soft sweep
Burke MK 2012 How does adaptation sweep through the genome? Insights from long-term selection experiments. Proc R Soc Lond B 279:5029-5038.
- a classic 'hard' selective sweep describes the process of a novel, major effect mutation arising on a single haplotype in a population and ultimately reaching fixation
- a 'soft' selective sweep, by contrast, describes the dynamics of selection acting on beneficial alleles present on many haplotypes in a population with standing genetic variation
- sweep patterns are hereafter classified as four specific types
- (i) a complete hard sweep
- (ii) an incomplete hard sweep
- (iii) a complete soft sweep
- (iv) an incomplete soft sweep
- in an incomplete hard sweep, zero heterozygosity is not achieved, for reasons that could include insufficient time-elapsed, epistatic interactions influencing sweep trajectory, or selection coefficients changing over time
- soft sweeps involve multiple copies of a selective allele contributing to a substitution
- these multiple copies may or may not be identical by descent
- when alleles are identical by descent, this is considered a single-origin soft sweep
- cases where beneficial alleles enter the population independently, either through recurrent mutation or migration, are considered multiple-origin soft sweeps
- a single-origin soft sweep should result in a narrower footprint of reduced heterozygosity than a hard sweep
- the variant under selection is older and has been exposed to more recombination events with nearby neutral sites
