title: The fossil record of phenotypic integration and modularity: A deep-time perspective on developmental and evolutionary dynamics.
abstract: Variation is the raw material for natural selection, but the factors shaping variation are still poorly understood. Genetic and developmental interactions can direct variation, but there has been little synthesis of these effects with the extrinsic factors that can shape biodiversity over large scales. The study of phenotypic integration and modularity has the capacity to unify these aspects of evolutionary study by estimating genetic and developmental interactions through the quantitative analysis of morphology, allowing for combined assessment of intrinsic and extrinsic effects. Data from the fossil record in particular are central to our understanding of phenotypic integration and modularity because they provide the only information on deep-time developmental and evolutionary dynamics, including trends in trait relationships and their role in shaping organismal diversity. Here, we demonstrate the important perspective on phenotypic integration provided by the fossil record with a study of Smilodon fatalis (saber-toothed cats) and Canis dirus (dire wolves). We quantified temporal trends in size, variance, phenotypic integration, and direct developmental integration (fluctuating asymmetry) through 27,000 y of Late Pleistocene climate change. Both S. fatalis and C. dirus showed a gradual decrease in magnitude of phenotypic integration and an increase in variance and the correlation between fluctuating asymmetry and overall integration through time, suggesting that developmental integration mediated morphological response to environmental change in the later populations of these species. These results are consistent with experimental studies and represent, to our knowledge, the first deep-time validation of the importance of developmental integration in stabilizing morphological evolution through periods of environmental change.
authors.affiliation: Department of Genetics, Evolution & Environment and Department of Earth Sciences, University College London, London WC1E 6BT, United Kingdom; a.goswami@ucl.ac.uk.
authors.forename: Anjali
authors.initials: A
authors.lastname: Goswami
authors.affiliation: Department of Biology, Loyola Marymount University, Los Angeles, CA 90045;
authors.forename: Wendy J
authors.initials: WJ
authors.lastname: Binder
authors.affiliation: Department of Anatomy, Des Moines University, Des Moines, IA 50312; and.
authors.forename: Julie
authors.initials: J
authors.lastname: Meachen
authors.affiliation: Department of Biological Sciences, Marshall University, Huntington, WV 25755.
authors.forename: F Robin
authors.initials: FR
authors.lastname: O'Keefe
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002415:Cats; D005580:Fossils; D058685:Genetic Pleiotropy; D009929:Organ Size; D010641:Phenotype; D013997:Time Factors; D019596:Wolves
keywords: Late Pleistocene; carnivorans; macroevolution; modularity; phenotypic integration