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
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.
1 Measuring gene flow
2 Barriers to gene flow
2.1 Allopatric speciation
2.2 Sympatric speciation
Gene flow between species
3.1 Horizontal gene transfer
3.2 Genetic pollution
5 See also
7 External links
Measuring gene flow
Decrease in population size leads to increased divergence due to
drift, while migration reduces divergence and inbreeding.
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
). This formula accounts for 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.
displaystyle Fst=1/(4N_ e m+1)
The formula can be modified to solve for the migration rate when
, Nm = number of migrants .
Barriers to gene flow
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
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.
Examples of speciation affecting gene flow.
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
Gene flow between species
Horizontal gene transfer
Main article: Horizontal gene transfer
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) .
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
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
Main article: Genetic pollution
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.
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.
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. 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
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
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 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.
Horizontal gene transfer
Horizontal gene transfer (between species)
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