In this paper we review the characteristics of the floral biology, life cycle and breeding system in the genus Quercus. The species of this genus are self-incompatible and have very long life spans. The focus of our review is on the effects of gene flow on the structuration of genetic variation in these species. We have examined the influence of gene flow in 2 ways: 1) by measuring the physical dispersal of pollen, seed and vegetative organs; and 2) by using nuclear and cytoplasmic markers to estimate genetic parame (Fis, Nm). These approaches have shown that nuclear (isozyme markers) als well as cytoplasmic (chloroplastic DNA) gene flow is usually high, so that very low interspecific differentiation occurs. However, intraspecific differentation is higher for the cytoplasmic DNA than for the nuclear isozyme markers.
