Final answer:
Criterion 4 for character displacement involves greater phenotypic differences between species in sympatric populations than in allopatric ones, suggesting adaptation due to competition.
Step-by-step explanation:
The criterion 4 for demonstrating character displacement, as outlined by Schluter & McPhail (1992), involves the evidence of displacement between sympatric and allopatric populations. Specifically, this criterion states that phenotypic differences between species should be more pronounced in sympatric populations (where the two species coexist) than in allopatric populations (where the species are separated geographically). This is considered evidence of adaptive changes due to competitive interactions when species coexist, which distinguishes character displacement from similar phenotypic changes that might occur due to other forces such as genetic drift or similar selective pressures in similar environments.
Criterion 4 for demonstrating character displacement, as discussed by Schluter & McPhail in 1992, focuses on ecological models. Specifically, it examines how resource competition between closely related species leads to divergence in their traits and characteristics. This can be observed through differences in the morphology, behavior, or habitat preferences of these species.