Talk:Evolution/Gene flow

This page is for work on the "Gene flow" subsection of the Evolution article, so as to cause minimal interruption while the basic structure and content is hashed out.

Gene flow
Gene flow is the exchange of genetic variation between populations which are most commonly of the same species. Examples of intraspecial gene flow include the migration of organisms and the exchange of pollen between populations.

However, gene flow can also occur between different species. Suppose that two closely-related species have acquired adaptations suitable for different environments. In this situation, hybrids can form along the border between those environments. Plants easily form hybrids in this way, and bacteria can share plasmids (small rings of DNA) coding for beneficial traits even between very distantly-related species. As well, viruses can become incorporated into the genome, and can take DNA between hosts, allowing transfer of genes even across biological domains.

Migration
Migration into or out of a population may be responsible for a marked change in allele frequencies; that is, the number of individual members carrying a particular variant of a gene can change because of migration. Immigration may result in the addition of new genetic material to the established gene pool of a particular species or population, and conversely emigration may result in the removal of genetic material. As reproductive isolation is a necessary condition for speciation, gene flow within a species may delay speciation by partially homogenizing two otherwise diverging populations.

Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, vast deserts or something so simple as the Great Wall of China, which has hindered the natural flow of plant genes, with samples of the same species from different sides of the wall having been shown to be genetically different.

Hybridisation
Depending on how far two species have diverged since their last common ancestor, it may still be possible for them to mate and produce viable offspring. For example, horses and donkeys can be mated to produce mules and hinnys (named based on which species is the mother: with a few genes, the copy used depends on which parent it comes from). Mules and hinnys are largely infertile, though a few rare cases of successful mating with a donkey have been seen. However, as donkeys and horses have different numbers of chromosomes, the pairing up of chromosomes during meiosis, the process that produces eggs and sperm, usually fails to provide a viable set of chromosomes due to mispairing, which is usually lethal.

However, two more closely-related species may, in some cases, regularly interbreed, with natural selection strongly discriminating against the hybrids and thus keeping the populations distinct. This has been noted in, among other species, toads, butterflies, clams, and mussels. In rare cases, hybrids may be well adapted to a zone between the extremes favoured by the two parents, and may fill that zone.

Hybridisation, however, rarely leads to new species in the animal kingdom. However, it is a common and important method of producing new species in plants, where polyploidy, having more than two copies of each chromosome, is much more tolerated than in animals (where it is usually lethal). This allows hybrids to simply double their total number of chromosomes (not a particularly unusual circumstance in plants), and gain the ability to reproduce. One classic example spelt wheat and common wheat:

The basic precursors of wheat are all diploid, having two chromosomes with two copies each. The first hybridisation produced wild emmer, T. dicoccoides from T. urartu and some unknown wild goatgrass similar to Aegilops searsii or Ae. speltoides. This produced a plant with four chromosomes, but only one copy of each. In one such hybrid, by chance, a chromosomal duplication occured, allowing it to reproduce freely. Einkorn wheat (T. monococcum) is diploid (2 chromosomes, 2 copies of each). As grasses are generally self-fertile, it could then reproduce freely. Domestication developed this hybrid into emmer and durum wheat. Finally, either emmer or durum wheat hybridised with the wild grass Aegilops tauschii within farmer's fields, and, with another chromosomal duplication event, produced the ancestor of spelt wheat (Triticum spelta) and common wheat (Triticum aestivum). All of these hybrids have been reproduced experimentally.

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), which is also known as "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) or passes genetic material to its offspring. 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.

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.

Population Structure
''This isn't very good. Leaves out really important effects like, for instance, dominant males. However, it's also what we have in the article now.''

The free movement of alleles through a population may be impeded by population structure: the size and geographical distribution of a population. Most real-world populations are not actually fully interbreeding, as geographic proximity has a strong influence on the movement of alleles within the population. Population structure has profound effects on possible mechanisms of evolution.

An example of the effect of population structure is the founder effect, in which a population temporarily has very few individuals as a result of a migration or population bottleneck, and therefore loses much genetic variation. In this case, a single, rare allele may suddenly increase very rapidly in frequency within a specific population if it happened to be prevalent in a small number of "founder" individuals. The frequency of the allele in the resulting population can be much higher than otherwise expected, especially for deleterious, disease-causing alleles. Since population size has a profound effect on the relative strengths of genetic drift and natural selection, changes in population size can alter the dynamics of these processes considerably.

=For Genetic drift section=

The effect of genetic drift depends strongly on the size of the population: drift is important in small mating populations, where chance fluctuations from generation to generation can be large. The relative importance of natural selection and genetic drift in determining the fate of new mutations also depends on the population size and the strength of selection. Natural selection is predominant in large populations, while genetic drift is in small populations. Finally, the time for an allele to become fixed in the population by genetic drift (that is, for all individuals in the population to carry that allele) depends on population size&mdash;smaller populations require a shorter time for fixation.