1 March 2009 The Ontogenetic Trajectory of the Phenotypic Covariance Matrix, with Examples from Craniofacial Shape in Rats and Humans
Philipp Mitteroecker, Fred Bookstein
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Many classic quantitative genetic theories assume the covariance structure among adult phenotypic traits to be relatively static during evolution. But the cross-sectional covariance matrix arises from the joint variation of a large range of developmental processes and hence is not constant over the period during which a population of developing organisms is actually exposed to selection. To examine how development shapes the phenotypic covariance structure, we ordinate the age-specific covariance matrices of shape coordinates for craniofacial growth in rats and humans. The metric that we use for this purpose is given by the square root of the summed squared log relative eigenvalues. This is the natural metric on the space of positive-definite symmetric matrices, which we introduce and justify in a biometric context. In both species, the covariance matrices appear to change continually throughout the full period of postnatal development. The resulting ontogenetic trajectories alter their direction at major changes of the developmental programs whereas they are fairly straight in between. Consequently, phenotypic covariance matrices—and thus also response to selection—should be expected to vary both over ontogenetic and phylogenetic time scales as different phenotypes are necessarily produced by different developmental pathways.

© 2009 The Society for the Study of Evolution.
Philipp Mitteroecker and Fred Bookstein "The Ontogenetic Trajectory of the Phenotypic Covariance Matrix, with Examples from Craniofacial Shape in Rats and Humans," Evolution 63(3), 727-737, (1 March 2009). https://doi.org/10.1111/j.1558-5646.2008.00587.x
Received: 4 September 2008; Accepted: 1 November 2008; Published: 1 March 2009

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Craniofacial growth
geometric morphometrics
morphological integration
Riemannian metric
space of covariance matrices
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