the frequency of an allele due to the randomness of what other non-neutral alleles it happens to be found in association with. Assuming genetic drift is the only evolutionary force acting on an allele, after one generation in many replicated idealised populations each of size N, each starting with allele frequencies of p and q, the newly added variance in allele frequency across those populations (i.e. the degree of randomness of the outcome) is . This equation shows that the effect of genetic drift is heavily dependent on population size, defined as the actual number of individuals in an idealised population. Genetic draft results in similar behavior to the equation above, but with an effective population size that may have no relationship to the actual number of individuals in the population. Instead, the effective population size may depend on factors such as the recombination rate and the frequency and strength of beneficial mutations. The increase in variance between replicate populations due to drift is independent, whereas with draft it is autocorrelated, i.e. if an allele frequency goes up because of genetic drift, that contains no information about the next generation, whereas if it goes up because of genetic draft, it is more likely to go up than down in the next generation. Genetic draft generates a different allele frequency spectrum to genetic drift. Applications Sex chromosomes The Y chromosome does not undergo recombination, making it particularly prone to the fixation of deleterious mutations via hitchhiking. This has been proposed as an explanation as to why there are so few functional genes on the Y chromosome. Mutator evolution Hitchhiking is necessary for the evolution of higher mutation rates to be favored by natural selection on evolvability. A hypothetical mutator M increases the general mutation rate in the area around it. Due to the increased mutation rate, the nearby A allele may be mutated into a new, advantageous allele, A* --M------A-- -> --M------A*-- The individual in which this chromosome lies will now have a selective advantage over other individuals of this species, so the allele A* will spread through the population by the normal processes of natural selection. M, due to its proximity to A*, will be dragged through into the general population. This process only works when M is very close to the allele it has mutated. A greater distance would increase the chance of recombination separating M from A*, leaving