| Summary: | <p>Abstract</p> <p>Background</p> <p>Hybridization, or the interbreeding of two species, is now recognized as an important process in the evolution of many organisms. However, the extent to which hybridization results in the transfer of genetic material across the species boundary (introgression) remains unknown in many systems, as does the length of time after initial divergence that the species boundary remains porous to such gene flow.</p> <p>Results</p> <p>Here I use genome-wide genotypic and DNA sequence data to show that there is introgression and admixture between the <it>melpomene</it>/<it>cydno </it>and silvaniform clades of the butterfly genus <it>Heliconius</it>, groups that separated from one another as many as 30 million generations ago. Estimates of historical migration based on 523 DNA sequences from 14 genes suggest unidirectional gene flow from the <it>melpomene</it>/<it>cydno </it>clade into the silvaniform clade. Furthermore, genetic clustering based on 520 amplified fragment length polymorphisms (AFLPs) identified multiple individuals of mixed ancestry showing that introgression is on-going.</p> <p>Conclusion</p> <p>These results demonstrate that genomes can remain porous to gene flow very long after initial divergence. This, in turn, greatly expands the evolutionary potential afforded by introgression. Phenotypic and species diversity in a wide variety of organisms, including <it>Heliconius</it>, have likely arisen from introgressive hybridization. Evidence for continuous gene flow over millions of years points to introgression as a potentially important source of genetic variation to fuel the evolution of novel forms.</p>
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