Final answer:
Gene flow reduces the differences between populations by allowing the exchange of genes between them, while the bottleneck and founder effects, types of genetic drift, increase differences due to random selection of alleles.
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 move from one population to another through various means such as migration of individuals or gametes. This genetic exchange can increase genetic variation within a population and can lead to populations becoming more genetically similar over time if individuals are migrating between them.
Conversely, both the bottleneck effect and the founder effect are examples of genetic drift, which is a change in the gene pool of a small population due to chance. The bottleneck effect occurs after a large portion of a population is wiped out, often due to a natural disaster, leaving a reduced genetic diversity. The founder effect happens when a new population is established by a few individuals who are not representative of the original population's genetic diversity. Both effects result in a decrease in genetic variation and an increase in genetic differences between populations.
Natural selection also can lead to differences between populations as it allows for the survival and reproduction of individuals with favorable inherited variations, leading to a change in the genetic structure of a population over time.