• Eucalyptus;
  • hybridization;
  • landscape genetics;
  • pollen dispersal

In chemistry, the law of mass action describes how variations in the concentrations of chemical compounds lead to different chemical reaction outcomes. Does the extent of hybridization, or more particularly, the formation of hybrid offspring, likewise depend on the local abundance of pollen from compatible species in systems as complex as tall trees which depend on (insect) pollinators? In this issue of Molecular Ecology, Field et al. (2011a) present a study involving two ecologically divergent eucalypt species (Fig. 1). By comparing several contrasting settings with different local densities and geographical arrangements of adult trees and by studying parentage in progeny arrays, they show that on top of pre-mating barriers like flowering time differences, local demography and varying scales of pollen dispersal, which in themselves depend on pollinator behaviour in reaction to flowering abundance, all interact in a somewhat predictable way. In other words, these factors can explain some of the variation in hybrid formation observed. In this way, the study introduces important progress towards a quantitative description of hybridization potential. Therefore, let me tell you about the birds and the bees and the flowers and the trees (Newman 1964).


Figure 1.  The black gum tree of south-eastern Australia (Eucalyptus aggregata—left), which is associated with poorly drained flats, and candlebark (Eucalyptus rubida—right), which is associated with well-drained skeletal soils. Where the species cooccur, both F1 and later-generation hybrids have been identified, particularly in transition zones between the parental habitats (Field et al. 2011b) (Photographs courtesy of David Field).

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