two problems. First, piecemeal explanations go against reformist characterizations of the debate as a general debate about the structure and content of evolutionary theory. Moreover, reasons for why the reformist-conventionalist controversy should be treated piecemeal have not yet been provided. Second, piecemeal explanations tend to be unduly deflationary. The reformist-conventionalist debate is not taken at face value, which requires taking the epistemic considerations presented by reformists and conventionalists as the primary difference-makers between them, at least prima facie. Instead, these differences are explained away as consequences of relative significance, pluralism, idealization, or naiveté.

Contrary to these types of accounts, I suspect that the extension controversy is mainly epistemic and in fact turns on competing standards of scientific understanding held by reformists and conventionalists. That is to say: Differences in what constitutes scientific understanding of evolution are the primary fuel of this scientific controversy. An understanding-based account would go directly against a piecemeal and deflationary view of the extension controversy. However, formulating such an account is a substantial task. It requires identifying two scientific communities who take opposing positions based primarily on epistemic considerations that must play a crucial role throughout the controversy (although they need not be immutable). Additionally, for the reformist-conventionalist controversy to be a scientific controversy proper, these epistemic considerations must be more important than social or political considerations.

In this project, my primary focus is on reformist thought, particularly as it was developed in palaeontology, evolutionary developmental biology (EvoDevo), and the ‘Extended Evolutionary Synthesis’ (EES). My aim is to lay an important part of the foundation of an understanding-based account of the extension controversy by (i) determining the leading epistemic considerations that have motivated reformist critiques of the modern synthesis and (ii) to determine the extent to which these critiques constitute, or came to constitute, a coherent reformist position. Thus, this project consists in a comparative analysis of various phases of reformist critique of the modern synthesis, starting in the 1970s with palaeontology, followed by EvoDevo, and leading up to the EES. This comparative analysis of reformist thought relies on recent advances in integrated history and philosophy of science and the epistemology of scientific understanding.