Abstract: Recent surveys of the human genome have shown that thousands of recent mutations have strong advantages and have increased greatly in frequency since their origins during the last 40,000 years. The rate of such changes appears to have increased by a factor of 100 times over the rate that characterized most of human evolution. Natural selection, as Darwin recognized, is fundamentally a demographic phenomenon: individuals with one allele have a higher intrinsic rate of growth than those having alternative alleles, resulting in the replacement of such alleles over time. When this process occurs across a population spread over geographic space, a wave of population growth and migration tends to disperse a selected allele outward from its source. This process is well understood when modeled for a single gene. However, the introduction of many (perhaps thousands) of simultaneously selected alleles may lead interactions between genes to outweigh the fitness consequences of individual genes.