Studying morphological integration and modularity at multiple levels: concepts and analysis

Abstract

Although most studies on integration and modularity have focused on variation among individuals within populations or species, this is not the only level of variation for which integration and modularity exist. Multiple levels of biological variation originate from distinct sources: genetic variation, phenotypic plasticity resulting from environmental heterogeneity, fluctuating asymmetry from random developmental variation and, at the interpopulation or interspecific levels, evolutionary change. The processes that produce variation at all these levels can impart integration or modularity on the covariance structure among morphological traits. In turn, studies of the patterns of integration and modularity can inform about the underlying processes. In particular, the methods of geometric morphometrics offer many advantages for such studies because they can characterize the patterns of morphological variation in great detail and maintain the anatomical context of the structures under study. This paper reviews biological concepts and analytical methods for characterizing patterns of variation and for comparing across levels. Because research comparing patterns across level has only just begun, there are relatively few results, generalizations are difficult and many biological and statistical questions remain unanswered. Nevertheless, it is clear that research using this approach can take advantage of an abundance of new possibilities that are so far largely unexplored.

Keywords: allometry; covariation; evolution; geometric morphometrics; modularity; morphological integration.

Figure 1.

Different levels of integration and modularity. The diagram contains four related species, each with three ontogenetic stages. Ontogenetic integration and modularity concern the variation across stages within each species, whereas the evolutionary level focuses on the variation among species at any given stage. The static level is within one species and stage. Functional, developmental, genetic and environmental integration and modularity are usually studied in a static context, that is, at one particular ontogenetic stage for a given species. Pooled within-group analyses can be used to summarize patterns.