Question

Why is reproductive isolation required for speciation to occur?

Answer 1

If populations can interbreed, they are considered one species.

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Answer 2

Let's say a geographical barrier separates a species into two different population. Which also cause a reproductive isolation ( the two pupolations cannot interbred)

Throughout the time (lets say a million years), the two populations will undergo a lot of changes in their  respective genomes and eventually will form 2 different species.

If there is no reproductive isolation, the 2 population could interbred with one another and there will be no speciation