What mechanism of evolution becomes more powerful as population size lowers?

Answer: Genetic drift

Explanation: Genetic drift is a device of evolution in which allele frequency of a population change over generations due to chance. Genetic drift occurs in all populations of non-infinite size, but its consequences are strongest in small populations. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation. It can also cause initially rare alleles to become much more frequent and even fixed.