Genetic exchange between divergent evolutionary lineages, from introgressive hybridization between locally adapted populations to insertion of retroviral sequences into eukaryotic genomes, has now been documented. The detection of frequent divergence-with-gene-flow contrasts the neo-Darwinian paradigm of largely allopatric diversification. Nevertheless, of even greater significance is the growing wealth of data suggesting that the recipients of the transferred genomic material gain adaptive phenotypes from the donor lineages. This adaptive enrichment is reflected by changes in pathogenicity in viruses and bacteria, the transformation of ecological amplitude in eukaryotes, and adaptive radiations in extremely diverse lineages. Although genetic exchange may produce maladaptive consequences, most of the recently reported examples suggest increases in fitness, and many such adaptive trait transfers have been identified in our own species. Although the exchange of genetic material between lineages as diverse as viruses and mammals has been recognized for decades, particularly through the analysis of genomic datasets, it has remained an active debate as to whether such exchanges can lead to adaptive evolution. Recently, the growing wealth of examples of genetic transfer involving organisms from all domains of life has provided the means to test the hypothesis of adaptive genetic exchange. The repeated testing of this hypothesis has revealed not only adaptive effects among viral and prokaryotic lineages but also for plants, animals, and fungi. Indeed, the data now available indicate how profoundly important ancient and more recent gene exchange has been in the evolution of even humans. Adaptive evolutionary diversification can now be seen as being often facilitated by the addition of standing genetic variation from one divergent lineage to another.