Genetic drift

Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles are subject to sampling error.[125] As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.[126] It is usually difficult to measure the relative importance of selection and neutral processes, including drift.[127] The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.[128] The neutral theory of molecular evolution proposed that most evolutionary changes are the result of the fixation of neutral mutations by genetic drift.[5] Hence, in this model, most genetic changes in a

population are the result of constant mutation pressure and genetic drift.[129] This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.[130][131] However, a more recent and better-supported version of this model is the nearly neutral theory, where a mutation that would be neutral in a small population is not necessarily neutral in a large population.[93] Other alternative theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft.[125][132][133] The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[134] The number of individuals in a population is not critical, but instead a measure known as the effective population size.[135] The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest.[135] The effective population size may not be the same for every gene in the same population.