Morphological Integration at 50: Patterns and Processes of Integration in Biological Anthropology

Abstract

The articles published in this issue of Evolutionary Biology were originally presented in a symposium entitled ‘‘Patterns and Processes of Morphological Integration in Primate and Human Evolution’’, held at the 77th annual meeting of the American Association of Physical Anthropologists in Columbus, Ohio, in April 2008. The symposium marked the 50th anniversary of Everett Olson and Robert Miller’s seminal book ‘‘Morphological Integration’’, and sought to capture the current state of affairs in studies of morphological integration, particularly in relation to primate and human evolutionary biology. In its simplest form, morphological integration is the inter-dependence between sets of traits within an individual, reflecting a common influence from functional and/or developmental factors. The concept of integration is not a new one. In fact, the Principle of the Correlation of Parts, the idea that in order to be viable, organisms must be comprised of organs that are functionally coordinated with each other, was a major theme in the work of French anatomist Georges Cuvier (1769–1832, see Mayr 1982:460). Perhaps not surprisingly, Charles Darwin also discussed this ubiquitous feature of complex organisms in the Origin of Species under the term ‘‘Correlation of Growth’’: ‘‘I mean by this expression that the whole organization is so tied together during its growth and development, that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified’’ (On the Origin of Species, 1859: 147). Thus, whereas Cuvier’s work emphasized the influence of shared functions on covariation and correlation among traits, Darwin recognized that interactions between these structures during ontogeny could also bias the production of variation within and among traits, and understood the consequences of such a bias on the evolution of complex organisms. Olson and Miller synthesized these earlier views, suggesting that both developmental and functional interactions were important sources of correlation among parts, each contributing towards building ‘integrated’ organisms in which different parts function in harmony with all others. The process by which such holistic phenotypes are built is what Olson and Miller called morphological integration, i.e., ‘‘the summation of the totality of characters which, in their interdependency of form, produce an organism’’ (1958, p. v). Olson and Miller were also aware of the importance of integration—as a variational property of populations—for the capacity of organismal traits to evolve independently of one another. Olson and Miller also deserve credit for providing the first quantitative methods, mostly based on statistical correlation, for empirically identifying groups of phenotypic traits that are more strongly integrated on the basis of shared developmental and/or functional factors (q-groups). Despite this important insight into a fundamental emergent property of organisms, Olson and Miller’s theory of integration did not immediately gain traction among evolutionary biologists. Reasons for this might be that their approach was pattern-based, relying heavily on statistical rather than biological criteria for grouping sets of correlated traits, and inferring in a post-hoc manner developmental and functional reasons for increased correlation among C. Rolian (&) Department of Cell Biology and Anatomy, University of Calgary, G503-3330 Hospital Dr NW, Calgary, AB T2N 4N1, USA e-mail: cprolian@ucalgary.ca