Some phenotypes within a population are better adapted to some environment changes and hence ae more likely to survive and reproduce successfully than other phenotypes . This is called directional selection. In due course of time this variant population outgrows the others and appears as new species. This would happen within days or in years. The pesticides DDT resistance in mosquitoes is common example of directional selection
In the original population, some individuals were DDT resistant because of gene mutation. They were fever because they had no advantage over DDT sensitive mosquitoes when DDT pesticides was not being used. Once the use of DDT pesticide started to control mosquitoes. only DDT resistant mosquitoes survived and they reproduced . In due course of time, their resistant genotype spread to almost the entire population because of directional selection
