The Andes are the longest and most diverse mountain range on Earth. They span about 9,000 kilometers from north to south and host a variety of habitats and species.
But how and when did they form? And how did they influence the evolution and diversity of South American biota?
These are some of the questions that scientists have been trying to answer using different methods and data sources.
The role of tectonics and climate in shaping the Andes
The Andes are the result of a long and complex process of tectonic collision between the South American and Nazca plates, which began about 200 million years ago and continues to this day.
The uplift of the Andes was not uniform or continuous, but rather occurred in different phases and rates, creating a mosaic of mountain ranges, plateaus, basins, and valleys.
The highest and widest section of the Andes is the Altiplano-Puna Plateau, which covers parts of Bolivia, Peru, Chile, and Argentina.
This plateau has an average elevation of four kilometers and is the second-largest mountain plateau in the world, after Tibet.
The uplift of the plateau has been a matter of debate among geologists, who have proposed different scenarios and timings for its formation.
A recent study by Dr. Heiko Pingel, a geoscientist at the University of Potsdam used stable hydrogen isotopes in volcanic glass to reconstruct the uplift history of the plateau and its surroundings.
Their results showed that the plateau was uplifted by about two kilometers between 13 and 9 million years ago, much later than previously thought.
This uplift coincided with a period of intense volcanic activity in the region, which may have contributed to the thickening of the crust and the elevation of the surface.
The uplift of the Andes not only affected their geology but also their climate.
The rising mountains created a barrier for atmospheric circulation, blocking moisture from reaching the western side of South America and creating one of the driest deserts in the world: the Atacama.
On the eastern side, however, moisture from the Atlantic Ocean was able to reach higher altitudes and generate abundant rainfall over the Amazon basin.
The Andes also influenced global climate by altering ocean currents and wind patterns.
The climate of South America has also changed significantly over time, especially during the Pleistocene epoch, which spanned from about 2.5 million to 11,700 years ago.
During this period, Earth experienced several cycles of glacial and interglacial phases, which caused dramatic fluctuations in temperature and ice cover.
The Andes were particularly affected by these changes, as glaciers advanced and retreated across their slopes and valleys. These climatic oscillations had profound impacts on the environment and biodiversity of South America.
Also Read: Beneath the Andes Mountains: Earth's Crust 'Dripping' Into the Planet's Interior
The origin and diversification of South American biota
The biodiversity of South America is one of its most distinctive features. The continent hosts about 40% of all plant species and 30% of all vertebrate species in the world.
Many of these species are endemic, meaning that they are found nowhere else on Earth. For example, about 90% of all bird species in South America are endemic.
The high diversity and endemism of South American biota are largely explained by its complex geological and climatic history.
One of the key factors that shaped South America's biodiversity was its long isolation from other continents.
Until about 3 million years ago, South America was separated from North America by a marine barrier: the Central American Seaway.
This isolation allowed South American biota to evolve independently from other regions, resulting in unique lineages and adaptations.
However, isolation was not complete or constant. Throughout its history, South America had occasional connections with other landmasses, such as Antarctica, Australia, Africa, and North America.
These connections allowed the exchange of biota between continents, creating opportunities for colonization and diversification.
Another important factor that influenced South America's biodiversity was the formation and change of the Andes.
The Andes created a variety of habitats and environmental gradients along their length and width, providing niches for different species to occupy and adapt to.
The Andes also acted as barriers for gene flow and dispersal, isolating populations and promoting speciation.
Speciation is the process by which new species arise from existing ones, usually as a result of geographic or ecological separation.
A recent study by Palacio et al. used bird species as a model to investigate the patterns and processes of speciation in the Andes.
The authors identified 15 biogeographical regions resulting from Andean uplift and Pleistocene climatic oscillations.
They also developed three main models of speciation that explain the origin of the biodiversity within these regions. These models are:
Vicariance
This mode occurs when a population is split into isolated subpopulations by a physical barrier, such as a mountain range or a river.
Over time, these subpopulations diverge genetically and morphologically until they become distinct species.
Dispersal
This mode occurs when a population expands its range into new areas that are not connected to its original habitat.
The colonizing individuals may encounter different selective pressures and adapt to new conditions, becoming different from their source population.
Parapatry
This mode occurs when a population occupies a continuous habitat that varies gradually along a spatial gradient, such as elevation or rainfall.
The individuals at opposite ends of the gradient may experience different environmental conditions and diverge adaptively, forming distinct populations that are partially isolated by distance or habitat preference.
These modes of speciation are not mutually exclusive or exhaustive, but rather represent general scenarios that can be modified by other factors, such as hybridization, polyploidy, or ecological interactions.
These modes may also operate at different temporal scales, depending on the evolutionary history and life history traits of each lineage.
Related article: Andes Mountain Faces Low Snowfall Due to a Mega-drought Linked to Global Heating