Option (1): The Hardy-Weinberg law applies to large populations, not small populations. It assumes that allele frequencies in a population remain constant over generations in the absence of evolutionary influences.
Option (2): The Sewall Wright effect (also known as genetic drift) is more significant in small populations, not large ones.
Option (3): The development of resistance to DDT by mosquitoes is an example of directional selection. Over time, mosquitoes with resistance to DDT survive and reproduce, increasing the frequency of resistant alleles in the population.
Option (4): The process of one species diverging into two or more species is known as cladogenesis, not anagenesis. Anagenesis refers to the evolution of a single species over time without branching.
Thus, option (3) is the correct answer, as the resistance to DDT in mosquitoes is a clear example of directional selection.