第20章系統と生命の歴史

Phylogenetic modeling concepts are constantly changing. It is one of the most dynamic fields of study in all biology. Over the last several decades, new research has challenged scientists’ ideas about how organisms are related. The scientific community has proposed new models of these relationships.

Many phylogenetic trees are models of the evolutionary relationship among species. Phylogenetic trees originated with Charles Darwin, who sketched the first phylogenetic tree in 1837 (Figure 20.12a). This served as a prototype for subsequent studies for more than a century. The phylogenetic tree concept with a single trunk representing a shared ancestry, with the branches representing the divergence of species from this ancestry, fits well with the structure of many common trees, such as the oak (Figure 20.12b). However, evidence from modern DNA sequence analysis and newly developed computer algorithms has caused skepticism about the standard tree model's validity in the scientific community.

Classical thinking about prokaryotic evolution, included in the classic tree model, is that species evolve clonally. That is, they produce offspring themselves with only random mutations causing the descent into the variety of modern-day and extinct species known to science. This view is somewhat complicated in eukaryotes that reproduce sexually, but the laws of Mendelian genetics explain the variation in offspring, again, to be a result of a mutation within the species. Scientists did not consider the concept of genes transferring between unrelated species as a possibility until relatively recently. Horizontal gene transfer (HGT), or lateral gene transfer, is the transfer of genes between unrelated species. HGT is an ever-present phenomenon, with many evolutionists postulating a major role for this process in evolution, thus complicating the simple tree model. Genes pass between species which are only distantly related using standard phylogeny, thus adding a layer of complexity to understanding phylogenetic relationships.

The various ways that HGT occurs in prokaryotes is important to understanding phylogenies. Although at present some do not view HGT as important to eukaryotic evolution, HGT does occur in this domain as well. Finally, as an example of the ultimate gene transfer, some scientists have proposed genome fusion theories between symbiotic or endosymbiotic organisms to explain an event of great importance—the evolution of the first eukaryotic cell, without which humans could not have come into existence.

Horizontal gene transfer (HGT) is the introduction of genetic material from one species to another species by mechanisms other than the vertical transmission from parent(s) to offspring. These transfers allow even distantly related species to share genes, influencing their phenotypes. Scientists believe that HGT is more prevalent in prokaryotes, but that this process transfers only about 2% of the prokaryotic genome. Some researchers believe such estimates are premature: we must view the actual importance of HGT to evolutionary processes as a work in progress. As scientists investigate this phenomenon more thoroughly, they may reveal more HGT. Many scientists believe that HGT and mutation are (especially in prokaryotes) a significant source of genetic variation, which is the raw material in the natural selection process. These transfers may occur between any two species that share an intimate relationship (Table 20.1).