Phenotypic plasticity receives increasing interest from evolutionary biologists due to its putative role in evolutionary processes. Plasticity might promote the evolution of morphological novelty. Through secondary restriction of the plasticity, new specialized species occupying respective niches may arise. Cichlids have undergone adaptive radiation, potentially due to plasticity in basal lineages. Therefore they represent excellent models for studying the role of phenotypic plasticity in evolutionary processes. Diet-induced phenotypic plasticity has been studied in the lower pharyngeal jaw (LPJ) of the cichlid A. alluaudi. This study investigated the degree of phenotypic plasticity in the LPJ induced by diet, amongst an array of cichlid species, ranging from riverine generalists to lacustrine specialists that are part of adaptive radiations. Five species were included in our study, split in two groups, which compared the influence of two standardized diets that differed only in the mechanical robustness. Morphological plasticity was investigated using linear and geometric morphometrics. The expressions of candidate genes were investigated using qRT-PCR. Three of the five species displayed levels of plasticity namely A. alluaudi, A. burtoni and H. ishmaeli. A. alluaudi displayed the highest degree of morphological plasticity of all species studied, followed by A. burtoni, and H. ishmaeli. Accordingly, the expression of candidate genes also displayed highest variability in A. alluaudi, followed by A. burtoni and H. ishmaeli. Thus, this study shows that phenotypic plasticity is highest in species that did not undergo adaptive radiations. Therefore, my findings indicate that ancestral phenotypic plasticity in the LPJ underwent events of genetic assimilation in several lineages. By providing alternative jaw morphologies and thus trophic niches for these lineages, genetic assimilation contributed to the rapid adaptive radiation of East African cichlids.