this works for one or two generations before the 50-50 gender split catches up and you start seeing foreign ydna males in the gene pool. you'd need a complete mdna replacement for this to happen without it showing in the ydna maps. it doesnt work in all cases (if a migration is mostly women it skews the results) but it works in most, and is useful for seeing the historic european races. also historically it was usually the opposite. almost complete ydna replacements after conquest or migration, what ydna maps are good for.
Spain