Botanic Gardens Trust, Sydney, Australia

Proteaceae - speciation

Combining genetic structure and bio-climatic modelling to understand temporal changes in within- and between-species contacts

Dr Maurizio Rossetto - Principal Research Scientist and Manager, Evolutionary Ecology

Four of the five species of Telopea (Proteaceae) are distributed in a pattern in south-eastern Australian mainland where one species is replaced by another as the latitude becomes more southerly. In similar circumstances, it has general been assumed that this is the result of a simple allopatric speciation model where genetic isolation occurs as a result of geographic separation. However, secondary contact between differentiated lineages can result in similar distributional patterns to those arising from a process of parapatric speciation (where gene flow between lineages remains uninterrupted during differentiation). We have used the characteristic distributional patterns in Telopea to test whether it reflected the evolutionary models of allopatric or parapatric speciation. Using a combination of genetic evidence and environmental niche modelling, we focused on three main questions: do currently described geographic borders coincide with genetic and environmental boundaries; are there hybrid zones in areas of secondary contact between closely related species; did species distributions contract during the last glacial maximum resulting in distributional gaps even where overlap and hybridisation currently occur?

Environmental niche models were produced for each of the Telopea species using environmental parameters from both the present day and the last glacial maximum (LGM), approx-imately 20,000 to 25,000 years ago and molecular markers (nuclear and chloroplast SSR) were used to infer demographic patterns. DNA analyses recognised the four species with the additional division of T. speciosissima into pop-ulations north and south of the Shoalhaven River valley. Unexpectedly, the northern disjunct population of T. oreades grouped with T. mongaensis. Present day and LGM environmental niche models differed dramatically, suggesting that distributions of all species had repeatedly expanded and contracted in response to climatic oscillations in the Pleistocene period and confirming strongly marked historical distributional gaps among taxa (Figure 1).

Genetic structure and bio-climatic modelling results are more consistent with a history of allopatric speciation followed by repeated episodes of secondary contact and localised hybridisation, rather than with parapatric speciation. This study on Telopea shows that the evidence for temporal exclusion of gene flow can be found even outside obvious geographical contexts, and that it is possible to make significant progress towards excluding parapatric speciation as a contributing evolutionary process in these plants.  





Figure 1: Modelled bio-climatic distribution (LGM right column; current left column) for Telopea aspera (top row) and Telopea speciosissima (bottom row) and measured population-level genetic diversity (chloroplast and nuclear).