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.

Abstract. Although there are many studies of the evolution and ecology of hybrid zones, few extend over long enough time periods to track evolutionary changes in the zones or assess the ultimate outcome of hybridization. Here we describe the current genetic and morphological composition of a hybrid sunflower population relative to its initial makeup 50 years ago. It appears that few genetically pure parental plants remain in the hybrid population and the average phenotype has shifted from an initial bias toward Helianthus bolanderi to a predominance of H. annuus-like plants. The similarity to H. annuus is more pronounced for morphology than for neutral genetic markers. In contrast to the shift in morphology that occurred primarily in the past 40 years, overall pollen viability increased to its current level during the first 10 to 15 years of hybridization, indicating the presence of strong fertility selection. Dramatic differences are seen in morphology, genetics, and pollen viability between the eastern and western halves of the population, thus confirming observations by previous authors and suggesting that selection pressures in the two halves differ. The trends seen in this hybrid population over the past 50 years suggest that H. bolanderi is undergoing genetic assimilation, and this trend may be representative of its fate throughout its range.

Pollen-tube growth and seed siring ability were measured in crosses between the Louisiana iris species Iris fulva and Iris hexagona and their F, and F, hybrids. Flowers of the parental species were pollinated with self, outcross intraspecific, and interspecific pollen. Pollen-tube lengths were similar for all three pollen types in I. fulva, but in I. hexagona interspecific pollen tubes were longer than intraspecific pollen tubes. Pollen-tube lengths also differed for F, and F, flowers pollinated with I. fulva, I. hexagona, and hybrid pollen. For both hybrid classes I. fulva pollen tubes were the shortest while pollen tubes from I. hexagona and hybrids grew the furthest. Mixtures of genetically marked pollen were used to assess the seed siring ability of intra-and interspecific pollen in the parental species by varying the proportion of each pollen type in a replacement series design. For both species, the observed proportions of hybrid seeds were lower than the expected based on the frequency of each pollen type in the mixtures across all treatments. Flowers of I. fu/va produced less than 10% hybrid progeny even when 75% of the pollen applied to stigmas was derived from interspecific flowers. The frequency of hybrid seed formation was somewhat greater in I. hexagona, but was still significantly lower than expected across all mixture treatments. Seed set per fruit remained constant across the mixture treatments for both species, but in I. fulva fruit set decreased with an increase in the proportion of interspecific pollen. The data indicate that both pre-and postfertilization processes contribute to discrimination against hybrid seed formation.

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We investigated the indirect effects of Argentine ant (Linepithema humile) invasions on patterns of seed dispersal and predation in the myrmecochorous tree poppy Dendromecon rigida in coastal San Diego County, California. Significantly more seeds were removed from ant-accessible seed stations at sites numerically dominated by a common harvester ant (Pogonomyrmex subnitidus), a native disperser of these seeds and a species sensitive to displacement by L. humile, than from those stations at sites where L. humile was in the majority. Predation of seeds was high, but variable, across sites, suggesting that reduced dispersal could result in increased seed predation in some habitats. Removal of elaiosomes did not affect the frequency with which predators removed seeds, but ants removed significantly more seeds with elaiosomes than without. In behavior trials, only P. subnitidus was able to carry seeds of Dendromecon rigida effectively. L. humile and a small native ant species, Dorymyrmex insanus, while displaying interest in the diaspores, were seldom able to carry whole seeds and, when they did, only carried them a few centimeters. Displacement of native harvester ants by L. humile appears to decrease the dispersal of Dendromecon rigida seeds and may be increasing loss of seeds due to predation.