The gene pool of a population of bighorn sheep in the southwest United States is altered when several sheep cross over a mountain pass and join the population. This would be an example of:

Answer: Gene flow

Explanation:

Gene flow is the process of transfer of variation in the genetic material from one population to another. This results in the change in the composition of the gene pool of the population which receives the genetic material.

The introduction of the new alleles by the process of gene flow increases the variation within the population and leads to the development of new traits in the new population. The gene flow begins when the population of a species translocate or migrate to a new location.

According to the given situation, the bighorn sheep population is migrating or shifting from the southwest United States to mountain pass and join the another population. Hence, the gene flow will occur due to inbreeding of population of the southwest United States and the population of mountain pass.