Genetic hitchhiking

Genetic hitchhiking, also known as genetic draft, is a phenomenon in population genetics where an allele (a variant form of a gene) increases in frequency not because it confers a selective advantage to the organism, but because it is located near another gene on the same chromosome that is undergoing positive selection. This process can lead to the spread of neutral or even slightly deleterious alleles throughout a population.

Overview

Genetic hitchhiking occurs when a beneficial mutation arises in a gene, increasing the fitness of individuals carrying that mutation. If this advantageous allele becomes more common in the population through natural selection, alleles at nearby loci on the chromosome can "hitchhike" along with it. This is because recombination between chromosomes during sexual reproduction is less likely to separate alleles that are close together. As a result, these linked alleles can increase in frequency as a byproduct of the selection acting on the beneficial mutation.

Mechanisms

The key mechanisms behind genetic hitchhiking are linked to the structure and function of DNA and how genetic variation is inherited. Two main processes play roles in this phenomenon:

• Linkage Disequilibrium: This occurs when alleles at different loci are associated more frequently than would be expected by chance. In the context of genetic hitchhiking, linkage disequilibrium between a beneficial allele and its neighboring alleles means that these alleles will be inherited together more often than not.
• Selective Sweep: This is the process by which a beneficial allele rapidly increases in frequency, leading to a reduction in genetic variation around that allele. If a selective sweep is strong enough, it can cause nearby neutral or deleterious alleles to increase in frequency as well, due to their physical proximity to the advantageous allele.

Consequences

The consequences of genetic hitchhiking can be significant for the genetic diversity of populations. While it can lead to the rapid spread of beneficial alleles, it can also reduce overall genetic variation, especially in the regions of the genome near the selected allele. This reduction in variation can have long-term effects on the population's ability to adapt to future environmental changes.

Examples

One of the most well-known examples of genetic hitchhiking is the spread of the allele for sickle-cell anemia in regions where malaria is prevalent. The sickle-cell allele confers resistance to malaria, and despite its deleterious effects when present in two copies, it has increased in frequency in these regions due to the strong selective advantage it provides against malaria.

Research and Implications

Research into genetic hitchhiking is ongoing, with scientists using both theoretical models and empirical data to understand its role in evolution. Understanding this phenomenon is crucial for fields such as conservation genetics, where maintaining genetic diversity is a key concern, and medical genetics, where it can have implications for the spread of genetic diseases and the evolution of drug resistance.

See Also

• Population Genetics
• Selective Sweep
• Linkage Disequilibrium
• Evolutionary Biology