The BJPS Popper Prize 2023

The concept of homology—concerning the sameness of characters across groups of organisms—has long played a central role in biological reasoning. In recent decades, it has been argued that a new theory of homology is needed, which goes beyond phylogeny to take account of organismal development, especially developmental mechanisms. Such a theory is needed, it is claimed, in order to identify and individuate homologues and to explain the phenomenon of homology. In ‘Toward a Theory of Homology: Development and the De-coupling of Morphological and Molecular Evolution’, James DiFrisco addresses a significant challenge to the construction of a successful developmental theory of homology: the fact that developmental mechanisms and phenotypic characters can be de-coupled in evolution to a significant degree. The occurrence of this de-coupling via evolutionary change in the genes and/or network underlying the same character in related taxa is known as developmental system drift (DSD). Synthesizing a wide range of empirical and theoretical findings, DiFrisco offers a qualitative model of the factors and causal processes at work in DSD and argues that DSD is a widespread phenomenon that might well be (probabilistically) predictable. Far from undermining the possibility of a successful theory of homology, this points to the need for a broader, integrative theory that encompasses multiple explanatory models—a theory that can provide developmental–genetic explanations when they apply but, guided by models of DSD, can seek other explanations, such as stabilizing selection, when necessary. DiFrisco goes on to argue that this integrative approach will be most successful if the different explanatory models and perspectives share the same definition and criteria of homology. In other words, alongside a pluralism regarding explanatory models, there should be monism regarding definitions and criteria of homology.

For its impressive contribution to the theory of homology, informed by a masterful synthesis of wide-ranging work on this important topic, the BJPS Co-Editors-in-Chief and the BSPS Committee finds ‘Toward a Theory of Homology’ to be worthy of the 2023 Popper Prize.