Asian plentitude
PLANTS of same genera can be found in eastern Asia as well as in parts of the temperate zone of North America. But in Asia there is more variety of plants within the genus.
The temperate landmass was once contiguous. Landmasses making up Asia and North America today were severed apart in the mid to late Tertiary period of the Cenozoic era, at the Bering land bridge (now Bering strait). The number of species of these plant genera found in eastern Asia today are far greater than in North America. The plant diversity of eastern Asian temperate zones outstrips that of plants of same genus growing in the similar environment of the temperate zone of eastern North America.
A recent study suggests that the plant biodiversity has been enriched in eastern Asia by the changes occurring in the climate and the sea level coupled with the variations in the environment within the Asian temperate zone (Nature, Vol 407, pp 108-109).
Scientists at University of British Columbia in Canada and University of Missouri-St Louis in usa have studied 58 plant genera common to both the regions to find that 33 of them each have more species in Asia, 10 have more species in North America while 15 have equal number of species in both the regions. The researchers suggest that this bias in favour of Asia exists because greater degree of change in the climate and diverse habitats of eastern Asia forced allopatric speciation (evolution into new distinct species when members of one species disperse to occupy geographically different locations and evolve while adapting to the new environment).
The change in climate conditions and sea level changes have caused repeated fragmentation and rejoining of habitat. This has led to adaptive radiation, when species evolve over time to adapt to the changing environment.