Migration by seeds or pollen between populations is an example of gene flow.

• A. Pollen from a plant population moves to a nearby population and fertilizes plants there
• B. A population experiences a drought and selects for drought tolerance
• C. A population experiences a bottleneck due to a flood
• D. New mutations arise in a population

Answer: (A)

Gene flow is the movement of alleles between populations through the transfer of individuals or their gametes. When pollen from one plant population fertilizes plants in a neighboring population, the genetic material carried by that pollen enters the new population’s gene pool, spreading alleles and often making the populations more genetically similar. This transfer of genes between populations is exactly what gene flow describes.

The other scenarios illustrate different evolutionary processes: drought-driven changes are natural selection acting on existing variation; a bottleneck is genetic drift caused by a sharp reduction in population size that reduces genetic diversity; new mutations arise within a population and introduce new alleles independently of gene flow.