Question

how do specials of two isolate subpopulation become two different species

Answer 1

Sometimes two isolated population diverges into two different species due to evolution.
The formation of two different species is known as the speciation .
This may occur due to many number of reasons such as natural selection, genetic drift, geographic isolation etc.
Sometimes nature select a particular species may be due to some trait that is needed for better survival, and allows the elimination of weaker species.
Sometimes geographic isolation separates the species from exchanging genetic material and this will also leads to speciation.