How similar do DNA sequences need to be for 2 organisms to be considered the same species? And how is this decided

According to the biological species concept, organisms belong to the same species if they can interbreed to produce viable, fertile offspring. Species are separated from one another by prezygotic and postzygotic barriers, which prevent mating or the production of viable, fertile offspring.

Except hold on to his or her beliefs even if there is no evidence for those beliefs.