Gene Flow Between Species
Gene flow can occur between species, either through hybridization or gene transfer from bacteria or virus to new hosts.
Gene transfer, defined as the movement of genetic material across species boundaries, which includes horizontal gene transfer, antigenic shift, and reassortment is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can incorporate genes from other dead bacteria, exchange genes with living bacteria, and can exchange plasmids across species boundaries. "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes."
Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.
"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT . Combining the simple coalescence model of cladogenesis with rare HGT events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times."
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