Acknowledgements
We thank R. Butôt for laboratory assistance, B. Wielstra for constructive comments on earlier versions of the manuscript, and O. Schaap and K. Vrieling for running the SNP line. JLD thanks the “Jan Joost ter Pelkwijk fonds” for a travel grant and G. Clifton, A. Delgado, J. López and A. Mateo for fieldwork assistance. Permission to access genetic resources in Portugal was granted by the Instituto da Conservação da Natureza e da Biodiversidade, Lisbon (permit no. 94263/13.11.2017).