Larcombe et al. present a 20-year assessment of reproductive isolation in a forest tree system. They show that intermediate inheritance of floral morphology results in the loss of strong mechanical barriers to zygote formation between Eucalyptus globulus and E. nitens – enabling F1 hybrids to act as a bridge for gene flow between the taxa.
However, intrinsic postzygotic barriers cause significant outbreeding depression in growth, survival and reproductive capacity at 20 years of age across multiple hybrid classes. Thus, in contrast to many herbaceous systems, intrinsic postzygotic barriers seem to play a key role in maintaining species integrity in this system.