As you told that, reproductive isolation allows each individual to evolve along different paths according to specif pressure present in different environment, but evolution is caused by variations which are brought about by the offspring. But as the offspring is individual one of its own species, so how does it reproduce? And with whom because if it reproduces with individuals of other species it will produce sterile organisms?
Reproductive isolation allows an individual or a group of individuals to diverge and eveolve along different paths. These individuals then separate and form a unique species over a course of time due to accumulation of genetic variations.
Now, reproductive isolation is essential for the accumulation of genetic variations. It helps in promoting genetic variability and evolutionary divergence. In the absence of reproductive isolation, various mutant forms will interbreed freely leading to the intermixing of their genotypes.
Whenever, individuals diverge due to reproductive isolation, they do not mate with the individuals of different species. They accumulate genetic variations leading to speciation and then reproduce increasing the number of their species.
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