Reproductive isolation: Natural selection at work

The crucial step in the separation of two populations of a sexually reproducing species into two new species — speciation — is the acquisition of reproductive isolation between them. Such isolation ensures that they can evolve independently of each other, and so end up with radically different genotypes and phenotypes. This causes the biological world to be divided up into the discrete taxonomic groups which we take for granted. Many different ways in which populations can fail to interbreed have been described, ranging from simple ecological separation of time or place of breeding, through behavioural barriers to interbreeding, to inviability or infertility of hybrids [1].