Showing papers by "Brad M. Potts published in 1986"

Population dynamics and regeneration of a hybrid zone between Eucalyptus risdonii Hook. f. and E. amygdalina Labill.

Abstract: Regeneration of a hybrid zone between E. amygdalina and E. risdonii and pure species stands following wildfire is reported, as well as the reproductive and vegetative fitness of parental and hybrid phenotypes. E. risdonii phenotypes dominated the seed rain and seedling cohort and there was clearly a marked fitness differential between E. amygdalina and E. risdonii at their boundary. When the F1 type hybrid is in competition with both parental types it is generally reproductively the least fit, although frequently vegetatively vigorous. Reduced fitness appears to extend to advanced generations as hybrid phenotypes tending.toward either species are, on average, less fit than the corresponding parental type. The pattern of phenotypic fitness suggests that the species' boundary is in disequilibrium and it is argued that E. risdonii is invading the range of E. amygdalina by both pollen and seed migration. There is an asymmetric distribution of F1 type hybrids across the boundary and the hybrid swarm examined is being invaded by E. risdonii genes. It is suggested that hybridization may be associated with natural disequilibrium and, where seed migration is limited, boundary movements may be preceded by a wave of hybridization due partly to pollen swamping of the least fit species. Hybrid swarms may develop but, at the boundary of large stands, are probably transitory. There is a marked inertia in the population response to the prevailing selective regime due to the extremely slow population turnover and limited dispersal potential. This is discussed in the broader context of non-equilibrium models where it is argued that dispersal may be the factor limiting population response to perturbation of a shallow environmental gradient. This is due to large geographical shifts in the position of the null point and would be accentuated in a patchy environment where migration as a front is prevented.

Inbreeding and Interspecific Hybridization in Eucalyptus gunnii

Abstract: Results of a hybridization programme in France showed that the success and vigour of progeny of self-fertilization and close intraspecific crosses of Eucalyptus gunnii were significantly lower than for wide intraspecific crosses and a range of interspecific crosses. E. gunnii females demonstrated wide crossability, with the success of most interspecific hybrid combinations tried with species from the section Maidenaria being n.s.d. from that of wide intraspecific crosses. The only major barrier to interspecific hybridization found was with E. globulus. There was no strong association between crossability and the taxonomic/genetic distance between parents, although after 1 year of plantation growth F1 hybrids from the only interseries cross examined (E. gunnii x E. ovata) showed reduced vigour. The implications of these results for the development of breeding strategies as well as for gene flow and hybridization in natural populations are discussed.

Evolutionary processes in the Tasmanian high altitude eucalypts

Abstract: The rugged Tasmanian topography results in a patchy distribution of treeline habitat, which can be viewed as a mosaic of "habitat islands" varying considerably in size and degree of isolation. In recent geological time the Pleistocene glacial cycles have subjected this mosaic to dramatic perturbation involving habitat relocation, fragmentation, coalescence and, on a local scale, extinction and birth of unique adaptive modes. In this paper we examine the adaptive response of populations to a fluctuating mosaic of selective forces through a discussion of the evolution of clines in the Tasmanian high altitude eucalypts. The eucalypts primarily associated with the treeline habitat in Tasmania (i.e. E. coccifera, E. urnigera, E. vernicosa and E. gunnii) are all endemic. Populations of these species are distributed as mosaics or disjunctions within more continuous and widespread populations of subalpine and lowland taxa (e.g. E. delegatensis, E. dalrympleana, E. pauciflora, E. nitida). In many cases, there is a geographic and genetic continuum between stunted treeline variants and surrounding subalpine forest variants resulting in complex genetic mosaics. The range of variation along these altitudinal gradients is often large, transgressing several recognized taxa. The origin of these multi-character, clinal mosaics is of particular theoretical interest. These may arise as a result of either primary or secondary intergradation and it is important from an evolutionary viewpoint to differentiate these alternatives, although this is difficult from population structure alone (Anderson 1953; Barber and Jackson 1957; Mayr 1963; Endler 1977; Potts and Reid 1985b). Specific hypotheses accounting for the origin of clinal mosaics include: Primary intergradation (parapatric differentiation) parallel evolution fragmentation of once geographically extensive clines Secondary intergradation range fragmentation and introgression associated with a break down in geographic isolation local reproductive isolation long distance migration by seed dispersal pollen dispersal Possible examples of virtually all these alternatives can be found in the Tasmanian eucalypts, and the intricate variation patterns in specific complexes probably integrate the full spectrum of these evolutionary processes.

The breeding system and gene flow in Eucalyptus urnigera

Abstract: Introduction The stepped cline from green to glaucous morphs of E. urnigera with increasing altitude on Mt. Wellington, Tasmania, is one of the classic examples of clines in forest trees (Barber and Jackson 1957; Endler 1977). A study of the breeding system of E. urnigera was undertaken to provide a basis for investigations of gene flow and the dynamics of this cline.