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The relative fitness of parental and hybrid Kunzea (Myrtaceae): The interaction of reproductive traits and ecological selection¹

School of Biological Sciences, The University of Sydney, NSW, 2006, Australia

ABSTRACT

Up to 22% of plant species are the result of breeding among species—hybridization—directly conflicting with the prediction that hybrids, compared to parental species, are intermediate in character and of low fitness and little consequence. Few studies, however, have compared the fitness of hybrids and parental species under field conditions. This study evaluates components of fitness in the field for naturally occurring hybrids of the shrub Kunzea, relative to the parental speciesKunzea rupestris. Hybrid plants did not differ from the parental species in the level of effective pollination. Thus, we found no support for Grant’s model (Evolution: International Journal of Organic Evolution3: 82–97) of reduced fitness of hybrids via reduced pollination level (the intermediate hypothesis). Hybrids displayed variable fitness across the measured fitness components. Seed set levels for hybrids were structured among populations, suggesting genetic structuring for this fitness component at this scale. The response of hybrids to fire (a major selective force in the study system) was partly consistent with a resource trade-off model. Hybrids were large robust plants but most did not resprout after fire. Hence, the fitness of hybrids was complex. We developed a model for relative fitness to estimate fitness for species and hybrids with complex life histories.

Key Words: disturbance • fitness • hybrid • Myrtaceae • pollination • resprouting • seed set