Phenotypic plasticity is considered a catalyst for speciation through two alternative mechanisms: the Baldwin effect and genetic assimilation. Midas cichlids – Nicaraguan freshwater fishes where multiple species diverged in sympatry in crater lakes after colonization from larger source lakes – are the ideal model to study the influence of plasticity in speciation. The presence of trade-offs in the morphology of Midas cichlids’ pharyngeal jaw – a trophic structure used to crush hard food items which plastically modifies its size and shape when subjected to hard food – allows to generate specific hypotheses. In fact, according to theoretical models, in presence of trade-offs genetic assimilation is more likely than the Baldwin effect. I will use common garden experiments involving two Midas species from Lake Apoyo and the species from Lake Nicaragua (which acted as a source for Lake Apoyo) subjecting the fish to hard and soft diet treatments, that are nutritionally identically and only differ in their hardness. I will then measure the levels and directions of plastic response in each species/treatment using advanced geometric morphometric analyses and I will assess their transcriptional correlates using a combination of Illumina RNA next generation sequencing and RT-qPCR. I will, therefore, be able to test the following three hypotheses: 1. that the derived populations from Lake Apoyo show lower levels of plasticity than source species (as expected as a consequence of genetic assimilation); 2. that the plastic response in the source population produces the same phenotypes found in natural populations of the derived species (as expected according to the “flexible stem hypothesis”); 3. comparing the results with published data from African cichlids, that the genes differentially expressed in pharyngeal jaw plastic response are the same in both closely and distantly related cichlid lineages (thus suggesting a conserved molecular basis for pharyngeal jaw plasticity).