Final answer:
A selective sweep is indicated by an allele at high frequency with high linkage disequilibrium with surrounding loci, reflecting a rapid increase in allele frequency due to strong positive selection and reduced genetic variation around the allele.
Step-by-step explanation:
The scenario that provides evidence that an allele has experienced a selective sweep in the recent past is the one where the allele is at high frequency and linkage disequilibrium with surrounding loci is high (Option 2). A selective sweep occurs when a beneficial allele rapidly increases in frequency due to strong positive selection, and as it does so, it 'sweeps' along closely linked alleles due to reduced recombination.
This results in a pattern of reduced genetic variation and increased linkage disequilibrium around the beneficial allele. If a selective sweep has recently occurred, we expect to find a high-frequency allele that is also in high linkage disequilibrium with its neighboring loci because there has not been sufficient time for recombination and mutation to reintroduce variation and to break down the linkage disequilibrium.
A selective sweep occurs when a particular allele experiences a rapid increase in frequency in a population due to strong positive selection. In the context of the question, option 1) The allele is at high frequency and linkage disequilibrium with surrounding loci is low is the scenario that provides evidence of a recent selective sweep. This is because a high frequency of the allele implies positive selection and low linkage disequilibrium indicates that the allele has spread rapidly through the population, suggesting recent selection pressure.