What does modularity mean?

Abstract

Modularity is now generally recognized as a fundamental feature of organisms, one that may have profound consequences for evolution. Modularity has recently become a major focus of research in organismal biology across multiple disciplines including genetics, developmental biology, functional morphology, population and evolutionary biology. While the wealth of new data, and also new theory, has provided exciting and novel insights, the concept of modularity has become increasingly ambiguous. That ambiguity is underlain by diverse intuitions about what modularity means, and the ambiguity is not merely about the meaning of the word—the metrics of modularity are measuring different properties and the methods for delimiting modules delimit them by different, sometimes conflicting criteria. The many definitions, metrics and methods can lead to substantial confusion not just about what modularity means as a word but also about what it means for evolution. Here we review various concepts, using graphical depictions of modules. We then review some of the metrics and methods for analyzing modularity at different levels. To place these in theoretical context, we briefly review theories about the origins and evolutionary consequences of modularity. Finally, we show how mismatches between concepts, metrics and methods can produce theoretical confusion, and how potentially illogical interpretations can be made sensible by a better match between definitions, metrics, and methods.

Consilience of methods for phylogenetic analysis of variance

“…An alternative solution is to use the independent solutions as a starting point for an iterative procedure that finds the optimal combination of values that maximizes likelihood, sensu Adams (2013). Such an approach might have intrigue as an analysis that considers the modularity of anatomical subconfigurations for landmark shape data (sensu Klingenberg 2009; Adams 2016; Zelditch and Goswami 2021), as suites of contrasting λ values for groups of variables might be evidence for natural selection acting differently on anatomical components.…”

Flexible conservatism in the skull modularity of convergently evolved myrmecophagous placental mammals

“…However, unlike maximum-likelihood, CR effect sizes were not statistically different between modular architectures with six and seven modules (Table 5 and Additional file 3: Table S6). These apparently more conservative results obtained with Z CR might be explained by two factors, one associated with the dataset, and the other related to how Z CR determines the favoured modularity hypothesis [72,139]. Firstly, the differences between architectures III to X, varying between six to seven modules, try to capture significant changes in covariance associated with a few landmarks, instead of large modules (see Additional file 3: Table S2; in Z CR analyses, architecture III = architecture IV, as 'NAs' are interpreted as a module).…”

“…Given that major modules are conserved, it is not surprising that modular strength is imperceptible to Z CR , provided that a few landmarks are likely not enough to increase the modular strength (lower Z CR ) in one specific module. On the other hand, and most importantly, a six-module architecture might separate two highly correlated landmark clusters and still yield a lower than or significantly indistinguishable Z CR than any other architecture with a finer modular architecture (e.g., 7 modules; [139], p. 16), which gives a better account of the between-modules correlations.…”

Evolution of median fin patterning and modularity in living and fossil osteichthyans

“…Modularity is a fundamental property of organisms playing an important role in their evolution [83][84][85]. Anatomical modules refer to an internal organization of anatomical structures into distinct units, or modules, which develop and vary in quasi-autonomy, but within which the constituents interact and vary together [83,85,86]. This quasi-autonomy among modules allows three main evolutionary processes: dissociation, duplication/divergence, and co-option [83].…”