There are two populations of the same species., The formation of a physical barrier results in the two populations becoming geographically isolated from each other., The environmental conditions on each side of the barrier may be different, so the separate populations may experience different or changing selection pressures., There is genetic variation within the two populations and some characteristics will be more beneficial than others. New mutations might arise that produce characteristics that are advantageous in each environment., The advantageous characteristics make it more likely for an organism to survive. The alleles for these characteristics will be passed on when the organism reproduces., Over many years, the genetic variation between the populations will increase. Eventually, they will have changed so much that they can no longer interbreed to produce fertile offspring..

Speciation

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