Differentiate between gene flow and genetic drift. give an example of each.

Genetic drift is the changes in allele frequency in a gene pool. The bottleneck effect and founder effect are prime examples of genetic drift. 

Gene flow, not to be confused with genetic drift, is what happens when organisms move into or out of a population. Gene flow either eliminates or introduces new alleles to the gene pool. 

In short: 

1. Genetic Drift is the changes in allele frequency in a population. 
2. Gene Flow is the process of alleles moving from one population to another. 

I hope this helps!