**Background:** Neural crest cells (NCCs) are a multipotent embryonic lineage unique to vertebrates, often called the 'fourth germ layer.' They give rise to a wide array of adult cell types, including craniofacial bones and cartilage, pigment cells (melanocytes and chromatophores), neurons of the peripheral and enteric nervous systems, chromaffin cells of the adrenal gland, and odontoblasts. While the role of NCCs in human disease (neurocristopathies) and animal domestication is recognized, their contribution to adaptive phenotypic variation in wild populations is less understood. This review takes a trait-based approach to assess how phenotypes shaped by natural and sexual selection are diversified through variation in NCC development.
**Methods:** The authors conducted a narrative review of the literature, synthesizing evidence from developmental biology, molecular genetics, evolutionary ecology, and comparative studies. They focused on traits under natural and sexual selection that originate from NCCs or their derivatives, including trophic specialization, intrasexual competition, mate choice, aggression and social behavior, social communication, predator avoidance, and thermoregulation/photoprotection. They also examined coordinated changes in multiple NCC-derived traits using domestication and cavefish as case studies.
**Key Results:** The review presents numerous examples linking NCC development to phenotypic diversity. For trophic specialization, variation in NCC migration (e.g., more cells migrating to the jaw in ducks vs. quail), proliferation (e.g., Bmp4 and Wnt signaling in finch beaks), and differentiation (e.g., altered bone deposition in cichlids) generate craniofacial diversity. In pigmentation, changes in genes like MITF, MC1R, TYR, and ASIP affect melanocyte survival and eumelanin biosynthesis, producing color patterns used in mate choice (e.g., cichlid egg spots via edn3b/ednrB1a) and camouflage (e.g., Peromyscus beach mice via Agouti and Mc1r). For behavior, NCC-derived chromaffin cells secrete catecholamines that mediate stress responses and aggression; reduced adrenal function is linked to tameness in domestication. Coordinated changes are exemplified by domestication syndrome (reduced NCC migration leading to docility, white fur, shortened snouts) and cavefish (Astyanax mexicanus), where NCC transplantation experiments directly link NCCs to eye size and pigmentation. The authors note that many behavioral and neurological traits remain underexplored.
**Clinical Implications:** Understanding NCC development as a source of microevolutionary variation has implications for human health, particularly neurocristopathies (e.g., Waardenburg syndrome, Hirschsprung disease). The review suggests that the same developmental mechanisms generating natural diversity may underlie disease phenotypes. Additionally, insights into NCC plasticity and multipotency could inform regenerative medicine. The authors call for integrating evolutionary ecology with developmental biology to better understand trait covariation and evolutionary trajectories, which may reveal how NCC-derived traits evolve under multiple selective pressures.