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Gene Flow

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Biology

You have probably seen or heard about migratory animals, like some birds and mammals. Migration, or dispersal, is a common event in wild populations of animals and plants. What is the effect of an organism carrying its genetic material to another population? This receiving population is probably exposed to different environmental conditions than the migrating organism came from. We will discuss how these migrating organisms affect population evolution and genetics.

Gene Flow definition

Gene flow refers to the movement (in and out) of genes and alleles caused by the migration of organisms, or their gametes, between populations.

Although we usually think of animals when we talk about migration, plants can also interchange genetic material with other populations, so sometimes, the term dispersal is preferred over migration. In plants, it is not the adult organism that moves, but the gametes carried in pollen or the seeds carried in fruits (through wind, water, or by animals like birds and mammals). Remember that gene flow occurs only when a gene or allele enters a receiving population's gene pool. Thus, an organism not only has to reach and live in the receiving population, but it must reproduce and pass on its genes to this population.

The level of gene flow between two populations can vary. Populations of organisms that are not very mobile (that do not or cannot move great distances) will have a relatively low gene flow. For example, plants pollinated by birds or bats will get their gametes further away than wind-pollinated plants. Animals like frogs do not move a lot either; thus, they will have a lower rate of gene flow than more mobile organisms like birds.

When there is a significant rate of gene flow between two populations, they can eventually share the same gene pool. Hence gene flow tends to homogenize the genetic makeup of populations.

Individuals adapt to their local environment through natural selection. Will a trait or allele that makes an individual more fitted to its environment have the same beneficial effect in the receiving population? The introduction or reintroduction of a trait to a population through gene flow is considered a random event in population genetics; therefore, it does not necessarily mean that it will benefit the population. This will depend on the selective pressures for each population.

Examples of Gene Flow and diagram

The water snake, Nerodia sipedon, inhabits the area of Lake Erie in Ohio and Ontario. The species is divided into several subpopulations that differ in their coloration. Most snakes on the mainland have a strong band pattern, which is advantageous for camouflage in marshes.

On the other hand, most snakes that reside on the islands in the lake have a uniform coloration that is more advantageous along rocky shores. In islands, snakes without bands survive at higher rates and are favored through natural selection. However, a small fraction of snakes with bands are still found on these islands.

Why haven’t the alleles for the banding pattern disappeared from these island populations? About 3-10 snakes are estimated to migrate to the islands per year, reintroducing the alleles for the band pattern into the islands. Thus, gene flow from the mainland maintains these alleles in the islands despite them being disadvantageous in that environment (Figure 1).

Gene Flow, Simplified diagram of intraspecific gene flow, StudySmarterFigure 1. Simplified diagram of intraspecific gene flow. Thick arrows represent natural selection over many generations, thin arrows represent gene flow. Source: Liza, StudySmarter Originals.

Based on the water snake example, we can infer that gene flow from the mainland tends to homogenize the genetic diversity of these populations and tends to make them more similar by the continuous reintroduction of the band pattern alleles (and, of course, all the genetic material these mainland individuals carry with them).

However, in this case, the introduced alleles are disadvantageous in the rocky environment of the receiving population. Therefore, their frequency does not increase due to natural selection. But what would happen if the introduced allele were advantageous in a receiving population?

Another well-documented case of adaptation through gene flow is the acquisition of insecticide resistance in Anopheles coluzzii, the African malaria mosquito. The insecticide resistance allele arose as a new mutation in another mosquito, A. gambiae.

This mutation rapidly increased in frequency in several subpopulations of A. gambiae, due to its strong selective advantage for these mosquitoes exposed to the insecticide. The allele eventually entered the gene pool of A. coluzzi through interspecific gene flow with A. gambiae, in an area where both species are present. The allele then spreads to other subpopulations of A. coluzzi through intraspecific gene flow.

Gene flow can occur between populations of the same species (intraspecifically) and between populations of different species (interspecifically). It is important to notice that gene flow between these two Anopheles species was possible because these are young species (they separated relatively recently). They still reproduce in areas where both species live. In these areas, they produce hybrid individuals (organisms whose parents are from different species). These hybrids mated with A. coluzzi individuals, introducing the allele to the gene pool of A. coluzzi (Figure 2).

Gene Flow, Simplified diagram of interspecific gene flow, StudySmarter

Figure 2. Simplified diagram of interspecific gene flow. Thick arrows represent natural selection over many generations, thin arrows represent gene flow, and colored dots represent alleles. Source: Liza, StudySmarter Originals.

These mosquito hybrids have reduced fitness, though, signifying that strong reproductive isolation is in process. If gene flow stops between these populations, they will keep diverging genetically while each species continues to adapt to local conditions. For subpopulations of the same species, as with the water snakes, lack of gene flow would eventually lead to speciation.

Human evolution is also an example of gene flow between species. Tibetans carry a gene associated with differences in hemoglobin concentration at high altitudes. This gene is hypothesized to come from Denisovan or Denisovan-related populations and would have helped Tibetans adapt to the low oxygen conditions where they live.

Denisovans are extinct hominins whose origins are still unclear; they may have been ancestors of both Neanderthals and modern humans, or possibly only Neanderthals. We also postulated that we acquired the gene related to human diabetes from Neanderthals (Homo neanderthalensis). In fact, we all share some percentages of genes derived from the Neanderthal lineage. Thus, gene flow events between Homo species were not that rare.

