Sympatric and Allopatric Diversification in the Adaptive Radiations of Midas Cichlids in Nicaraguan Lakes

Abstract

The Nicaraguan Midas cichlid species complex is a natural experiment where fish from a large source population from turbid and shallow great lakes very recently (<20,000 years ago) colonized eight small crater lakes. The colonizers experienced completely novel environments in the clear and deep calderas. So far, 13 Midas cichlid species have been described, but more genetic clusters were identified. Although some of these species arose in allopatry, many more evolved in the absence of barriers to gene flow within two crater lakes. They contain small radiations of four and six endemics, respectively. These radiations constitute one of the few generally accepted empirical examples for sympatric speciation making them an ideal system for studying repeated evolution of adaptations and speciation at different levels of biological organization, including the genome level. Diversification occurred repeatedly in parallel including body morphology, coloration, color perception, and trophic structures such as pharyngeal jaws and hypertrophied lips. Additionally, parallel speciation happened in the two small crater lake radiations, where ecomorphologically similar species evolved repeatedly. Genomic differentiation associated with oligogenic traits (e.g., hypertrophic lips and coloration) is shallow, remaining polymorphisms, but much higher for polygenic traits (e.g., body shape and pharyngeal jaw morphology) that distinguish new species.