Step 2. Reproductive isolation occurs between two populations of Ancestor Species A.

Let us say that through some type of environmental change (for example, mountain-building, continental drift, or drying of the climate) members of Ancestor Species A are geographically divided into two separate populations. There will be no exchange of genetic information between the two populations; hence, they are reproductively isolated from each other.

In terms of the bead model, the original string has been duplicated. Each of the new strings will be adding beads from the pool independently (See step 3) and the genetic information in each population will begin to become unique to that population.

For the sake of our example, let's assume that reproductive isolation within Ancestor Species A occurred after two units of evolutionary time had passed since the original prototype ancestral species originated. The duplicate strings therefore look like:

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

   

Go to Step 3: Independent evolution of isolated populations and speciation

Return to Step 1: Evolution in prototype common ancestor species, A0.

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