[ELI5] How is the reproductive isolation formed in allopatric speciation?


Like I get there is a geographic barrier, the natural selection works and the populations’ gene pools differ in terms of adaptation to their new geographic location but how exactly is reproductive isolation formed? What mechanism works here?

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It’s the differentiation of their genes/chromosomes. Once the two populations are separated, they experience different mutations and genetic drift that they cannot share with the sister populations. Eventually enough differing mutations and other genetic differences will build up and the two populations will no longer be reproductively compatible.