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Originally published as Genetics Published Articles Ahead of Print on October 14, 2008.

Genetics, Vol. 180, 2151-2161, December 2008, Copyright © 2008
doi:10.1534/genetics.108.092452

Effects of Selection and Drift on G Matrix Evolution in a Heterogeneous Environment: A Multivariate Qst–Fst Test With the Freshwater Snail Galba truncatula

Elodie Chapuis*,{dagger},1, Guillaume Martin*,{ddagger} and Jérôme Goudet*

* Département d'Ecologie et Evolution, Bâtiment Biophore, Université de Lausanne, CH 1015 Lausanne, Switzerland, {dagger} Institut des Sciences de l'Évolution de Montpellier, UMR 5554, Université Montpellier 2, 34095 Montpellier Cedex 5, France and {ddagger} GEMI UMR 2724, 34394 Montpellier Cedex 5, France

1 Corresponding author: Institut des Sciences de l'Évolution, UMR 5554, Place Eugene Bataillon, CC 65, Université Montpellier 2, 34095 Montpellier Cedex 5, France.
E-mail: elodie.chapuis{at}univ-montp2.fr

Unraveling the effect of selection vs. drift on the evolution of quantitative traits is commonly achieved by one of two methods. Either one contrasts population differentiation estimates for genetic markers and quantitative traits (the Qst–Fst contrast) or multivariate methods are used to study the covariance between sets of traits. In particular, many studies have focused on the genetic variance–covariance matrix (the G matrix). However, both drift and selection can cause changes in G. To understand their joint effects, we recently combined the two methods into a single test (accompanying article by Martin et al.), which we apply here to a network of 16 natural populations of the freshwater snail Galba truncatula. Using this new neutrality test, extended to hierarchical population structures, we studied the multivariate equivalent of the Qst–Fst contrast for several life-history traits of G. truncatula. We found strong evidence of selection acting on multivariate phenotypes. Selection was homogeneous among populations within each habitat and heterogeneous between habitats. We found that the G matrices were relatively stable within each habitat, with proportionality between the among-populations (D) and the within-populations (G) covariance matrices. The effect of habitat heterogeneity is to break this proportionality because of selection for habitat-dependent optima. Individual-based simulations mimicking our empirical system confirmed that these patterns are expected under the selective regime inferred. We show that homogenizing selection can mimic some effect of drift on the G matrix (G and D almost proportional), but that incorporating information from molecular markers (multivariate Qst–Fst) allows disentangling the two effects.