We are indebted to Professor J. C. Avise (University of California Irvine, CA) for his help in interpreting our results and preparing the manuscript. Prof. F. Juanes (University of Massachusetts, MA) revised the final version of the manuscript. S. Glise (INRA, France) and I. González Pola (University of Oviedo, Spain) collaborated in field and laboratory work. We are grateful to Dr G. Luikart and to 3 anonymous reviewers of Journal of Heredity for valuable comments on the manuscript. Spanish National Grant CGL2009-08279.