The origin and evolution of tertiary relict floras

Abstract

Tertiary relict floras contain survivors from plant communities that were distributed throughout a large part of the Northern Hemisphere during much of the Tertiary (i.e. 65 – 15 million years ago (Ma)). They are now mainly restricted to warm humid areas (refugia) in southeastern and western North America, East Asia and southwest Eurasia. Recent molecular phylogenetic studies show that within East Asia the Tertiary relict flora is best divided into two distinct refugial groups, with geographical distributions centred on the Japan/Korea/northeast China and southeast China/Himalayas regions respectively. Recognition of this division leads to a significant improvement in our understanding of the origins and evolution of Tertiary relict floras in East Asia and elsewhere. Molecular studies also indicate two putative clusters of divergence times for East Asian-North American Tertiary relict disjuncts occurring at 5 and 10 Ma. These clusters might reflect a break in the continuity of the Tertiary flora between East Asia and North America across Beringia during a cold period 6–8 Ma, i.e. before the Bering Land Bridge was severed approximately 5 Ma. In addition, there is some evidence that evergreen disjuncts diverged earlier than their deciduous counterparts, possibly due to the high latitude of Beringia. Molecular studies further suggest that divergence times for transatlantic Tertiary relict disjuncts generally fall between 10 and 40 Ma, even though most geological evidence shows that the North Atlantic Land Bridge (NALB), which connected the floras of Europe and North America, was severed around 50 Ma. This raises the issue of whether a partial NALB allowed migration of floras between Eurasia and North America throughout much of the Tertiary. Tertiary relict floras are notable for exhibiting slow morphological evolution (stasis). This might result from large-scale allopatric speciation, together with stabilising selection.