The killifishes of Lake Titicaca (genus Orestias) provide a spectacular example of rapid adaptive radiation. Unlike any other genus of killifishes, the recently diverged species of Orestias have evolved to occupy a broad variety of trophic niches, including pelagic piscivores, macro- and micro planktivores, benthic insectivores, and specialized molluscivores. It is considered likely (by me, at least) that the absence of any other fishes from Lake Titicaca (save for a small Trichomycterid catfish and the recently introduced brown trout) has permitted this degree of morphological and ecological diversification, which is unprecedented in any other group of Cyprinodontiformes.

This is an individual of the most widespread and generalized species, Orestias agassii.
This taxon can be found both in Lake Titicaca proper.....

.....and in glacier-fed mud puddles at almost 17,000 feet. This pond ices over every night, and reaches bathwater temperatures every sunny day (remember, 17,000 feet at 18 degrees south latitude - that sun is bright). It also supports an incredible density of O. agassii .

This is O. luteus, one of the more morphologically unique forms that is restricted in distribution to Lake Titicaca proper. Unfortunately, due to their rapid speciation, it is proving impossible to unravel the phylogenetic relationships among these taxa using mtDNA. This is similar to the situation that obtains with many groups of East African Cichlidae.

On the whole, the killifishes of Lake Titicaca are faring better than many unique faunas located in impoverished parts of the world, although the introduction of these tasty exotics is thought to have precipitated the extinction of at least one species, the large pelagic piscivore O. cuvieri, which has not been found in the lake since 1939.