Natural selection is essentially a bookkeeping process. If there is heritable variation in phenotypic traits that affect fitness, then by definition the genetic composition of a population will change over time, over generations. Natural selection can be directional, stabilizing or disruptive. For qualitative traits, selection will lead to defined changes in allele and genotype frequencies. For qualitative traits, the response to selection will be determined by both the strength of the selection and the heritability of the trait.
Selection can occur at any stage of an organisms life cycle.
Sexual selection is a special mode of selection that affects traits that determine the access of individuals to mates. Sexual selection primarily affects traits that determine the access of individuals of the limted sex to mates. Drift is most important in small populations, and often results from populaion bottlenecks and founder effects. Drift can lead to loss of genetic variation within populations due to the chance loss of specific alleles. Gene flow is the transfer of alleles from one population to another, due to the migration of individuals or dispersal of gametes.