==Gene Flow== Gene flow is the movement of genes into or out of a population. This may involve '''immigration''' (when new individuals enter the population) and '''emigration''' (when individuals leave the population). A population may gain or lose alleles through gene flow. [image:] Gene flow tends to reduce the differences between populations because members of each population can interbreed and genes are essentially shared or transferred between the two populations. This makes the two gene pools become more similar. Gene flow can have a greater effect on smaller populations. In a small population some individuals might be the only ones to carry a particular allele. If they leave the population that allele might be lost entirely. In a larger population it is likely that some other individuals will still possess the same allele. Similarly when an individual immigrates into a population they might introduce a new allele. Therefore gene flow can result in the loss or introduction of new alleles. ==Speciation== [image:] Organisms are considered to be members of the same species if they can produce fertile offspring. If two populations become isolated from one another and there is no gene flow between the two populations, they may develop into two distinct species. This process is known as "''speciation"''. As the two populations evolve under slightly different selection pressures they can become so different that ultimately they can no longer interbreed or produce fertile offspring. This only occurs if there is no gene flow between the two populations. Note that two populations don't have to be geographically isolated from one another for speciation to occur. Speciation occurs when there is little or no inter-breeding (gene flow) between the two groups. Therefore we can say speciation is the result of '''reproductive isolation'''.
Credit: Edward, Ben Himme