Résumé
A key issue in evolutionary studies is the means by which evolution can be channeled by intrinsic processes such as genetic and development. Studying the phenotypic variation in a population can shed light on these constraints, because phenotypic variation, being the product of genetic and developmental processes, is the target of both selective screening and random sampling. The main phenotypic variance in populations ("Pmax") could thus act as a "line of least resistance to evolution". Based on morphometric analysis of molar evolution in several fossil lineages and modern murine rodents, the role of Pmax as line of least resistance to evolution is investigated: Does evolution along lineages actually occur along Pmax? Does this line of least resistance facilitate parallel evolution? What is the relationship of Pmax to developmental processes and functional constraints? Case studies on murine rodent teeth are complemented by examples focusing on mouse mandibles. Compared to teeth, which are mineralized early during development, the mandible, as a bone, is prone to shape changes through remodeling in relation to masticatory muscles and other tissues. Mandible shape may thus vary throughout an animal's life due to allometric growth and, more generally, because of environmental influences. This may lead the mandible's Pmax to align with the direction of plastic and allometric variation. However, the same kind of shape change may also be produced by genetic changes. These examples illustrate how studying patterns of phenotypic variance using geometric morphometrics can help to identify evolutionary processes, bridging several evolutionary levels from intra-group variation to inter-group evolution, and therefore can contribute to an integrated view of phenotypic evolution.