About 55 million years ago, when the Indian Subcontinent collided with Asia, many plants and animals migrated from India to other parts of Asia. Now, by studying the evolutionary history of a spider family, researchers have tried to map the dispersal event from India to the islands of Southeast Asia 55–38 million years ago.
The team from the Institute of Zoology at the Chinese Academy of Sciences conducted extensive field surveys in Tibet, Nepal and Sri Lanka, and genetic studies were carried out on over 100 global spider species. Based on the climate and the current distribution of these spiders, they concentrated on 12 regions including South and Central America, Africa, Madagascar, India, Sumatra, Fiji and Borneo.
Common ancestor
The constructed histories show that a spider subfamily called Theotiminae might have come from Africa, whereas a sister family called Ochyroceratinae had a South American origin. This means they had more than one common evolutionary ancestor and the taxonomy might even need revisions.
Why not to East Asia?
One question the researchers were trying to answer was: Why did the spiders not migrate from India to East Asia? They note that, in contrast to the tropical and wet regions of Southeast Asia, East Asian countries like China, Japan, Korea and Mongolia, had an arid climate with less than 800 mm of annual precipitation during that period.
“Despite no obvious geographical barrier, this dry or seasonal climate acted as barriers to the colonisation of East Asia by theotimines,” explains the paper published in Systematic Biology.
Role of climate
Also, during that period, the Tibetan Plateau developed into a mountain belt with almost 4000 m height and the colder and drier climate was also unsuitable for the spiders.
“Our analyses suggest that climate plays a more important role than geology in biological migration from the Indian subcontinent to Southeast Asia, providing new insights into the Indian–Asian biogeographic link,” notes the paper.
During the field study the team noted that in some areas, the spiders were found in leaf litter in the forests, while in others they were occupying caves.
Habitat shifts
The researchers write that these habitat shifts also occur depending on the climate of the area. “Most caves have an almost constant temperature and high humidity throughout the year, providing suitable shelters for theotimines in seasonally variable environments. Species in a wet environment can live in the litter or caves, whereas species in an environment with low precipitation are more often found in caves,” adds the paper.
The team writes that an “integrative approach combining different sources of evidence, such as biogeography and ecology,” can provide a more realistic reconstruction of evolutionary history. They predict that other organisms distributed in the tropical zones may also exhibit similar mechanisms of migration.
