A gene that controls an observable trait in birds has three different alleles. Which of the following best describes how gene flow would affect the genetic makeup of this population?

A. Allele frequencies would shift until the population has only one allele.

B. Birds would choose to express the allele that provides them with the greatest survival advantage.

C. The allele that provides the greatest survival advantage would increase, while the other two allele frequencies would decrease.

D. The bird population would have new alleles introduced into the gene pool, increasing genetic variation.