section_path: Change/Speciation
section_name: Speciation
paragraph_text: Speciation
  
  The evolutionary process by which biological populations of sexually-reproducing organisms evolve to become distinct or reproductively isolated as species is called speciation. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species. Speciation depends on a measure of reproductive isolation, a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model. A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form (a chronospecies), without increasing the number of resultant species.