Web and Network Models
Recognizing the importance of HGT, especially in prokaryote evolution, has caused some to propose abandoning the classic “tree of life” model. In 1999, W. Ford Doolittle proposed a phylogenetic model that resembles a web or a network more than a tree. The hypothesis is that eukaryotes evolved not from a single prokaryotic ancestor, but from a pool of many species that were sharing genes by HGT mechanisms. As Figurea shows, some individual prokaryotes were responsible for transferring the bacteria that caused mitochondrial development to the new eukaryotes; whereas, other species transferred the bacteria that gave rise to chloroplasts. Scientists often call this model the “web of life.” In an effort to save the tree analogy, some have proposed using the Ficus tree (Figureb) with its multiple trunks as a phylogenetic way to represent a diminished evolutionary role for HGT.