# Gene flow

(Redirected from Flow of genes)

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.[1] 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.[2]

Gene flow is the transfer of alleles from one population to another population through immigration of individuals.

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.[3][4] 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.[5]

## Measuring gene flow

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 (${\displaystyle N_{e}}$) 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(${\displaystyle Fst}$).[6] This formula accounts for the proportion of total molecular marker variation among populations, averaged over loci.[7] When there is one migrant per generation, the inbreeding coefficient (${\displaystyle Fst}$) 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 (${\displaystyle Fst}$ = 1). The most common ${\displaystyle Fst}$ 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[1].

${\displaystyle Fst=1/(4N_{e}m+1)}$

The formula can be modified to solve for the migration rate when ${\displaystyle Fst}$ is known: ${\displaystyle Nm=1(1/Fst-1)/4}$, Nm = number of migrants [1].

## Barriers to gene flow

### Allopatric speciation

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.[8] 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.[8] This is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects.[8] 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.

Examples of speciation affecting gene flow.

### Sympatric speciation

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.[9] 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 species

### Horizontal gene transfer

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 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) .[10][11]

Viruses can transfer genes between species.[12] Bacteria can incorporate genes from dead bacteria, exchange genes with living bacteria, and can exchange plasmids across species boundaries.[13] "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."[14]

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.[15]

"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."[16]

### Genetic pollution

Naturally-evolved, region-specific species can be threatened with extinction[17] 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.[18] 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.[19][20] 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.[21]

## Examples

Marine iguana of the Galapagos Islands evolved via allopatric speciation, through limited gene flow and geographic isolation.
• Fragmented Population: fragmented landscapes such as the Galapagos Islands are an 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.[22]
• 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.
Theorized historic radiation of the first humans throughout the world and various species of homoinids that may have contributed to the modern day humans.
• 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.[23][24] 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.[24]
• Butterflies: Comparisons between sympatric and allopatric populations of Heliconius melpomeneH. cydno, and H. timareta revealed a genome-wide trend of increased shared variation in sympatry, indicative of pervasive interspecific gene flow.[25]
• 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.[26] 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 an international breeding program in which genetic materials are 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.[27]

## References

1. ^ a b c Frankham, Richard; Briscoe, David A.; Ballou, Jonathan D. (2002-03-14). Introduction to Conservation Genetics. Cambridge University Press. ISBN 9780521639859.
2. ^ Daniel I. Bolnick and Patrik Nosil. "NATURAL SELECTION IN POPULATIONS SUBJECT TO A MIGRATION LOAD".
3. ^ A Hastings; Harrison, and S. (1994). "Metapopulation Dynamics and Genetics". Annual Review of Ecology and Systematics. 25 (1): 167–188. doi:10.1146/annurev.es.25.110194.001123.
4. ^ Hamrick, J. L.; Godt, M. J. W. (1996-09-30). "Effects of Life History Traits on Genetic Diversity in Plant Species". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 351 (1345): 1291–1298. ISSN 0962-8436. doi:10.1098/rstb.1996.0112.
5. ^ Eldridge, M. D., King, J. M., Loupis, A. K., Spencer, P., Taylor, A. C., Pope, L. C., & Hall, G. P. (1999). Unprecedented Low Levels of Genetic Variation and Inbreeding Depression in an Island Population of the Black‐Footed Rock‐Wallaby. Conservation Biology13(3), 531-541.
6. ^ Neigel, J. E. (1996). Estimation of effective population size and migration parameters from genetic data. Molecular genetic approaches in conservation, 329-346.
7. ^ Rogers, D. L., & Montalvo, A. M. (2004). Genetically appropriate choices for plant materials to maintain biological diversity. University of California. Report to the USDA Forest Service, Rocky Mountain Region, Lakewood, CO. www. f s I ed. u s/ r2.
8. ^ a b c Su H, Qu LJ, He K, Zhang Z, Wang J, Chen Z, Gu H (March 2003). "The Great Wall of China: a physical barrier to gene flow?". Heredity. 90 (3): 212–9. PMID 12634804. doi:10.1038/sj.hdy.6800237.
9. ^ Savolainen, Vincent; Anstett, Marie-Charlotte; Lexer, Christian; Hutton, Ian; Clarkson, James J.; Norup, Maria V.; Powell, Martyn P.; Springate, David; Salamin, Nicolas (2006-05-11). "Sympatric speciation in palms on an oceanic island". Nature. 441 (7090): 210–213. ISSN 0028-0836. doi:10.1038/nature04566.
10. ^ Johnston C, Martin B, Fichant G, Polard P, Claverys JP (March 2014). "Bacterial transformation: distribution, shared mechanisms and divergent control". Nature Reviews. Microbiology. 12 (3): 181–96. PMID 24509783. doi:10.1038/nrmicro3199. line feed character in `|journal=` at position 17 (help)
11. ^ Lang, A. S.; Zhaxybayeva, O.; Beatty, J. T. (2012). "Gene transfer agents: Phage-like elements of genetic exchange". Nature Reviews Microbiology. 10: 472–82. PMC . PMID 22683880. doi:10.1038/nrmicro2802.
12. ^
13. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2006-02-18. Retrieved 2005-12-31.
14. ^ "Archived copy". Archived from the original on 2005-10-16. Retrieved 2005-12-31.
15. ^ Horizontal Gene Transfer - A New Paradigm for Biology (from Evolutionary Theory Conference Summary), Esalen Center for Theory & Research
21. ^ https://web.archive.org/web/20130221052009/http://www.talking-naturally.co.uk/hybird-mallards-theyre-everywhere/. Archived from the original on February 21, 2013. Retrieved January 23, 2013. Missing or empty `|title=` (help)