In population genetics, gene flow (also known as gene migration) is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent genetic diversity and therefore effectively a single population. It has been shown that it takes only "One migrant per generation" to prevent population diverging due to drift. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants into or out of a population may result in a change in allele frequencies (the proportion of members carrying a particular variant of a gene), changing the distribution of genetic diversity within the populations. Immigration may also result in the addition of new genetic variants to the established gene pool of a particular species or population. High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason,gene flow has been thought to constrain speciation by combining the gene pools of the groups, and thus, preventing the development of differences in genetic variation that would have led to full speciation.
There are a number of factors that affect the rate of gene flow between different populations. Gene flow is expected to be lower in species that have low dispersal or mobility, occur in fragmented habitats, there is long distant between populations, and smaller populations sizes. Mobility plays an important role in the migration rate as a highly mobile individuals tend to have greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind. As dispersal distance decreases, gene flow is impeded and inbreeding, measured by the inbreeding coefficient (F), increases.For example, many island populations have low rates of gene flow due to geographically isolated and small population size. The Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia. The population is so strongly isolated that gene flow is not a possibility leading to high occurrences of inbreeding.
Measuring gene flowEdit
Decrease in population size leads to increased divergence due to drift, while migration reduces divergence and inbreeding. Gene flow can be measured by using the effective population size ( ) and the net migration rate per generation (m). Using the approximation based on the Island model, the effect of migration can be calculated for a population in terms of the degree of genetic differentiation( ). This formula accounts fro the proportion of total molecular marker variation among populations, averaged over loci. When there is one migrant per generation, the inbreeding coefficient ( ) equals 0.2. However, when there is less than 1 migrant per generation (no migration), the inbreeding coefficient rises rapidly resulting in fixation and complete divergence ( = 1). The most common is < 0.25. This means there is some migration happening. Measures of population structure range from 0 to 1. When gene flow occurs via migration the deleterious effects of inbreeding can be ameliorated.
The formula can be modified to solve for the migration rate when is known: , Nm = number of migrants .
Barriers to gene flowEdit
When gene flow is blocked by physical barriers, this results in Allopatric speciation or a geographical isolation that does not allow populations of the same species to exchange genetic material. Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba, Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China where Ulmus pumila grows. This is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools.
Barriers to gene flow need not always be physical. Sympatric speciation happens when new species from the same ancestral species arise along the same range. This is often a result of a reproductive barrier. For example, two palm species of Howea found on Lord Howe Island were found to have substantially different flowering times correlated with soil preference, resulting in a reproductive barrier inhibiting gene flow. Species can live in the same environment, yet show very limited gene flow due to reproductive barriers, fragmentation, specialist pollinators, or limited hybridization or hybridization yielding unfit hybrids. A cryptic species is a species that humans cannot tell is different without the use of genetics. Moreover, gene flow between hybrid and wild populations can result in loss of genetic diversity via genetic pollution, assortative mating and outbreeding.
Gene flow between speciesEdit
Horizontal gene transferEdit
Horizontal gene transfer (HGT) refers to the transfer of genes between organisms in a manner other than traditional reproduction, either through transformation (direct uptake of genetic material by a cell from a its surroundings), conjugation (transfer of genetic material between two bacterial cells in direct contact), transduction (injection of foreign DNA by a bacteriophage virus into the host cell) or GTA-mediated transduction (transfer by a virus-like element produced by a bacterium) .
Viruses can transfer genes between species. Bacteria can incorporate genes from dead bacteria, exchange genes with living bacteria, and can exchange plasmids across species boundaries. "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."
Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists [should] instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.
"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT. Combining the simple coalescence model of cladogenesis with rare HGT events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times."
Naturally-evolved, region-specific species can be threatened with extinction through genetic pollution, potentially causing uncontrolled hybridization, introgression and genetic swamping. These processes can lead to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal. Nonnative species can threaten native plants and animals with extinction by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones which can occur between island and mainland species. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that supplant the native stock. The extent of this phenomenon is not always apparent from outward appearance alone. While some degree of gene flow occurs in the course of normal evolution, hybridization with or without introgression may threaten a rare species' existence. For example, the Mallard is an abundant species of duck that interbreeds readily with a wide range of other ducks and poses a threat to the integrity of some species.
- Fragmented Population: fragmented landscapes such as the Galapagos Islands are a ideal place for adaptive radiation to occur as a result of differing geography. Darwin's Finches likely experienced allopatric speciation in some part due to differing geography, but that doesn't explain why we see some many different kinds of finches on the same island. This is due to adaptive radiation, or the evolution or varying traits in light of competition for resources. Gene flow moves in the direction of what resources are abundant at a given time.
- Island Population: The Marine Iguana is en endemic species of the Galapagos Islands, but it evolved from a mainland ancestor of land iguana. Due to geographic isolation gene flow between the two species was limited and differing environments caused the Marine Iguana to evolve in order to adapt to the island environment. For instance, they are the only iguana that has evolved the ability to swim.
- Human Populations: Two theories exist for the human evolution throughout the world. The first is known as the multiregional model in which modern human variation is seen as a product of radiation of Homo erectus out of Africa after which local differentiation led to the establishment of regional population as we see them now. Gene flow plays an important role in maintaining a grade of similarities and preventing speciation. In contrast the single origin theory assumes that there was a common ancestral population originating in Africa of Homo sapiens which already displayed the anatomical characteristics we see today. This theory minimizes the amount of parallel evolution that is needed.
- Butterflies: Comparisons between sympatric and allopatric populations of Heliconius melpomene, H. cydno, and H. timareta revealed a genome-wide trend of increased shared variation in sympatry, indicative of pervasive interspecific gene flow.
- Plants: Two species of Monkeyflowers, mimulus lewsii and mimulus cardinalis, were found to have highly specialized pollinators that acted on major genes resulting in a contribution to the floral evolution and reproductive isolation of these two species. The specialized pollination limited gene flow between the two species, eventually resulting in two different species.
- Human-mediate gene flow: The captive genetic management of threatened species is one way in which humans attempt to induce gene flow in ex situ situation. One example is the Giant Panda which is part of a international breeding program in which genetic materials is shared between zoological organizations in order to increase genetic diversity in the small populations. As a result of low reproductive success, artificial insemination with fresh/frozen-thawed sperm was developed which increased cub survival rate. A 2014 study found that high levels of genetic diversity and low levels of inbreeding were estimated in the breeding centers.
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