Gene Flow Can Cause Evolution

Gene flow produces changes to allele frequencies in a population; therefore, it is one of the main mechanisms that drive evolution, along with genetic drift and natural selection. However, these changes will only be adaptive if the introduced genes or alleles are beneficial to the receiving population, as with the insecticide resistance allele in mosquitoes. In this sense, introducing new variants through gene flow is random, similar to a mutation. And similarly, after being introduced, it depends on the effect of natural selection to increase or decrease the frequency of an introduced allele.

How do Gene flow and Genetic drift differ?

As mentioned, gene flow and genetic drift are mechanisms that drive evolution, along with natural selection. Unlike natural selection, changes in alleles’ frequencies caused by gene flow or genetic drift are random (not related to an allele’s advantageous or disadvantageous effect on fitness). For example, the continuous reintroduction of the banded coloration pattern to islands’ populations is disadvantageous for water snakes.

However, gene flow and genetic drift differ in their specific causes and the direction of their outcomes.

Gene flow is caused by the dispersal of organisms or their gametes to a different population.

Genetic drift is caused by random shifts of allele frequencies from one generation to the next (known as sample error).

Concerning their outcomes, we discussed how gene flow could increase the genetic diversity in a population (through the introduction of new alleles) and decrease the differences between populations (by sharing alleles). On the other hand, genetic drift (and natural selection) tend to reduce genetic diversity within a population and increase the differences between populations.

Gene Flow - Key takeaways

  • Gene flow is one of the main mechanisms driving evolution, genetic drift, and natural selection.
  • Gene flow occurs between populations of the same species (intraspecific) or between populations of different species (interspecific).
  • The level of gene flow between populations depends on the mobility or dispersal capabilities of adult individuals or their gametes to reach another population.
  • A disadvantageous allele can be maintained in a population if it is continuously being reintroduced through gene flow.
  • Gene flow can counteract the effects of natural selection and genetic drift. It increases the genetic variation in a population (through the introduction of new alleles) and decreases the genetic differences between populations (by sharing alleles).
  • When gene flow is reduced or stopped, each population adapts to local conditions, and their gene pool keeps diverging, eventually leading to speciation.

References

1. Anna Tigano and Vicky Friesen, Genomics of local adaptation with gene flow, Molecular Ecology, 2016.

2. Campbell et al., Biology 7th edition, 2020.

3. Huerta-Sánchez et al., Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA, Nature, 2014.

4. The deep roots of diabetes, Understanding Evolution, 2014. (https://evolution.berkeley.edu/evo-news/the-deep-roots-of-diabetes/)

Gene Flow

Gene flow refers to the movement (in and out) of genes and alleles caused by the dispersal of organisms, or their gametes, between populations. 

Both gene flow and genetic drift are evolutionary mechanisms (along with natural selection), they can produce changes in an allele frequency. However, gene flow can counteract the effects of the other two, as it tends to increase the genetic variation in a population and decrease the genetic differences between populations.

An example of gene flow is the acquisition of insecticide resistance in Anopheles coluzzii, the African malaria mosquito. The resistance allele arose as a new mutation in another mosquito, Anopheles gambiae, and then entered A. coluzzi’s gene pool through gene flow between the two species.

Yes, gene flow can increase genetic variation if it introduces new alleles in the receiving population.

Gene flow is caused by individuals breeding with individuals from different populations. This implies the movement (migration or dispersal) of adult individuals or their gametes from one population to another. 

Final Gene Flow Quiz

Question

Gene flow refers to the movement of only adult individuals between populations

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Answer

False, gene flow also refers to the movement of gametes between populations (like with wind-pollinated plants) 

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Question

Would you expect more gene flow between populations of frogs or between populations of birds? 

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Answer

Between populations of birds, as they can move greater distances (they have more mobility) 

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Question

Gene flow _____ the genetic variation in a population, and ____ genetic differences between populations? 



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Answer

increases, decreases 

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Question

A gene or allele introduced in a population will always be beneficial and therefore its frequency will increase with next generations. 


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Answer

False

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Question

When gene flow is reduced or stops between two populations: 


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Answer

each population continues to adapt to their local conditions and their genetic pool keeps diverging 

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Question

Gene flow, genetic drift, and natural selection are similar because they: 


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Answer

can produce changes in allele frequencies in a population 

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Question

Contrarily to natural selection, both gene flow and genetic drift:  


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Answer

are random and usually lead to nonadaptive evolution 

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Contrarily to gene flow, both genetic drift and natural selection (select all that apply): 

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reduce genetic variation within a population

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Question

Imagine a population of birds subdivided into two populations A and B.  Each population lives at the top of a mountain and are separated by a valley. Initially, there were blue birds in both subpopulations and green individuals were only found in population A. Twenty years later, green individuals are also present in population B. What is the most probable scenario?

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Answer

an increase in gene flow between the two populations

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How does reducing gene flow cause speciation?

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When gene flow is reduced or stops between two populations, each population continues to adapt to local conditions and their genetic pool keeps diverging, eventually leading to speciation.

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Question

In the water snake example from the article, snakes with a band pattern coloration are maintained in the islands’ populations because: 

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Answer

The band pattern allele is reintroduced constantly by migrant snakes 

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Question

How is the introduction of new variants through gene flow similar to a mutation?

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Answer

Both are random. The effect of natural selection to increase or decrease the frequency of an introduced allele will depend on the allele’s impact (beneficial, neutral, or disadvantageous).

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Question

Gene flow can only occur between populations of the same species


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Answer

False

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Question

A conservation strategy for small populations recovery can be to maintain gene flow with other populations. Why do you think this can help a small population? 


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Answer

Gene flow can increase or maintain genetic diversity, enhancing a population’s probability to adapt to changes in the environment. 

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Question

Which of the following can introduce new alleles into a population's gene pool?              

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Answer

mutation

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