On the coenospecies concept and tree migrations during the oscillations of the Pleistocene climate

The expansion and contraction of forest tree populations during the climatic oscillations of the Pleistocene implies changes in rates of mortalitity and survival in different parts of a species' range at different times, Different genotypes are favoured in expanding versus contracting populations. Hybridisation is common among many tree and shrub genera in New Zealand. A group of related, more or less frequently hybridising, species forms a common gene‐pool or “coenospecies”, whose constituent taxa vary, at anyone time, in their ecological isolation and taxonomic distinction. The evolutionary significance of this lies in the possibility that forms which are now rare or intermediate may have been more common at other times, as the altitudinal and geographical range of a coenospecies has fluctuated in response to climatic change. Glacial and interglacial climates select for different physiological tolerances, while the transition periods select for demographic characteristics favouring population expansion or contraction. I propose that, during the prolonged cooling phases of the Pleistocene, sub‐alpine and montane species invaded more diverse forest at lower altitudes, while components of the latter became progressively restricted to the most favourable niches within their ranges. These processes imply a ‘K‐selection ’ regime. The much shorter warming phases witnessed rapid population expansion in some taxa, implying ‘r‐selection’. These ideas are discussed with reference to the ‘fusca’ beeches in New Zealand.