Final answer:
The mechanism of evolution occurring when captive-bred trout are introduced into a river, affecting wild trout allele frequencies, is gene flow, which involves the mixing of genetic material between populations.
Step-by-step explanation:
The mechanism of evolution taking place in the scenario where captive-bred trout are introduced into a river and cause a change in the allele frequencies of the wild trout population is gene flow. Gene flow occurs when there is a transfer of genetic variation from one population to another. In the given example, the captive-bred trout introduce new alleles into the wild trout population's gene pool, altering the wild population's allele frequencies over time.
This can occur when the captive-bred trout breed with wild trout, allowing their genetic material to mix and become part of the wild population. Thus, the introduction of these trout and the resulting genetic change in the wild population exemplify gene flow, influencing the evolutionary dynamics of the population.