In the absence of other factors, a person with an advantageous version of a gene, such as one that helps you digest food better or makes you immune to certain diseases, will generally survive to have more children. Given enough generations, this will result in a large number of people having that version. This is called a selective sweep, as the gene under selection quickly increases in frequency in a population.
If a selective sweep occurs quickly enough—for example, because of the spread of a contagious disease—recombination does not have a chance to unlink neighboring genes and SNPs. These “hitchhikers” are swept through the population together with the advantageous version. The extent of linkage can be calculated as a number and used to measure the magnitude and timing of a selective sweep by comparing it to the known mutation and recombination rates for the entire genome. It’s important to note that the dates provided are rough estimates—there will be considerable “fuzziness” in the calculated age of the sweep.
FOXP2 is an example of a gene that has swept through modern humans. It is a gene that is known to be associated with proper language development in the brain. Although people the world over have the same allele, the human FOXP2 is completely different from the versions seen in any other mammal or bird measured. The version all humans have today appears to have swept through the population at some point in the past 200,000 years, and is suggested as a reason for why we have more complicated language than other animals.