Surfing by tortoises
Genetic surfing that is. It's created among a population that is subject to many founding events and lots of genetic drift! When a population has an advancing frontier, the genetic complement of the members is different from the genome of a population surrounded by other populations of the same species.
This strange "surfing," phenomenon was seen even in humans colonising new countries, so there's little argument about the truth of the situation. These spur-thighed tortoises, Testudo graeca reach the edge of their range in parts of North Africa and the expansion they must have made recently to south eastern Spain. The North African population has a genetic pattern with little genetic drift, while the prediction by Eva Gracia and her colleagues was that the Spanish "colonists" would show quite a lot of drift.
The slow-coach reptiles seemed to disperse from a single point in the south of Spain and progressed according to two possible scenarios. They could have either been selected by environmental factors, then interbred, creating a diverse population; or dispersed from a single point and then maintained a population with much less diversity at the centre.
The results of the research in the Biology Letters clearly show 13 alleles occurring frequently in certain localities in Spain, while these particular alleles are rare in the North African population. The natural expansion of Spanish T.graeca seems to have progressed from just a single point.
Diversity decreases as you travel to that arrival area in south central Spain. The rare African alleles are present more frequently, the further you travel from the arrival area. More uniform populations occurred in the arrival area.
Founding groups which are less diverse must have established from a few individuals. The varied Spanish landscape, the low population at the front of, "the great tortoise expansion," and lack of ability to disperse quickly all seem to have conspired to cause genetic drift. As for surfing, I'm afraid it's a "no", to the theory about wooden planks across the Mediterranean Sea.