Mutation leads to the formation of new alleles. Natural selection will favour advantageous alleles and lead to an increased allele frequency. Sexual selection will lead to an increase in characteristics that encourage mating.Immigration and emigration will lead to gene flow (the flow of alleles between different populations) whilst genetic drift in small populations will lead to the loss of alleles by chance and any mutations will have a larger impact as there will be fewer alleles in the population.Density dependent factors depend on population size e.g. competition, predation, parasitism and communicable disease. Natural disasters, climate change and human activities are density independent.