Integrating integration: The intrinsic mechanisms and biological processes that shape patterns of trait covariation over evolutionary time

One ubiquitous pattern of organismal form that has long fascinated biologists is the covariation of seemingly unrelated traits across the body. This has led many to study phenotypic integration - the tendency of a biological or developmental system to give rise to such correlations. While integration has been observed across broad phylogenetic and broad temporal scales, our understanding of the underlying mechanisms is limited to broad categories of causation, such as development and selection. However, we believe that developing a more granular understanding of these mechanisms will be critical to more fully elucidate the evolutionary consequences of integration and to resolve past discrepancies in empirical data. To this end, we offer a list of intrinsic and molecular mechanisms that we hypothesize could drive integration of organismal form. We also present a list of biological processes, the set of intra- and inter-individual interactions affecting an organism, which may shape the deployment of these intrinsic mechanisms. Finally, we discuss how understanding these mechanisms could lead to different predictions about the temporal patterns of integration and even the evolvability of a system. Neither our list of mechanisms, nor our proposed consequences, are comprehensive; rather we hope that this discussion will encourage evolutionary and molecular biologists alike to build a deeper mechanistic understanding of organismal covariation, from cell-cell communications to macro-evolutionary trends.