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, 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); and bacteria can share plasmids (small rings of DNA) coding for beneficial traits even between very distantly-related species.

Migration

Migration into or out of a population may be responsible for a marked change in allele frequencies (the number of individual members carrying a particular variant of a gene). 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 [1]. Examples of the same species which grow on either side have been shown to be genetically different.

Hybridisation

Include antigenic shift, reassortment, and hybridisation of plants and animals.

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.[2].

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!]}

Population Structure

This isn't very good. Leaves out really important effects like, for instance, dominant males. However, we ought to be able to fix it too.

The free movement of alleles through a population 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.

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—smaller populations require a shorter time for fixation.

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.