Isolation by distance in equilibrium and nonequilibrium populations of four talitrid species in the Mediterranean Sea

Allozymic variation at 21-23 loci was studied in 28 populations of Talitrus saltator, 23 populations of Orchestia montagui, 13 populations of O. steph enseni, and five populations of Platorchestia platensis from the Mediterran ean Basin. Different levels of gene flow (Nm(e)) were detected within each species at the scale of the whole Mediterranean: O. montagui and P. platens is had low population structure, with levels of Nm(e) greater than or equal to 1, whereas the T. saltator and O. stephenseni populations have values o f Nm(e), < 1. The relationship between Nm(e), and geographic distance was a nalyzed to test for the presence of an isolation by distance pattern in the spatial genetic variation within each species. A model of isolation by dis tance is useful to describe the pattern of genetic structuring of study spe cies at the scale of the whole Mediterranean: geographic distance explained from 28% to 70% of the variation in gene flow. In the Aegean area all spec ies showed an island model of genetic structuring regardless of the levels of gene flow.