The Mantel test is routinely used in many areas of biology and environmental sciences to assess the
signicance of the association between two or more matrices of distances relative to the same pairs of indi-
viduals. This test is a valid statistical procedure to test the auto-correlation of a single (possibly multivariate)
variable. This includes the widely used test of isolation-by-distance in population genetics. However, we
show that contrarily to a widely shared belief, the simple and partial Mantel tests are not valid statistical
procedures to assess the signicance of the correlation between two variables structured in space. Under
a fairly general model, simulations show that the Mantel tests provide an excess of Type I error whose
magnitude increases with the intensity of the spatial auto-correlation. The Mantel tests should not be used
in case auto-correlation is suspected in both variables compared under the null hypothesis.
This is a preprint of an article whose final and definitive form will be published in Systematic Biology (c) 2011 Society of Systematic Biologists: Systematic Biology is available online at informaworld.