Fay and Wu’s H: Distinguishing Natural Evolution from Positive Selection

In the field of population genetics, understanding how DNA sequences evolve is crucial for unraveling the history of life and identifying the forces driving adaptation. Researchers Justin Fay and Chung-I Wu developed a statistical test, known as Fay and Wu’s H, to differentiate between DNA sequences evolving randomly (neutrally) and those undergoing positive selection. This test builds upon earlier methods like Tajima’s D, offering a more refined approach to detect selective sweeps in evolutionary history.

Understanding the Challenge: Neutral vs. Non-Random Evolution

A DNA sequence with few variations across different populations can arise from several scenarios. It could be subject to strong purifying selection, where harmful mutations are quickly eliminated. Alternatively, it might have recently experienced a selective sweep – a rapid rise in the frequency of a beneficial allele, homogenizing the sequence. A population bottleneck, where a small group of individuals establishes a new population, can also lead to reduced genetic diversity.

Tajima’s D, a previous method for identifying non-random evolution, can indicate an excess of rare polymorphisms, suggesting selection or a recent sweep. However, it doesn’t pinpoint the specific cause. This is where Fay and Wu’s H comes into play.

How Fay and Wu’s H Works: Incorporating Outgroup Data

Fay and Wu’s H goes beyond analyzing population polymorphism data; it also incorporates data from an outgroup species – a closely related species that branched off earlier in evolutionary history. By comparing the sequences to the outgroup, researchers can determine the ancestral state of the allele before the lineages diverged. This allows for a more accurate assessment of whether observed patterns are due to recent positive selection or other factors like demographic changes.

Applications and Findings

Studies utilizing Fay and Wu’s H, alongside other tests like Tajima’s D and measures of Fst, have revealed population-specific selection signals in various genes. Research has indicated that positive selection signals are more prevalent in European (CEU) and East Asian (CHB) populations compared to Yoruba (YRI) populations.

Genes Under Selection

Several genes have been identified as potential targets of positive selection:

• Pigmentation-related genes: MC1R and MFSD12
• Odor reception-related genes: OR6C1 and TAS1R3
• Immune-related gene: TLR1
• Thyroid hormone transport: SLCO4A1 (particularly in YRI populations)
• Glucose and lipid metabolism: PASK, CPT1A, and EXOC7
• Neural development: COMT, TAS1R3, and ALMS1

These genes suggest adaptations in gene expression regulation in response to changes in diet and environment. Signatures of positive selection have been detected in genes associated with diet, pigmentation, and immunity when examining ancient genomes.

Distinguishing Fay and Wu’s H from Tajima’s D

Whereas both tests are used to detect selection, they perform differently depending on the timing of selection. Research suggests that Tajima’s D is more effective at identifying selection that occurred before a population expansion (prefixation), while Fay and Wu’s H is better at detecting selection that occurred after a population expansion or sweep (postfixation). This difference stems from the weighting of allele frequencies in each test.

Conclusion

Fay and Wu’s H is a powerful tool for understanding the forces shaping genetic diversity in populations. By incorporating outgroup data and providing a nuanced approach to detecting positive selection, it helps researchers unravel the complex interplay between evolution, adaptation, and population history. Continued application of this test, alongside other genomic analyses, will undoubtedly reveal further insights into the genetic basis of human phenotypic and physiological differences.