When population increases, it results in the competition between the individuals of that population for the available resources. When there is an increase in the numbers of individuals of two populations that are dependent on similar type of resources, then competition persists between the individuals of different species.
When competition occurs between the individuals of different species, it puts both individuals into pressure of winning the competition. Such pressure ultimately forces the individuals to develop certain adaptations that will make the individuals of one population superior to the other so as to win the competition.
Such adaptations developed in the members of the population are naturally selected and are often the doors to produce evolutionary changes. Therefore, these ultimately lead to the formation of new species. Since no two species can occupy the same niche, they evolve to occupy separate niches.