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Interspecific hybridization has contributed significantly to land diversity, species evolution, and crops' domestication, including upland cotton, the cultivated form of Gossypium hirsutum. Being the world's most important fiber crop species, Gossypium hirsutum belongs to the allotetraploid Gossypium consisting of six additional tetraploid species. The lint fiber evolved once in diploid parent A-genome species in the Gossypium's history and passed on during hybridization of the A-genome with the D-genome and was maintained in subsequent evolution. The domestication history of G. hirsutum involved the collection and use of lint fibers by indigenous people for the purpose of making strings and other textile products; hence, spinnable lint fibers were likely to have evolved under domestication. Crossing with G. barbadense has resulted in the development of multiple genetic lines in contemporary upland cotton. However, in later-generation hybrids between G. hirsutum and other polyploid species, reproductive barriers such as reduced fertility, segregation distortion, and hybrid breakdown are frequently observed, complicating the task of introgressing new, stably inherited allelic variation from inter-specific hybridization. Recent efforts in molecular genetics research have provided insights into the location and effects of QTLs from wild species that are associated with traits important to cotton production. These and future research efforts will undoubtedly provide the tools that can be utilized by plant breeders to access novel genes from wild and domesticated allotetraploid Gossypium for upland cotton improvement. [ABSTRACT FROM AUTHOR]