14 DAY TRIAL // Tropical forests have much more biodiversity than the temperate forests. And biodiversity decreases furthermore as one gets nearer to the poles. But why is that?
Scientists from Auckland University of Technology in New Zealand now have an answer. Evolutionary ecologists Len Gillman, Shane Wright and their colleagues looked at the DNA of 45 pairs of related plants, one from the tropics and one from the temperate zone, and compared it with the DNA of a third more distantly related plant. This allowed them to determine how long ago the paired plants had diverged from one another and to see how many mutations each of them had accumulated since the separation point.
The conclusion couldn't be clearer: on average, the tropical plants had more than twice as many mutations as temperate species in the same genus.
This proves that the high biodiversity in the tropical areas is caused mainly by the faster metabolism of tropical species, stimulated by hotter temperatures and more sunlight. The mechanism at work is this: "Warmer temperatures speed up metabolism by allowing chemical reactions to occur at a faster rate, but this increased efficiency comes at a price: it produces higher quantities of charged atoms or molecules called 'free radicals,' which can damage proteins - including DNA," explains Ker Than at LiveScience. "Higher metabolism also speeds up DNA replication, which is just another chemical reaction, and this can increase the number of copying mistakes that can occur. Together, damage to DNA by free radicals and replication mistakes could result in mutations that, over time and through natural selection pressures, can form new species."
This theory was proposed by biologist Klaus Rohde in 1992, but some doubted that faster metabolism was behind the high tropical biodiversity - they speculated that the biodiversity could be triggered by smaller population sizes which make it easier for random genetic changes to accumulate. However, Gillman's team looked at species with large populations, so this hypothesis was ruled out. It turns out that faster metabolism, caused by the hotter climatic conditions, is indeed behind the differences between the tropics and the temperate zones. "This provides an explanation for the pattern [of biodiversity] that you see across the globe," said Gillman.