Allelic gene conversion softens selective sweeps.

Area of Science:

• Population Genetics
• Evolutionary Biology
• Genomics

Background:

• Positive selection drives adaptive evolution by favoring beneficial mutations.
• Selective sweeps, characterized by reduced genetic diversity, occur when adaptive alleles rapidly increase in frequency.
• Two models, hard sweeps (single origin) and soft sweeps (multiple origins), describe sweep dynamics, with soft sweeps often linked to rapid adaptation.

Approach:

• Investigated the impact of allelic gene conversion on selective sweeps originating from single de novo mutations.
• Conducted simulation studies across human, Drosophila, and Arabidopsis population models.
• Analyzed the frequency of multiple haplotypes arising from single-origin sweeps under varying demographic histories and gene conversion rates.

Key Points:

• Allelic gene conversion can 'soften' hard sweeps by copying the adaptive allele onto diverse genetic backgrounds, creating 'pseudo-soft' sweeps.
• Simulations show that gene conversion frequently results in multiple haplotypes, even from single-origin mutations.
• In large populations, sweeps involving multiple haplotypes are more common than hard sweeps, particularly with moderate selection.

Conclusions:

• Hard sweeps are likely exceptions, not the rule, in large populations, even with low mutation rates or no standing variation.
• The presence of soft sweep signatures does not automatically imply rapid adaptation or lack of mutation limitation.
• Gene conversion plays a crucial role in shaping the genetic landscape during positive selection.