Ice age climate change and ancient flooding -- but not barriers created by rivers -- may have promoted the evolution of new insect species in the Amazon region of South America, a new study suggests.
The Amazon basin is home to the richest diversity of life on earth, yet the reasons why this came to be are not well understood.
A team of American and Brazilian researchers studied three species of leafcutter ants from Central and South America to determine how geography and climate affect the formation of new species.
Their results was released Tuesday by the online journal PLoS ONE.
In order to evaluate the three most popular hypotheses as to why there's such a diversity of species in the Amazon, they collected genetic samples from 194 leafcutter ant colonies scattered throughout the Amazon basin.
By combining analysis of the genetic information with knowledge of the species' current ranges and "paleodistribution" models of what the species' ranges were during the last ice age, the researchers found support for both the "Pleistocene refugia hypothesis" and the "marine incursion hypothesis."
The genetic and the climatic results, however, both suggested that the "riverine barrier hypothesis" cannot explain insect biodiversity.
The Pleistocene hypothesis posits that a major decrease in rainfall during the last ice age (about 21,000 years ago) affected where many Amazonian species were able to survive. What was initially a single species would evolve into multiple, distinct species after being separated into isolated "refugia," each with its own set of selective environmental pressures.
The marine hypothesis suggests that some 10-15 million years ago, a combination of tectonic events and elevated sea levels flooded much of the Amazon Basin in salty or brackish water. This would have caused higher elevation regions to become like islands, in which species were able to evolve independently from species on other "islands."
The riverine hypothesis suggests that tropical rivers serve as barriers to gene flow for terrestrial organisms. These rivers would promote divergence of populations restricted to either side. But, the study shows that even the Amazon river has not kept winged leafcutter ant queens and males from flying across it.
However, "the results don't solve the great puzzle of why tropical insect diversity exceeds temperate insect diversity by several orders of magnitude," says Ulrich Mueller, one of the lead authors. " but they are significant, and they point to some key processes that other researchers will now test also on other organisms."
Source: Xinhua
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