The uncertainty of Late Pleistocene range expansions in the western Mediterranean: a case study of the colonization of south-eastern Spain by the spur-thighed tortoise, Testudo graeca
Correspondence: Eva Graciá, Ecology Area, Department of Applied Biology, Miguel Hernández University, Av. de la Universidad, Torreblanca, 03202, Elche, Spain.
Recent biogeographical studies have postulated a North African, Late Pleistocene, origin for some species of the Iberian Peninsula. However, a robust assessment of such range expansions requires high-resolution molecular tools to resolve overlapping biogeographical and cultural processes. Here we aim to determine whether the spur-thighed tortoise, Testudo graeca, arrived in south-eastern Spain during historical or prehistoric times, and whether its dispersal to the Iberian Peninsula was human-mediated.
The western Mediterranean Basin (south-eastern Spain, northern Algeria and north-western Morocco).
Using 428 samples from 19 sites in North Africa and 18 in south-eastern Spain, we obtained mitochondrial sequences from the cytochrome b gene and genotypes derived from seven microsatellite loci. These data were employed to obtain population genetics descriptors, haplotype networks, Bayesian cluster analyses and isolation-by-distance patterns. Moreover, we used a Bayesian demographic approach to delimit the dates involved in the range expansion.
We found lower levels of genetic variability and weak mitochondrial differentiation in the south-eastern Spanish tortoises compared with the North African ones. However, exclusive haplotypes occurred in the Iberian samples and microsatellite cluster analyses revealed moderate levels of admixture across both sides of the Mediterranean. A coastal area in the west of Algeria and the central-southern region in south-eastern Spain are suggested as the most probable founder and arrival places, respectively. Finally, we identified signatures of an ancient bottleneck event approximately 20,000–30,000 years ago.
The spur-thighed tortoise probably arrived in south-eastern Spain during Late Pleistocene sea-level low stands. The role that humans may have played as dispersers across the Mediterranean remains unclear. Our results are in accordance with other recent findings of trans-Mediterranean expansions during this period and highlight the importance of employing precise methodological approaches before a species can be considered as historically introduced.