Such events when populations become temporarily small are called population bottlenecks. An interesting result is that genetic drift that occurs while the population is small will impact the genetic variation in the population even when it has grown large again. This finishes the introduction to genetic drift. Remember from the lecture on Hardy-Weinberg Equilibrium that gene flow causes evolution because when individuals move into a population they may bring in alleles not already present in that population or in frequencies that differ from the allele frequencies of that population. First let's look at the effects of gene flow on a population.
Gene flow increases genetic variation within a population.
This increase occur because individuals from other populations will bring in alleles that would otherwise be absent or rare in the population. Note that the effect of gene flow on genetic variation in a population is the opposite of the effect of genetic drift. Now consider how gene flow affects similarities and differences among populations. Gene flow tends to make populations genetically similar to each other in that if gene flow occurs, the alleles that occur in one population will be introduced to the other population. The more gene flow occurs, the more similar the populations will become.