Figure 1. The classical case of character displacement is a pattern of enhanced morphological difference in sympatry: two species of Eurasian rock nuthatches differ more in beak size (and eye-stripe size) where they occur together in sympatry (a and c) than where they occur alone in allopatry (b and d). The species are Sitta neumayer (b, Greece; d, Iran) and Sitta tephtonota (a, Iran; c, Russia).

Figure 2. The classical case of character release is a pattern of morphological intermediacy where a species is alone. The medium ground finch (a, Geospiza fortis) is larger than a competitor, the small ground finch (b, G. fuliginosa) on the Galápagos island of Santa Cruz, but in the absence of G. fuliginosa on Daphne Major island G. fortis has become intermediate in size (c).

Figure 3. The classical experiment of character displacement. The growth of sticklebacks is reduced in the presence (broken line) of a competitor species compared with their growth in the absence of competitors (solid line). The effect of competitors is strongest on the fish that are most similar to the competitors in morphology (large index values).