Final answer:
The founder effect is the most likely reason for the higher incidence of polydactyly in a specific human population, due to the initial genetic makeup of a small group that established the population.
Step-by-step explanation:
The most likely explanation for a particular human population with a higher incidence of polydactyly than the human population as a whole is the founder effect. When a small group of individuals establishes a new population, the genetic makeup of this initial group can have a disproportionate influence on the gene pool.
If the founding members carry the polydactyly allele, it could become more common in the subsequent population. In the case of the Amish, where polydactyly is seen at a higher frequency, this can be attributed to a small number of individuals, including a ship captain with polydactyly, who found the population and had a different allele frequency for the polydactyly trait compared to the original population they left.