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Closely related species and populations within a species often display subtle morphological differences in skeletal systems. Such morphological variation probably results from slight differences in the timing or location or intensity of the action of specific genes. (A–C) Skeletal variation in the palate and neurocrania of three species of teleost fish. (A)Danio rerio (zebrafish), (B) Gasterosteus aculeatus (stickleback) and (C) Oryzias latipes (medaka). Note species-specific differences in the shapes of the palate, consisting of ethmoid plate and parasphenoid. (D–E) Anterior view of the premaxilla bone showing variation in shape between D. rerio (zebrafish) (D) and D. nigrofasciatus (dwarf danio) (E). The angle of zebrafish premaxilla bone is 81 ± 5° (n= 44) for D. rerio while the angle of D. nigrofasciatus is 74 ± 4° (n= 35). (F) Evolutionary relationships of some fish discussed in this review, including the basally diverging non-teleost Lepisosteus oculatus (spotted gar). Abbreviations: e, eye; ep, ethmoid plate; nt, notochord; ol, otolith; pa, palate; pc, parachordal; ps, parasphenoid and tr, trabeculum.