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.

The shapes of bird beaks are highly controlled by nondietary factors

“…Despite generating beak movement, the adductor muscles never exert force directly on to the upper beak. Further research is therefore warranted to investigate whether biomechanical function is similarly integrated (41) and how the shape of the upper beak is affected by the skull musculature as it develops. Finally, the phenomena of "many-to-one" and "one-to-many" mapping between form and function mean that similarities in shape do not necessarily imply similarity in function (42)(43)(44), further justifying the need for biomechanical analyses of avian skulls.…”

Analyzing Fluctuating Asymmetry with Geometric Morphometrics: Concepts, Methods, and Applications

“…In turn, the patterns of covariation can also provide information about the nature and developmental origins of asymmetry. A most promising approach combines analyses of fluctuating asymmetry with others such as individual and evolutionary variation into a multilevel approach [429]. This approach is feasible for many studies, as long as multiple specimens from a number of species are measured for left and right sides, and has yielded useful insights in the few examples where it has been used to date [168,172,173].…”

“…Analyses of fluctuating asymmetry can therefore be used as an integral part of multilevel analyses of morphological integration [429] including also individual variation, interspecific comparative data providing information on evolutionary diversification and perhaps other levels, if they are available. Some first examples of this kind of study exist [72,168,172,173], and it can be expected that others will soon appear.…”

Morphological integration in human skull and its possible role in etiogenesis of neuropsychiatric disorders: Deciphering the link between evolution, development and disease

“…Functional modules (1,15) are the sets of related functions which are coordinated together in a biological system (39,45).…”

“…(In morphological integration a set of phenotypic traits vary in common. Modularity bears a conceptual similitude, where simultaneously varying traits are compartmentalized as modulesshowing stronger intra-modular than inter-modular co-variance (2).) Morphological integration in human skull has evoked curiosity of the neuropsychiatry investigators for long.…”