Final answer:
Gene flow is the process that typically reduces differences between populations by allowing the exchange of genetic material, in contrast to the bottleneck and founder effects, which are forms of genetic drift that can increase genetic differences.
Step-by-step explanation:
The factor that tends to reduce the differences between populations is gene flow. Gene flow is the process in which genes flow from one population to another, typically via the migration of individuals or the movement of gametes. This exchange of genetic material can decrease the genetic differentiation between populations and increase genetic diversity within populations. In contrast, both the bottleneck effect and the founder effect are forms of genetic drift, where allele frequencies within a population change due to random sampling effects rather than migration or reproductive exchange between populations.
The bottleneck effect occurs when a large portion of a population is suddenly eliminated, often due to a natural disaster, which can lead to a random sampling of alleles and potential genetic change. The founder effect, on the other hand, occurs when a new population is established by only a few individuals from a larger population, carrying an unrepresentative set of alleles. While these effects can lead to increased genetic differences between populations, they work differently than gene flow.