Natural Selection and Genetic Drift
Evolution represents the process of change in the genetic content of populations. As such, the common denominator of evolution is the change in gene frequencies with time. Changes in allele (alternative forms of a gene) frequency can occur as a result of natural selection or genetic drift. Natural selection is the process by which organisms possessing the most favorable genetic adaptations outcompete other organisms in a population, resulting in their tendency to displace the less-adapted ones. Natural selection is dependent on the existence of variations of the same trait that confer different survival advantages or disadvantages (Graur and Li 2000). In contrast, genetic drift is a stochastic process by which changes in gene frequencies result from the accumulation of small, random, neutral mutations over time (Kimura 1983, 1989).
The forces of genetic drift and natural selection rarely act in isolation, and their relative strengths depend on several factors including effective population size. In large populations, genetic drift will occur slowly. Therefore even weak selection of an allele can have significant effects on its frequency; beneficial alleles will increase (positive selection), while harmful ones will be eliminated (negative or purifying selection) (Graur and Li 2000).
However, in species with a small effective population size, genetic drift will predominate. In small effective populations, deleterious alleles can increase in frequency leading to fixation in a population purely as a result of chance since the relative importance of genetic drift is greater. Species with a small effective population size are also subject to a greater probability of extinction because they are more vulnerable to genetic drift, leading to stochastic variation in their gene pool, demography, and environment. Any allele, regardless of its effect on fitness (deleterious, beneficial, or neutral), is more likely to be lost from a small population or gene pool than from a large one, resulting in a reduction in the number of variants of a given allele (Graur and Li 2000).