Plant hybridization

Most studies of plant hybridization are concerned with documenting its occu rrence in different plant groups. Although these descriptive, historical st udies are important, the majority of recent advances in our understanding o f the process of hybridization are derived from a growing body of experimen tal microevolutionary studies. Analyses of artificially synthesized hybrids in the laboratory or glasshouse have demonstrated the importance of gameti c selection as a prezygotic isolating barrier; the complex genetic basis of hybrid sterility, inviability and breakdown; and the critical role of fert ility selection in hybrid speciation. Experimental manipulations of natural hybrid zones have provided critical information that cannot be obtained in the glasshouse, such as the evolutionary conditions under which hybrid zon es are formed and the effects of habitat and pollinator-mediated selection on hybrid-zone structure and dynamics. Experimental studies also have contr ibuted to a better understanding of the biology of different classes of hyb rids. Analyses of morphological character expression, for example, have rev ealed transgressive segregation in the majority of later-generation hybrids . Other studies have documented a high degree of variability in fitness amo ng different hybrid genotypes and the rapid response of such fitness to sel ection - evidence that hybridization need not be an evolutionary dead end. However, a full accounting of the role of hybridization in adaptive evoluti on and speciation will probably require the integration of experimental and historical approaches.