Evolutionary genetics of the insular lacertid lizard Podarcis tiliguerta: genetic structure and population heterogeneity in a geographically fragmented species

Abstract

Podarcis tiliguerta is an insular Mediterranean lacertid lizard endemic to Corsica, Sardinia and many neighbouring small islands. The genetic structure and population heterogeneity of the species were studied by means of allozyme electrophoresis at 20 presumptive gene loci. The observed heterozygosity (Ho) decreases southwards towards more arid climatic regimes. No severe reduction in genetic variability was found in samples from the tiny satellite islands, except for a population inhabiting a very small island off the south-western coast of Sardinia (Meli Island), in which about 80 per cent of the genetic variability was lost. Population heterogeneity analysis carried out by the estimation of Wright's F-statistics demonstrated substantial genetic differentiation among populations. The value of FST (0.460) exceeds values known for other lizard species. F-statistics and genetic distance data show that genetic variation is distributed into three geographically coherent population groups. The first group includes populations from the northern part of the range (Corsica), the second includes populations from the small islands off the south-eastern coast of Corsica (Cerbicale and Lavezzi), and the third comprises populations from the southern part of the range (Sardinia and Meli Island). The pattern of genetic variability and the apparent clinal variation of alleles at a few loci (Idh-1, Gapd, Gpi) indicate that the subdivided genetic structure of P. tiliguerta is moulded by the interplay of stochastic processes and agents selectively affecting allele frequency changes.