By-Chih-Horng Kuo, Nancy A. Moran and Howard Ochman,
Abstract-Genetic drift, which is particularly effective within small populations, can shape the size and complexity of genomes by affecting the fixation of deleterious mutations. In Bacteria, assessing the contribution of genetic drift to genome evolution is problematic because the usual methods, based on intraspecific polymorphisms, can be thwarted by difficulties in delineating species' boundaries. The increased availability of sequenced bacterial genomes allows application of an alternative estimator of drift, the genome-wide ratio of replacement to silent substitutions in protein-coding sequences. This ratio, which reflects the action of purifying selection across the entire genome, shows a strong inverse relationship with genome size, indicating that drift promotes genome reduction in bacteria.
Genetic Drift-
- Genetic drift or allelic drift is the change in the relative frequency with which a gene variant (allele) occurs in a population that results from the fact that alleles in offspring are a random sample of those in the parents, and because of the role of chance in determining whether a given individual survives and reproduces. Genetic drift may cause gene variants to disappear completely, and thereby reduce genetic variability.
- Genetic drift is one of several evolutionary processes which lead to changes in allele frequencies over time. In contrast to natural selection, which makes gene variants more or less common due to their causal effects on reproductive success,[1] the changes due to genetic drift are not driven by environmental or adaptive pressures, and may be beneficial, neutral, or detrimental to reproductive success.
- The effect of genetic drift is larger in small populations, and smaller in large populations. Vigorous debates wage among scientists over the relative importance of genetic drift compared with natural selection.
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