Final answer:
Criterion 6 for demonstrating character displacement involves showing differences between species are a result of competition, as seen in the example of Galapagos Finches with different beak shapes adapted for varied diets.
Step-by-step explanation:
Criterion 6 for demonstrating character displacement, as outlined by Schluter & McPhail (1992), would involve showing that the differences observed between species are indeed a result of selection imposed by competition over resources, rather than mere chance or other ecological factors. This could be evidenced by instances where species that overlap geographically exhibit greater differentiation in resource-exploiting traits than those in allopatric populations (i.e., populations that are geographically separate and do not overlap). For example, the Galapagos Finches discussed in Figure 12.7.1 exhibit variations in beak shape that correlate with diet specialization, suggesting that character displacement has occurred as a mechanism to reduce competition for food resources. This example shows different types of bills adapted for eating different types of foods, reflecting an adaptation to ecological niches as a result of character displacement. Considering the provided text, it is essential to differentiate traits that arise due to character displacement from traits that may have arisen due to other pressures or from genetic drift, which is unrelated to competition.