(This series of three posts is taken from a review article I am planning to submit for publication. I have removed references from the posts, but will be happy to supply them on request.)
Natural Genetic Engineering (cont.)
There is strong evidence that a massive genomic alteration event occurred at about the time of the origin of the vertebrates. At some point between the origin of chordates and that of jawed vertebrates, an entire genome was duplicated, at two different times. Whole genome duplication (WGD) is an extremely useful (and rare) event in evolutionary terms, because it allows for a great deal of genomic trial and error in organisms, without interference from purifying or balancing selection. By providing an extra, non-essential copy of every gene, WGD allows for very rapid and dramatic evolutionary leaps, such as the development of new structures and functions (cartilage and bony skeletons being the relevant story here). There is also good evidence that similar WGD events have occurred in flowering plants, at the origin of teleost fishes, and probably at many other critical evolutionary transition points.
Duplication of individual genes or smaller groups of genes (such as a chromosome) can also lead to major evolutionary changes. Gene duplication is often mediated by a mechanism called retrotransposition, whereby a gene is duplicated at a new location thanks to the action of genetic elements called retrotransposons. Such events were found to occur during primate evolution, when the common ancestor of gorillas, chimps and humans split from the orangutan line. Gene amplification leads to a similar result – the production of many copies of a single gene, that is also likely to be induced by stress. Exon shuffling, repetitive elements play an important role in gene duplication and new gene creation in files.
Another mechanism for rapid large-scale genomic change is horizontal gene transfer, whereby one organism transfers a large chunk of genetic material to another organism. This is a well-known phenomenon in bacteria. It now appears that such genetic transfers have taken place between prokaryotes like bacteria and eukaryotes, like parasites and sponges. Horizontal gene transfer could also explain the origin of animal like alpha amylase from animals and plants to bacteria. Horizontal gene transfer from bacteria to eukaryotes has been linked to the origin of mineralization in sponges, which led to the eventual development of skeletons in modern animals.
All of the mechanisms described in this series are forming part of what has been called the Extended Evolutionary Synthesis (EES), which many evolutionary biologists believe should and will replace the standard neo Darwinian synthesis. There are many more aspects of the EES than have been covered here, including niche selection, epigenetics, and the control of gene expression during development (Evo Devo). These will be described in future posts.