Morph ratio clines (MRCs) are spatial gradients in morph frequencies. MRCs are often misunderstood, on the basis that the selection pressure gradient explains the phenomenon. Theoretically, however, negative frequency dependent selection (NFDS) or gene flow (GF) is indispensable for maintaining MRCs. Here, we estimated the relative significance of GF and NFDS on MRCs. A mathematical model that describes the dynamics of two different morphs in spatially structured populations predicted that NFDS affects MRCs more than does GF, although greater GF makes the MRC gradient gentler. To test these predictions, we compared the MRCs of a pygmy grasshopper Tetrix japonica across latitude and altitude, both having the same temperature gradients, but different degrees of GF due to the large difference in geographic scale. T. japonica is dimorphic and the different color morphs are under NFDS by avian predators and gradual climatic selection via differential body heating rates. Regardless of the geographic scale, MRCs were observed over the climatic gradient, suggesting that NFDS was the major determinant of the MRCs. In addition, the altitudinal MRC was gentler than the latitudinal MRC, confirming that GF also affected the MRC gradient.