This review addresses the question “Are interspecific and macroevolutionary trends observed in ruminant morphology found among populations of a single species?” Several case studies of the Japanese sika deer, Cervus nippon, are discussed. The Japanese sika deer is a suitable species for this purpose because it inhabits various environments from the northern subarctic forests to the southern subtropical forests. It shows conspicuous variations in feeding habits, body size, and other morphological and behavioral traits. It can be concluded that selective pressures governing interspecific variations among ruminants, e.g., tooth wear due to grazing or mountainous habitat, promote morphological adaptation of local sika deer populations. However, in some cases, genetic differentiation among local populations is relatively small, resulting in a small degree of change compared to that observed between species. Intraspecific comparison of an ecologically diverse species like the Japanese sika deer is useful for elucidating the evolutionary patterns within ruminants and the possible causes for those patterns.