Random genetic drift is the change in allele frequencies caused by chance events of survival, reproduction, and meiosis. Unlike selection, it does not systematically favor one allele or another.
Drift is stronger in smaller populations, causing faster evolution.
Drift tends to erode genetic variation within a population, and causes differences among populations to accumulate.
Some effects of drift are best understood by looking backward in time at the gene tree that reflects the genealogy of the copies of a gene that are carried by living individuals. Going backward in time along the gene tree, two genes coalesce at their most recent common ancestor. All the copies of a gene in any population, species, or group of species ultimately trace back to a single ancestral gene copy at some point in the past.
The strength of drift is determined by the effective population size, Ne. A natural measure for the strength of drift is 1/Ne, which determines the size of the random fluctuations in allele frequencies going forward in time. It also determines the average time in the past of the most recent common ancestor of two gene copies now carried by living individuals.
Ne is smaller (sometimes much smaller) than the actual population size, as the result of factors that include fluctuating population sizes and unequal sex ratios. Population bottlenecks are short, severe reductions in population size.
The heterozygosity at a DNA site that is evolving neutrally (with no selection) is expected to be proportional to Ne. This relation can be used to estimate Ne from genetic data. DNA bases at which deleterious mutations occur tend to be less polymorphic. As a result, introns and regions between genes are typically more variable than the coding regions of a genome. For the same reason, the third positions of codons are typically more variable than the first and second positions.
An allele will evolve largely as if selection is not acting when s << 1/Ne, while it will evolve largely as if drift is not acting if s >> 1/Ne.
Because the relative importance of selection and drift depends on the population size, many of the genetic differences among species with large Ne are expected to be adaptive, while in species with small Ne many of the genetic differences result from drift rather than adaptation. Drift has caused many deleterious mutations to be fixed in humans and other species.
Many (but not all) genes evolve at a relatively constant rate. A constant rate of molecular evolution is called a molecular clock, and it can be used to estimate the time since two species shared a common ancestor. Constant rates are expected when genes evolve neutrally, but relatively constant rates are also seen in some genes that are evolving by positive selection.
Several methods are used to detect selection acting on DNA sequences. Recent positive selection can be detected using variation within and differences between species. Loci identified this way are of interest because they tell us about the molecular basis of adaptive evolution.