Explain how natural selection can lead to new species forming (speciation)

Within a gene pool of a population, there is genetic variation, due to mutation. This leads to phenotypic variation. Some individuals will be better adapted to the environment than others and will therefore be more likely to breed and pass on their alleles to the next generation. This means that each generation will be slightly better adapted to the environment than the last, and thus evolution occurs. If a population is divided, and each fragment is isolated from each other (for example a river separating them, or continental drift), then they cannot interbreed. Each new populaion will face a different environment and therefore different selection pressures. This means that each population will begin to evolve different adaptations to suit their environment, and will eventually be so physically and genetically different that they can't breed and produce live, fertile offspring. This means that the two populations are now two seperate species, and speciation has occured. If they are reintroduced to each other (the isolating feature is lost), they will not interbreed.