Northern Hemisphere plant disjunctions: a window on tertiary land bridges and climate change?
OBJECTIVE
This botanical briefing examines how molecular systematics has contributed to progress in understanding the history of Tertiary relict genera, i.e. those that that now occur disjunctly in parts of Eurasia and N America, and how progress in understanding Southern Hemisphere biogeography paradoxically makes unravelling Northern Hemisphere biogeography more complex.


METHODS
Tertiary relict floras comprise genera of warm wet climates that were once circumboreal in distribution but are now confined to E Asia, south-eastern and western N America, and SW Eurasia. The intercontinental disjunctions among these genera have long been believed to result from land connections between Eurasia and N America, across Beringia and the N Atlantic. This view is reassessed in the light of new evidence for long dispersal of propagules across oceans being responsible for many plant disjunctions involving southern continents. The impact of molecular dating, which has been very different in Southern and Northern Hemisphere biogeography, is discussed.


CONCLUSIONS
For N America-Eurasia disjunctions involving Tertiary relict floras, land connections remain the more likely cause of disjunctions but data from fossils or infraspecific variation will be required to exclude long-dispersal explanations for disjunctions in any individual genus. Molecular dating of divergence between disjunctly distributed Tertiary relict floras can tell us which palaeoclimatic or palaeogeographic events impacted on them, and how, but only if migration over land and vicariance can be proved and molecular dating is sufficiently accurate.
Ian Milne
Richard
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Institute of Molecular Plant Sciences, School of Biological Sciences, The University of Edinburgh, Daniel Rutherford Building, The King's Buildings, Mayfield Road, Edinburgh EH9 3JH, UK. r.milne@ed.ac.uk
D001208:Asia; D002980:Climate; D005060:Europe; D014644:Genetic Variation; D005843:Geography; D006239:Haplotypes; D009656:North America; D010802:Phylogeny; D010944:Plants / Q000145:classification* / Q000235:genetics*