Parapatric speciation may be associated with differential landscape-dependent selection. Even if there is a gene flow between two populations, strong differential selection may impede assimilation and different species may eventually develop. Habitat differences may be more important in the development of reproductive isolation than the isolation time. Caucasian rock lizards "Darevskia rudis", "D. valentini" and "D. portschinskii" all hybridize with each other in their hybrid zone; however, hybridization is stronger between "D. portschinskii" and "D. rudis", which separated earlier but live in similar habitats than between "D. valentini" and two other species, which separated later but live in climatically different habitats.