Abstract
Integration and modularity refer to the patterns and processes of trait interaction and independence. Both terms have complex histories with respect to both conceptualization and quantification, resulting in a plethora of integration indices in use. We review briefly the divergent definitions, uses and measures of integration and modularity and make conceptual links to allometry. We also discuss how integration and modularity might evolve. Although integration is generally thought to be generated and maintained by correlational selection, theoretical considerations suggest the relationship is not straightforward. We caution here against uncontrolled comparisons of indices across studies. In the absence of controls for trait number, dimensionality, homology, development and function, it is difficult, or even impossible, to compare integration indices across organisms or traits. We suggest that care be invested in relating measurement to underlying theory or hypotheses, and that summative, theory-free descriptors of integration generally be avoided. The papers that follow in this Theme Issue illustrate the diversity of approaches to studying integration and modularity, highlighting strengths and pitfalls that await researchers investigating integration in plants and animals.
Highlights
Variation is a fundamental property of life
Interest in the covariation of phenotypic traits has a long history in evolutionary biology, dating back at least as far as Darwin’s discussion of multi-trait correlations in domestic animals ([4], ch. 1)
We focus on the evolution of variational and developmental integration
Summary
Variation is a fundamental property of life. phenotypic traits do not vary independently, but instead reflect webs of developmental, physiological and functional interactions of varying strengths [1,2,3]. We may recognize several distinct conceptual variants of integration These include (i) statistical or phenomenological integration, understood as patterns of strong phenotypic or genetic correlations in standing population variation; (ii) variational integration, defined as a tendency for covariation, as proposed by Hallgrımsson et al [8]; (iii) developmental or structural integration of organismal architecture, which includes developmental interactions and phenomena such as the partial or complete fusion of parts. Modularity can refer to patterns of standing (co)variation, variational independence, developmental/structural independence or evolutionary independence among sets of traits [10,11]. Integration may be a facilitator of adaptation 2 (see Riedl [26]) This is easy to imagine when parts of the organism need to function together in a coordinated manner. While some authors have considered allometry as a possible constraint for evolution resulting from the developmental process [16,17,38], other have argued that allometry is itself evolvable, the trajectory being maintained by selection [40,41,42,43]
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