Talk:Evolution/Gene flow

Gene flow is the exchange of genetic variation between populations, most commonly of the same species, in which case it is either the migration of organisms, or some substitute such as pollen, between populations. However, gene flow can also happen between different species: Where two closely-related species have adapted for different environments, hybrids may form along the border between those environments; [Source: One of Gould's books. Alas, I forget which.] plants commonly hybridize (for instance, most commercially-grown wheat is a hybrid of three different species); [expand this a bit, I think] and bacteria can share plasmids (small rings of DNA) coding for beneficial traits even between very distantly-related species.

Migration

To use

Ernst Mayr thought that gene flow is likely to be homogenising, and therefore counteracting selective adaptation. Obstacles to gene flow result in reproductive isolation, a necessary condition for speciation.

Horizontal Gene Transfer

Note: Hybridisation isn't exactly Horizontal Gene Transfer, as the genetic data goes to the offspring of the two species, as per normal. As the section currently in the article isn't very good, the section below is instead assembled from Horizontal gene transfer (which could itself benefit from a bit more work). It might also be nice to include the additions of parts of viruses into the DNA.

Horizontal gene transfer (HGT), also Lateral gene transfer (LGT), is any process in which an organism transfers genetic material to another cell that is not its offspring. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g. its parent or a species from which it evolved. Most thinking in genetics has focussed on the more prevalent vertical transfer, but there is a recent awareness that horizontal gene transfer is a significant phenomenon. Artificial horizontal gene transfer is a form of genetic engineering.

Horizontal gene transfer is common among bacteria, even very distantly-related ones. This process is thought to be a significant cause of increased drug resistance; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Enteric bacteria appear to exchange genetic material with each other within the gut in which they live. There are three common mechanisms for horizontal gene transfer in bacteria:

• Transformation, the genetic alteration of a cell resulting from the introduction, uptake and expression of foreign genetic material (DNA or RNA). This process is relatively common in bacteria, but less common in eukaryotes. Transformation is often used to insert novel genes into bacteria for experiments, or for industrial or medical applications. See also molecular biology and biotechnology.
• Transduction, the process in which bacterial DNA is moved from one bacterium to another by a bacterial virus (a bacteriophage, commonly called a phage).
• Bacterial conjugation, a process in which a living bacterial cell transfers genetic material through cell-to-cell contact.

Analysis of DNA sequences suggests that horizontal gene transfer has also occurred within eukaryotes, from their chloroplast and mitochondrial genome to their nuclear genome. As stated in the endosymbiotic theory, chloroplasts and mitochondria probably originated as bacterial endosymbionts of a progenitor to the eukaryotic cell. Horizontal transfer of genes from bacteria to some fungi, especially the yeast Saccharomyces cerevisiae has been well documented. There is also recent evidence that the adzuki bean beetle has somehow acquired genetic material from its (non-beneficial) endosymbiont Wolbachia; however this claim is disputed and the evidence is not conclusive.

"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." ^{[PROPERLY REFERENCE THIS QUOTE!]}