Natural cycles linked to environmental change result in new species forming and old species dying out.
New species arise when two populations become isolated.
Populations become isolated when they are separated geographically e.g. on islands.
In an isolated population alleles are selected that increase successful breeding in the new environment.
Speciation takes place when an isolated population becomes so different from the original population that successful interbreeding can no longer take place.
Speciation: isolation - genetic variation - alleles selected - interbreeding no longer possible - new species
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