An ancient lake is a lake that carried water without interruption for more than one million years. The vast majority of lakes, including very large ones such as the Great Lakes, are of much more recent origin. The short life of most of lakes is due to continued sedimentation by their tributaries, which typically leads to complete siltation after several thousand years. In contrast, the longevity of ancient lakes can mostly be attributed to geological factors, such as plate tectonics, which can counter siltation.
With respect to evolutionary biology, ancient lakes serve as natural isolation laboratories. In analogy to the Galápagos Islands, ancient lakes form isolated 'water islands' in the midst of 'oceans of land'. An ancient lake's long lifespan may give rise to the evolution of endemic species (that are particular to their location). In comparison with the expanse of the oceans, ancient lakes allow evolutionary biologists to examine the circumstances that conditioned the evolution of new species within a reasonably circumscribed area. In Lake Tanganyika, for instance, scientists study the adaptive radiation of cichlids, while in Lake Ohrid, research focuses on endemic mollusca.