Origin of Species

How does reproductive isolation evolve?The most common mode of speciation is allopatric speciation.In allopatric speciation, one species becomes distributed into twoor more populations occupying different geographic areas andexchanging few or no individuals with other populations. Thepopulations become geographically isolated.In isolation, the two populations evolve independently. Theybecome different ecologically, behaviorally, develop reproductivetiming differences, or develop some other difference that wouldkeep them reproductively isolated if they were brought backtogether.Often the evolution of the allopatric populations is associatedwith adaptation to different environments. But, the evolution canalso involve chance changes in chromosome number, orarrangement, that results in postzygotic isolation.12