selective sweep

Understanding Selective Sweeps and Patterns of Polymorphism

A selective sweep is a process by which a beneficial mutation increases in frequency in a population, resulting in a change to genetic diversity (polymorphism).
The discussion revolves around understanding how polymorphism is affected during and after a selective sweep.

Polymorphic Sites: We begin with nine polymorphic sites in a population, which includes the beneficial mutation at the origin of the selective sweep.
Effects of the Sweep:
- As the beneficial mutation increases in frequency, surrounding sites (e.g., one to the right of the beneficial mutation) may also rise due to genetic linkage, known as "hitchhiking."
- From nine polymorphic sites, the count may reduce to as few as seven sites as the sweep progresses. This scenario illustrates a partial sweep where not all sites are affected equally.

A selective sweep continues until the beneficial mutation becomes fixed in the population, meaning it becomes the only variant present.
Time Dynamics: Over generations, the effects of the sweep unfold. The longer the duration, the more pronounced the change in genetic variance.
Stability of Mutations: While mutations can occur continuously, the introduction of new polymorphisms is often low post-sweep, leading to a new region with excess polymorphisms at low frequencies.

Low Frequency: The term "low frequency" indicates that certain nucleotide variants remain present at low levels due to their fixation and surrounding genetic context.
Gene Tree Representation: Drawing a gene tree reveals short branches for these polymorphisms which indicate recent fixation events and a low diversity at adjacent sites.

Definition: Tajima's D is a statistical measure used to detect natural selection by comparing two estimators of gene diversity—π (nucleotide diversity) and θ (theta, or expected diversity).
Understanding Tajima's D Values:
- D ≈ 0: Signals neutral evolution where π and θ are comparable.
- D < 0: Indicates an excess of low-frequency polymorphisms (typical in a selective sweep).
- D > 0: Suggests an excess of intermediate-frequency polymorphisms.
Negative Tajima's D: If a region shows a negative Tajima's D, it implies there are low-frequency polymorphisms linked to a selective sweep.

Even after a context of a selective sweep, there can be mutations that are weakly deleterious. These mutations persist in the population at low frequencies, consistently detected through negative Tajima's D.

In the case of a complete selective sweep, linked nucleotide variants to the left and right of the selected mutation experience a rise in frequency since they are inherited together.
Hitchhiking Effect: This results in large blocks of linkage disequilibrium.
- Meiotic recombination does not have sufficient time to separate these linked variants completely, maintaining them as a block above the expected frequency due to linkage with the beneficial mutation.

Studying Multiple Genes: To draw valid conclusions regarding selective sweeps, researchers must study not only the gene of interest but also other genes in the genome.
- This prevents misinterpretation of patterns caused by population growth dynamics.
Exponential Population Growth: If an entire genome shows the same Tajima's D pattern as the gene of interest, it suggests the observed patterns may stem from population growth rather than selective sweeps.

The discussion underscores the importance of understanding the genetic architecture and evolutionary dynamics of populations under selection.
Maintaining an analytical approach to studying multiple genetically linked regions is crucial in interpreting selective sweeps and their broader implications for evolutionary biology.