Gene flow is a concept in population genetics to refer to the movement of genes or alleles between interbreeding populations of a particular species. When members of a gene pool mate with members of another gene pool it can alter the allele frequencies (which pertains to the proportion of members in a population carrying a specific variant of a gene). Thus, gene pool can affect the genetic variability within a population. For instance, members of a population of particular species migrating to a new habitat could cause gene flow to occur when they mate with the members of a population already existing in the habitat. Thus, species that are highly mobile would have higher tendency of altering the allele frequency following gene flow. However, there are certain factors that serve as barriers to gene flow. Some of the factors affecting the rate of gene flow include physical barriers, geologic events and geographical barriers. An example of a geographical barrier is the street separating the population of flowering plant species to opposite sides. If the pollen from the plant on the one side is able to fertilize the plant on the other and eventually produce viable offspring, then, this species is able to recombine gene pools. In contrast, if the plant is unable to reach the population of plants of the same species on the other side because of this barrier, then, as a result, there may be little gene flow between the two populations.