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The fossil record shows that in a short period from about 25 Myr ago, the ancestral Gondwanan rainforest gave way to the unique Australian 'sclerophyll' flora dominated by eucalypts, acacias and casuarinas. This coincided with a drying climate and the advent of fire. It is hypothesized that 'explosive' evolutionary radiations rapidly increased the number of sclerophyll species, rather than a mere range expansion of pre-existing species (Crisp et al., in press). In the main ARC-funded project in Mike Crisp's lab, molecular phylogenies of several distantly related plant groups are being used to test whether explosive evolutionary radiations took place simultaneously and to identify the environmental triggers. It is predicted that radiations of the arid zone flora may date from 10-5 Mya (Pliocene). This would correspond with the first onset of severe aridity in Australia, when sclerophyll forest abruptly gave way to communities dominated by grasses and daisies. The radiation of the 'spinifex' grasses (Triodia) may date from this period, and will be studied in the main project. If synchronous radiations are found to be localised to particular regions, this may help to refine the trigger hypothesis, e.g. if Pliocene radiations were restricted to the central Australian arid zone. |
| Therefore, for comparison with Triodia, Lindy Orthia has started a project on the Chenopodiaceae. This will be based on a molecular phylogenetic analysis of genera such as Atriplex, Maireana or the samphires. Like Triodia, this group is diverse in the arid zone, and the a priori prediction is that both groups radiated simultaneously, perhaps during the Pliocene. However, chenopods are semi-succulent and fire resistant, whereas Triodia is fire-adapted. Therefore, comparison of these taxa may provide a test of whether fire and aridity triggered radiations at different times. Another hypothesis that will be tested is Burbidge's idea that the arid-zone chenopods evolved from salt-tolerant coastal ancestors. |
Contact <Mike.Crisp@anu.edu.au> For more information about the graduate program in the School of Botany and Zoology, and at ANU, see http://online.anu.edu.au/EES_BOZO/ |
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