Both natural selection and genetic drift lead to evolution process by varying the gene frequency of a population over time. Both these processes are involved in evolution and are not mutually exclusive. However, natural selection is the only process, which selects the best adaptive organism to the environment, and genetic drift reduces the genetic variation.
These variations in genes or alleles are inheritable and genetic variation can be resulted by mutation, gene flow and sex.
Natural selection is a hypothesis proposed by Darwin, where most adaptive organisms are selected by the environment gradually. Natural selection occurs when individuals are genetically varied, that variation makes some individuals better than others, and those superior traits are heritable.
This process occurs through mutations, which occurs in individuals randomly due to various reasons. Because of these mutations, individual may have the advantage beyond the environmental challenges. Individual with this mutation may have better adaptation to the environment than others. For an example, the superior trait will help to escape from predators running faster than other individuals. They can reproduce more than other individuals and trait will pass to the second generation and the evolving of new species happens. The frequency of the new trait will increase in the genome and this process is called natural selection or survival of the fittest organisms.
The variation in allele frequencies within a population due to random sampling is simply called genetic drift or Sewall Wright effect. Due to random sampling, subset of the population is not necessarily a representative of the population. It might be skewed to either direction. Smaller the population, the effect of random sampling causes genetic drift than a larger population. Some alleles become more common while they are being selected over and over again, and some may disappear from the small, isolated populations. This genetic drift or disappearance of the allele is unpredictable (Taylor et al, 1998).
The new generations may be diverge form of the parental population thus resulting either extinction of the population or making more adaptive species to the environment. However, in a large population, this effect can be considered as negligible. Genetic drift does not select the adaptive organism like natural selection.