Is the outcome of evolution predictable?
If one would rewind the tape of life, would evolution result in the same outcome? The Harvard evolutionary biologist Stephen Jay Gould came up with this famous thought experiment. He suggested that evolution would not repeat itself: the role of random processes in the origin of biodiversity was too important and hence evolution was not predictable. Konstanz evolutionary biologists around Prof. Axel Meyer have now described parallel evolution of two closely related, but geographically isolated populations of cichlid fish in Nicaraguan crater lakes. This repeated outcome of evolution is best interpreted as evidence for similar adaptation to similar Darwinian natural selection pressure – and suggests somewhat deterministic evolutionary trajectories. The findings of the Konstanz study have been published in Nature Communications.
There are only very few circumstances in which one can investigate the repeatability of evolution, because spatially independent environments that are populated by the same species are extremely rare in nature. "The young and completely isolated crater lakes along the Central American Volcanic Arc in Nicaragua provide an ideal setting to study parallel evolution. Several crater lakes house populations of Midas cichlid fish that have developed independently from the ancestral population in the nearby great lakes of Nicaragua. This setting is like a natural experiment", explains Axel Meyer.
In two of these crater lakes, Apoyo and Xiloá, new types of Midas cichlids evolved, independently from each other, in less than 10,000 years. These new species show identical morphological adaptations that are not found in the ancestral population: from the shallow, murky water to the new habitat of the deep, clear water of the crater lakes. "In each of the two crater lakes new species of the Midas cichlid evolved with an elongated body – a phenotype that does not exist in ancestral lakes from which the colonisers of crater lakes came from", explains Meyer. His research team studied the morphological, ecological, population genetic, and phylogenetic patterns of these fish. "We found these cichlids to be strong evidence for parallel evolution that – interestingly – took place via different routes. Our findings show that parallel phenotypes can evolve in similar habitats and due to similar selection pressure, however, not necessarily in parallel evolutionary sequence", explains the evolutionary biologist from Konstanz. This indicates that parallel adaptation to similar environments may lead to the same result by natural selection, yet this evolution can proceed along different evolutionary genetic routes. This is because the equivalent endemic species in these two crater lakes originated in different sequences in both lakes. "Now we are looking for the genes and mutations that are the cause for this parallelism", says Axel Meyer.
"Our study shows that complex parallel phenotypes in similar environments can evolve very rapidly, repeatedly and yet via different evolutionary routes. This is a microevolutionary example of rewinding Gould's tape and resulting in the evolution of two very similar species, albeit by non-parallel evolutionary routes", sums up Axel Meyer.