Epithelial behaviors and threshold effects in the development and evolution of internal and external cheek pouches in rodents

The internal and external cheek pouches found in certain rodents arise early in development by an evagination of the buccal epithelium. Differences in the epithelial eva inations that produce the internal pouches of the Syrian hamster (Mesocketus auratus) and least ckipmunk (Eutamias minimus) are consistent with the view that they evolved independently. The external cheek pouches of rodents of the superfamily Geomyoidea represent a macroevolutionary phenoty e when com ared to the internal pouches of other rodents. Externalization of an internal ouci rudiment Eund in the geom oids Di odomys and Thomomys can be explained by a simple c&nge early in its development, the erect of wiich is greatly magnified by facial growth. In this example, the traditional dichotomy between microevolutlonary and macroevolutionary theories is bridged by an understanding of developmental dynamics.