Summary 599

I. Introduction 599

II. Concepts and terminology 600

III. Historical background 600

IV. Studies of experimental hybrids 601

1. Isolating mechanisms 601

2. Prezygotic barriers 602

(a) Gametic barriers to hybridization 602

3. Postzygotic barriers 603

(a) Chromosomal rearrangements 604

(b) Genic sterility or inviability 604

4. Hybrid vigour 605

5. Introgression 606

6. Hybrid speciation 607

V. Experimental manipulations of natural hybrid populations 609

1. Hybrid-zone formation 610

2. Pollinator-mediated selection 610

3. Habitat selection 612

VI. The biology of different classes of hybrids 612

1. Character expression 613

(a) Morphological characters 613

(b) Chemical characters 613

(c) Molecular characters 613

2. The fitness of different classes of hybrids 614

(a) The importance of variance 614

(b) Estimating hybrid fitness 615

3. Interactions with parasites and herbivores 616

4. Patterns of mating 617

(a) Outcrossing rate 617

(b) Hybridization frequency 618

(c) Mate choice 618

VII. Conclusions and future research 619

Acknowledgements 620

References 620

Most studies of plant hybridization are concerned with documenting its occurrence in different plant groups. Although these descriptive, historical studies are important, the majority of recent advances in our understanding of the process of hybridization are derived from a growing body of experimental microevolutionary studies. Analyses of artificially synthesized hybrids in the laboratory or glasshouse have demonstrated the importance of gametic selection as a prezygotic isolating barrier; the complex genetic basis of hybrid sterility, inviability and breakdown; and the critical role of fertility 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 zones are formed and the effects of habitat and pollinator-mediated selection on hybrid-zone structure and dynamics. Experimental studies also have contributed to a better understanding of the biology of different classes of hybrids. Analyses of morphological character expression, for example, have revealed transgressive segregation in the majority of later-generation hybrids. Other studies have documented a high degree of variability in fitness among different hybrid genotypes and the rapid response of such fitness to selection – evidence that hybridization need not be an evolutionary dead end. However, a full accounting of the role of hybridization in adaptive evolution and speciation will probably require the integration of experimental and historical approaches.