What Is Genetic Hitchhiking?

Genetic hitchhiking is a phenomenon in evolutionary genetics where an allele changes frequency in a population not because it is under direct natural selection, but because it is physically close to another gene that is experiencing a selective sweep on the same DNA strand. As the beneficial gene is rapidly favored and spreads through the population due to selection, nearby alleles—regardless of their own effect—are “dragged along” and also increase in frequency. This process is also known as the hitchhiking effect or genetic draft.

How Does Genetic Hitchhiking Work?

• Selective Sweep: When a new, advantageous mutation arises, it can quickly become common in the population due to strong positive selection.

• Physical Linkage: If another allele is located near this beneficial gene on the chromosome, recombination may not separate them, especially if they are very close together.

• Frequency Change: The linked allele, even if neutral or slightly harmful, rises in frequency simply because of its proximity to the selected gene. This is the essence of genetic hitchhiking.

Key Characteristics

• Not Under Direct Selection: The hitchhiking allele is not itself being selected for or against; its frequency changes due to its physical association with a selected gene.

• Reduction in Genetic Variation: Genetic hitchhiking can reduce genetic diversity near the beneficial gene, as the sweep brings linked alleles to high frequency or fixation.

• Role of Recombination: The closer the alleles are, the less likely recombination will separate them, making hitchhiking more effective.

Example

Imagine a plant population where a gene for disease resistance (under strong selection) is located near a gene with no effect on fitness. As the resistance gene spreads, the neutral gene’s frequency also increases—not because it is beneficial, but because it is “hitching a ride” with the selected gene.

Why Is Genetic Hitchhiking Important?

• Evolutionary Insight: It helps explain why some alleles become common even if they offer no advantage.

• Genetic Mapping: Patterns of reduced genetic variation can signal recent selective sweeps and help identify important genes.

• Population Genetics: Understanding hitchhiking is crucial for interpreting genetic data and evolutionary history.

Correct Answer

The phenomenon described—where an allele changes frequency not because it is under natural selection, but because it is near another gene undergoing a selective sweep on the same DNA chain—is called:

(3) Hitch hiking.

Genetic hitchhiking is a powerful concept that reveals how the fate of one gene can be tied to its neighbors, shaping the genetic landscape of populations through the indirect effects of selection.