Fertile F1 Hybrid which Occupies an Ecotone between R. ponticum

Definitions of species used by evolutionary biologists range from the rigid interpretation of the biological species concept, i.e. that no 2 populations should be considered as separate species if they are capable of forming fertile hybrids and so interbreeding, to those that allow interbreeding provided there are clear and consistent morphological differences (Mallett, 1995). Plant species are often capable of remaining separate in the wild even where fertile hybrids are produced in large numbers (Rieseberg & Wendel, 1993; Arnold, 1997; Rieseberg & Carney, 1998). Therefore, to properly understand species requires an accurate understanding of the processes that keep them separate in the wild, which in turn requires the study of hybrid zones. Most documented plant hybrid populations fall into 2 types, although there is a continuum of variation between these. At one extreme are those hybrid populations that consist only of a few entirely sterile F1s, in which case hybridisation can proceed no further. At the other extreme are populations that comprise large numbers of hybrid individuals, many or most of which are of advanced generations. Even the very rare production of an almost sterile F1 can lead to the formation of such a large hybrid population (Arnold, 1997, 2000). The parent species may have a variety of pre- and post-mating mechanisms that minimise hybrid formation. However, once an F1 has been formed, often none of these mechanisms will prevent the F1 from receiving, and bein