which of these is an example of gene flow?
A. German immigrants settle in a colony in Eastern Pennsylvania without mixing with the locals.
B. A few black sparrows join a population of brown sparrows and mate with them.
C.Indiscriminate hunting reduces the blue whale population, which later revives.

Question

which of these is an example of gene flow?
A. German immigrants settle in a colony in Eastern Pennsylvania without mixing with the locals.
B. A few black sparrows join a population of brown sparrows and mate with them.
C.Indiscriminate hunting reduces the blue whale population, which later revives.

anonymous · Answer

Gene flow is when genes from one population enter a new population.  It is good to remember that even though this sounds a lot like migration, the two concepts (gene flow and migration) do not completely overlap.  

We can see this in your question. in "A" migration occurred but there was never an exchange of genes.  For the gene flow concept, we can look at two hypothetical population: pop A, a stationary population of a bird species and pop B, a migratory population of the same bird species as pop A.  If pop B moves through pop A and mates with individuals from pop A, but continues on and does not stay in the pop A range, we have genes being introduced without migration. @Debster06