Mountains may be cradles of evolution
Tectonics seem to fuel the appearance of new species.
Growing mountains may give rise to new species — and not simply provide a refuge to species whose traditional habitats have been lost, US scientists suggest.
"The major times of [species] diversification directly coincide with times of large tectonic events," says Catherine Badgley, a palaeontologist at the University of Michigan in Ann Arbor, who presented the findings this week at the annual meeting of the Society of Vertebrate Paleontology in Bristol, UK.
Mountainous regions are known to harbour higher levels of species richness than other areas. The reason, ecologists argue, is because mountains offer many different habitats in a relatively small geographical area. For example, whereas ten square kilometres of plains offer just one habitat, the same area of mountain landscape can provide sloping meadows, peaks and cliffs, all with different temperatures, rainfall and vegetation.
The long-standing view among ecologists has been that mountainous areas act as refuges for species that have been driven out of their normal habitat by difficult environmental conditions. A typical example could be a species dwelling in plains near the base of mountains — a change in conditions on the plains might mean that mountain areas begin to suit the needs of the species better, causing it to migrate.
"Mountains have always been considered the places for species to make their last stands because they offer such diverse terrain," notes Russell Graham, a palaeoecologist at Pennsylvania State University in University Park.
Curious about the mechanisms responsible for making mountains so rich in diversity, Badgley and her collaborator John Finarelli, also at the University of Michigan, studied mountain and lowland speciation rates and species richness using the fossil record of the Rocky Mountains and the Great Plains in North America. The fossils they inspected date to the Miocene period, which began around 23 million years ago and ended about 5 million years ago. They found that there were bursts of speciation that took place only in the mountains during times of tectonic activity. During all other times, they discovered that speciation rates in the two regions were moderate and similar.
Although the highly varied terrain of mountains helps to explain why so many different species can live there, the question of where the species richness actually comes from has never been addressed, explains Badgely. She and Finarelli now propose that as mountains are lifted up by the tectonic processes of the Earth's crust, mountain-dwelling species become isolated from other members of the same species living at lower altitudes. This isolation ultimately leads to the two groups breaking apart to form individual species.
"We had never thought of mountains as the birthplaces of species before," says Graham.
However, mountains might not be the only areas where speciation is taking place, explains Elizabeth Hadly, a palaeoecologist at Stanford University in California. It is also possible for mountains to have risen up from plains where there were once just, say, 10 species, she explains. The division created by the newly formed mountains could result in 20 unique species in the plains — 10 on either side of the mountains. Some of these species may ultimately find their way into the mountains to contribute to the increased diversity that is observed following tectonic activity. Indeed, adds Hadly, it is difficult to tell whether the evolution of new species happens in the plains, in the mountains, or both.