When reproductive isolation occurs, for example, within a species of bird, if some bird started hunting at night instead of the day and eventually lived at night. Night birds will be more likely to mate with night birds instead of day birds. Eventually, the night birds with different traits adapted to fit their night time environment would be selected for and passed down to their offspring. This results in different traits being selected for in the night bird population vs the day bird population and over time, the differences accumulate until a point where the birds are so different and can no longer mate with each other ( according to the biological species concept). At this point, they will be considered as separate species.
In this example, you can see that reproductive isolation results in differential reproductive successes between organisms in the same species result in a difference of allelic frequency between these two groups. Over time, phenotypic differences accumulate causing speciation.
