A laboratory for evolution
The Parque Natural de las Lagunas de Ruidera is a chain of fifteen small lakes in the arid plains of La Mancha. The lakes are connected by a series of streams, waterfalls, and underground channels. This unique wetland environment is formed by natural dams, called travertine barriers, created by the precipitation of calcium carbonate from the water over millennia. These barriers, or "barreras tobáceas," separate one lake from another, creating distinct aquatic habitats.
Within this chain of lakes, a remarkable evolutionary process is unfolding. The park is home to several species of freshwater snails from the Hydrobiidae family. Genetic studies on these tiny mollusks have revealed a startling fact: populations in adjacent lakes, sometimes separated by a barrier just a few hundred meters wide, are genetically distinct. Molecular clock analysis suggests that these snail populations have been reproductively isolated from one another for as long as two million years. This means they are evolving on separate trajectories, a process known as allopatric speciation, but on an unusually small geographic scale.
The geology of isolation
The reason for this micro-evolution is the unique geology and hydrology of the Ruidera lakes. The water has high levels of dissolved carbonates. As it flows over the barriers from one lake to the next, changes in pressure and temperature cause the calcium carbonate to precipitate, forming the porous travertine rock that builds the dams. This process continuously reinforces the separation between the lakes.
While the lakes are connected, the specific water chemistry and physical structure of each basin create isolated ecosystems. The snails, which have limited mobility, remain within their home lake. Gene flow between the populations is virtually nonexistent. Over vast timescales, this isolation has allowed random genetic mutations to accumulate differently in each group. Each lake acts as a separate evolutionary experiment, leading to the snails' genetic divergence. Researchers have found aquatic gastropods such as Pseudamnicola and Valvata species in core samples of the travertine deposits, indicating a long history of these mollusks in the lake system.
The snails' situation shows how geographical barriers, even very small ones, can drive the formation of new species. The Ruidera Lakes system is a place to study studying the mechanics of evolution in near-real time, showing the slow, persistent processes that shape the diversity of life on Earth.