Evolution and development of shape: integrating quantitative approaches

Key Points Quantitative approaches are used increasingly in evolutionary developmental biology ('evo-devo'). In particular, geometric morphometrics is widely used to quantify the shape of organisms or their parts. A wide range of tools is available to address specific questions and interpret results in their anatomical context. Genetic studies of shape variation have shown that inheritance tends to be polygenic, with many loci of mostly small effects. Because developmental processes integrate variation from diverse sources, interactions of genes with each other and with environmental factors seem to be important. Shape variation tends to be integrated and often has a modular structure; there is usually strong integration within morphological modules but relatively weak integration among modules. Strong integration can act as an evolutionary constraint by hindering the independent evolution of different traits. Identifying constraints and their evolutionary effects is an active area of current research. Functional considerations are increasingly important in evo-devo and provide explicit links between the genetic and developmental basis of variation and its adaptive significance for evolving populations. Large-scale comparative analyses of shape with an explicit phylogenetic basis provide a means of examining the long-term evolutionary consequences of the processes observed in contemporary populations. Overall, integration of quantitative approaches into evo-devo promises to unify developmental and adaptive factors of morphological evolution. Evolutionary developmental biology is being advanced by quantitative methods for studying morphology. This Review considers such approaches and emerging insights into interactions between genetic and non-genetic factors, as well as the evolutionary constraints that influence shape. Morphological traits have long been a focus of evolutionary developmental biology ('evo-devo'), but new methods for quantifying shape variation are opening unprecedented possibilities for investigating the developmental basis of evolutionary change. Morphometric analyses are revealing that development mediates complex interactions between genetic and environmental factors affecting shape. Evolution results from changes in those interactions, as natural selection favours shapes that more effectively perform some fitness-related functions. Quantitative studies of shape can characterize developmental and genetic effects and discover their relat