Final answer:
Gene flow reduces genetic differences between populations, making speciation by reproductive isolation less likely. It hinders the process by allowing gene mixing, which contrasts with the genetic divergence needed for speciation. Thus, gene flow can prevent populations from becoming distinct, genetically independent species.
Step-by-step explanation:
Gene flow makes speciation by reproductive isolation less likely because it involves the exchange of genetic material between populations. This exchange reduces genetic differences between populations, which is necessary for speciation to occur. Reproductive isolation is a key factor in speciation, and gene flow counteracts this by allowing genes to mix between what could become separate species.
When populations become geographically isolated (called allopatric speciation), the free flow of alleles is prevented. Over time, due to different environmental pressures, these populations may diverge and become genetically independent species. Restricted gene flow is crucial for this process as it facilitates the populations to evolve along different trajectories, increasing the likelihood of speciation.
Therefore, the correct answer to the question is C) Gene flow can hinder speciation by preventing reproductive isolation.