. When birds migrate into or out of a defined population and mate with birds from another population, the existing pool of genes may be altered. Migration can increase or decrease gene
frequencies in populations, resulting in...

A. Greater genetic variation, which may enhance chances of survival

B. Less genetic variation, which will not influence chances of survival at all

C. Selective breeding, which may enhance chances of survival

D. Natural selection, which will not influence chances of survival at all